In sexually size-dimorphic species, physiological constraints derived from differences in body size may determine different food requirements and thus a trophic niche divergence between males and females. These relationships between sexual size dimorphism (SSD) and dietary overlap are not well understood in birds. We analyzed differences between the sexes in diet composition, dietary diversity, diet selection, and volume and density of droppings, as well as the dietary overlap between sexes, in the Great Bustard (Otis tarda), the species showing the highest SSD among birds. We discuss the differences that we found in relation to various predictions derived from ecological and physiological differences between the sexes, under the hypothesis that these differences are ultimately determined by the strong SSD of this species. As expected, our best linear mixed-effects food selection models included sex as a main factor explaining differences in diet composition and dietary diversity of Great Bustards throughout the annual cycle. Both sexes were mostly herbivorous, consuming legumes when they were available. Males consumed fewer arthropods, but of significantly larger size, than females. The droppings of males were larger, heavier, and slightly denser than those of females. Males showed higher dietary diversity than females, except during the postmating season. The mean dietary overlap between the sexes was 0.7, one of the smallest values among birds. Overall, our results suggest that the species' extreme SSD along with the distinct reproductive role of each sex might explain the trophic niche divergence in the Great Bustard.
Sexual size dimorphism (SSD) is widespread among animals. Male-larger dimorphism is extreme in some polygynous species, where sexual selection, operating through male competition and female choice, favors larger males (Shine 1989, Andersson 1994, Fairbairn 2013). Distinct reproductive roles, sex-specific metabolism, and sexual segregation behavior also contribute to dietary divergence between males and females. In birds, sexual differences in energetic and nutritional requirements are fulfilled through sex-specific habitat selection (Pasinelli 2000, Elson et al. 2007), foraging behavior (González-Solís et al. 2000, Markman et al. 2006), food intake patterns (Pepper et al. 2000, Odden et al. 2003), diet (Le V. Dit Durell et al. 1993, Forero et al. 2002), and even digestion (Markman et al. 2006).
In this study, we analyzed differences in the diets of male and female Great Bustards (Otis tarda) (Figure 1) through a detailed analysis of their food selection patterns during a complete annual cycle. We discuss the differences that we found in relation to various ecological and physiological differences between the sexes, which are ultimately determined by the strong SSD of this species. Indeed, Great Bustards show the most extreme SSD among birds, with adult males being 2.44 times heavier than adult females (Alonso et al. 2009a). This species is therefore an excellent candidate to investigate SSD-related differences in the diets of males and females.
Previous studies have described the species' diet, but have not analyzed sexual differences in the diet or diet selection patterns of males and females (Palacios et al. 1975, Lucio 1985, Lane et al. 1999). In a previous study, we found some sexual differences in the diet composition of juvenile Great Bustards (Bravo et al. 2012), and hypothesized that some of these differences could increase as SSD increases toward adulthood. In contrast, the less-dimorphic North African Houbara Bustard (Chlamydotis undulata undulata) does not show any apparent dietary divergence between the sexes (Bourass et al. 2012).
In our study area, Great Bustards of both sexes are partial migrants, with sedentary and migratory individuals living together in the same population. The migratory fraction of the population shows differential migration patterns by sex (Alonso et al. 2009b, Palacín et al. 2009), which obviously could lead to sexual differences in their diets. However, here we were interested in unravelling any subtle sexual differences in the diets of Great Bustards living in sympatry throughout the whole year. Like migratory individuals, sedentary individuals sharing the same area also live year-round in sexually segregated flocks, and meet only for mating. Previous studies have concluded that both the sexually differentiated migration patterns of migratory Great Bustards and the marked sexual segregation of sedentary individuals are consequences of the marked SSD (Alonso et al. 2009a, in press, Palacín et al. 2009), which is ultimately determined by the strong sexual selection operating in this species (Alonso et al. 2010). In this study, we suggest that the strong SSD might also be the primary ultimate cause of the dietary divergence between the sexes, probably mediated through subtle spatial segregation and sexual differences in microhabitat selection when males and females share the same area and main habitat (Alonso et al. in press).
It is known that males and females of species with marked SSD exploit different food resources, either because they have different nutritional and energetic requirements or because in this way they reduce resource competition between the sexes (Shine 1989, Ruckstuhl and Neuhaus 2006). Also, as a consequence of extreme SSD, many physiological and biochemical processes, such as heartbeat, respiration, and metabolic rate, are expected to show allometric scaling, primarily associated with the ratio between body surface and mass or volume (Karasov 1990, Witmer and Van Soest 1998, Brown and Downs 2003, van Gils et al. 2005a, Santiago-Quesada et al. 2009). Overall energetic requirements should be size-specific, and thus SSD-induced dietary differences between the sexes should be expected in sexually size-dimorphic species, in addition to differences imposed by their typically polygynous breeding system and associated differential reproductive roles of males and females.
Therefore, based on the marked sexual segregation in the Great Bustard, in this study we made the following predictions. First, we expected a noticeable divergence between the diets of the sexes due to their extraordinary SSD, and, consequently, a relatively low dietary overlap value compared with other birds with lower SSD. Second, we expected sexual differences in food selection values throughout the year, due to the marked differences in the reproductive roles of this strongly polygynous species (Fairbairn 1997). Third, as prey size is usually related to the size of the predator (Andersson 1994), we expected differences in the size of prey consumed by males and females. Such differences have been found in the Australian Bustard (Ardeotis australis; Ziembicki 2010) and in juvenile Great Bustards (Bravo et al. 2012), with males consuming bigger prey than females in both cases. Fourth, we expected sexual differences in the physical characteristics of droppings (weight, volume, and density) and in the seasonal patterns of variation in these characteristics. Our rationale was that sexual differences in diet composition could vary among seasons because food availability changes seasonally and nutritional requirements change throughout the annual cycle in a different way for each sex. In addition, dietary changes impose nutritional and physical limits on the digestive system and vice versa. Birds may survive on a low-quality diet by reducing their metabolism and altering their gut morphology (Geluso and Hayes 1999) to compensate for decreased digestive efficiency associated with a low-quality diet. Alternatively, the length of the intestinal tract may increase during periods of hyperphagia (Karasov et al. 1996), and gut morphology may change in relation to food quality and quantity (Castro et al. 1989, DeGolier et al. 1999). Because the surface and volume of the gut affect assimilation efficiency (Santiago-Quesada et al. 2009), these factors may also have an effect on the size and density of droppings.
Finally, extending the Jarman-Bell principle (a lower ratio of metabolic requirement to gut capacity allows larger herbivores to subsist on lower-quality diets compared with smaller herbivores) to the intraspecific level, we expected larger males to have a lower-quality diet (Bell 1970, Jarman 1974, Demment and Van Soest 1985). Although this prediction has been verified in herbivorous mammals, where males seem to have a simpler, lower-quality diet than females (Staines et al. 1982, Bowyer 1984, 2004, Beier 1987, Ruckstuhl and Neuhaus 2006, Pérez-Barbería et al. 2008), only a few studies with raptors and fish-eating species have explored sexual divergence in the diets of birds (Boal and Mannan 1996, Webster 1997, González-Solís et al. 2000, Ishikawa and Watanuki 2002, Lee and Severinghaus 2004, Xavier et al. 2006). In herbivorous birds, body size has been suggested as an important factor driving foraging behavior and diet (Durant et al. 2003, Mini 2012). Some trophic niche partitioning between the sexes has been recently suggested in the Greater Prairie-Chicken (Tympanuchus cupido; Blanco-Fontao et al. 2013). However, the relationships between SSD and dietary overlap between males and females are still poorly known in birds (Székely et al. 2007), and our study aimed to gain some insight into this topic.
We conducted our fieldwork in central Spain, where a population of ca. 1,500 Great Bustards has been intensively studied through 2 decades (Martín et al. 2012). Here, Great Bustards behave as partial migrants, but the numbers of males and females found in the study area year-round are still high (during the study period, numbers varied from a maximum of 507 males and 1,231 females in the spring of 2007 to a minimum of 235 males and 841 females in the summer of 2007; J. C. Alonso personal observation). Droppings and food availability were sampled in all 9 Great Bustard areas of Madrid province (Figure 2). The areal extents of these 9 Great Bustard areas were calculated by merging 500 m buffers around each bustard location recorded in our censuses of the species in Madrid province in 2004–2010. The habitat in these areas is mostly dedicated to cereal crop production (>80% of the land surface is occupied by common wheat [Triticum aestivum] and common barley [Hordeum vulgare] fields). Some olive (Olea europaea) groves and wine grape (Vitis vinifera) vines have been planted in the remaining area. Occasional sheep grazing occurs in stubble and fallow fields. Cereal is usually grown in a traditional 2-yr rotation; it is sown between October and December, harvested in late June–early July, and the field is later left fallow for a whole year.
The diets of Great Bustards were studied using 619 droppings (299 from females and 320 from males) collected in the 9 Great Bustard areas across 3 seasons: winter (November, 2006–January, 2007), mating (April, 2007), and postmating (July, 2007). We analyzed the composition of the diet, dietary overlap between the sexes, dietary diversity, diet selection by compositional analysis, and size of arthropod prey species.
Fresh droppings were collected at roosting sites at dawn (>75% of droppings), at midday resting sites after Great Bustards resumed foraging (>20%), and throughout the day at foraging sites (<5%). We assumed that droppings collected at roosting and resting sites accounted for the food intake of the previous evening and morning, respectively (see Bautista et al.  for collection method details). We also recorded the UTM coordinates of the collection site, the substrate type (stubble, ploughed, legume, sown, fallow, and field border), the date of collection, and sex of the flock. Male and female Great Bustards live in separate single-sex flocks, so droppings from each sex were easily distinguished (Bautista et al. 2013, Bravo et al. 2014). Droppings were mostly collected from stubble, ploughed, and cereal fields (>70% of droppings), with some collected from fallow and legume fields (<20% and >10%, respectively). All droppings were dried for 48 hr at 60°C and then weighed (accuracy: 0.001 g). The volume of each dropping was measured using a graduated cylinder and glass spheres (1.5 mm diameter). Dry droppings were introduced into a graduated cylinder and covered with glass spheres. The cylinder was shaken until there was no further decrease in total volume. The dropping volume was then calculated as the difference between the total volume and the volume of glass spheres. Dropping density (g per cm3) was calculated as dry weight divided by volume.
Each dropping was partitioned into green plant material, arthropods, seeds, and fruits, the latter of which were identified through their undigested seeds. These components were weighed separately and the dry weight proportion of each was calculated per dropping. Plant species were identified and quantified by microhistological techniques (Catán et al. 2003) using our reference collection of tissues from the study area. From each sample, 80 fields were examined with a microscope (40×), recording the presence or absence of each plant species. We calculated the percentage of each species in the diet as OD = ni × 100 / 80, where ni is the number of microscope fields in which species i occurred. We also estimated the percentage of each plant species in the diet as the percentage of dry mass in the droppings. All identified plants and their percentages in the diet are shown in Appendix Table 6. For statistical tests of dry weight percentages and compositional analysis, we grouped plant species into 3 functional groups: cultivated cereals, leguminous species, and weeds.
Arthropods were identified to order or family level using our reference collection and published identification keys (Calver and Wooller 1982, Moreby 1988). We estimated a minimum number of ingested individuals for each arthropod order or family using key body parts (Moreby 1988). In each dropping, recognizable prey remains were measured (e.g., head width, pronotum length, elytra length, and mandible length) with a pair of digital calipers (0.01 mm precision). Length (mm) and dry weight (mg) were estimated for each individual prey item by means of published linear regressions for each taxon (Hódar 1997). Average prey body size (mm) was calculated for each dropping. We calculated the percentage of arthropods in the diet as the percentage dry mass of each family or order in the droppings (Appendix Table 7).
Seeds and fruits were classified to family or species. We calculated the percentage of seeds and fruits in the diet as the percentage dry mass of each taxon in the droppings (Appendix Table 8). The fruit component of the diet was calculated as the dry weight of whole olives, grapes, and black nightshades (Solanum nigrum), estimated according to the number of their seeds found in the droppings and using dry masses and mean numbers of seeds obtained from samples of these fruits. For dietary diversity and overlap analyses, we used the percentages of all identified plants, arthropods, seeds, and fruits (Appendix Tables 6, 7, and 8).
For diet selection analyses, the availability of plants, arthropods, seeds, and fruits was estimated in the 9 Great Bustard areas (Figure 2). Within each Great Bustard area, a 400 m buffer around buildings and roads was excluded to obtain the total farmland surface available as foraging substrate (Torres et al. 2011). Buffers were calculated with ArcGIS 9.3 (ESRI, Redlands, California, USA). In the resulting total buffer area comprising all 9 Great Bustard areas, 3,108 fields were randomly chosen to determine the relative surface of each substrate per Great Bustard area and season. The main substrate types were legumes, sprouted cereals, cereal stubble, ploughed fields, fallow fields, and borders between cultivated fields. In each season, several fields were sampled from each substrate type (range: 14–33 fields per Great Bustard area and season), totaling 479 fields with availability data for plants, seeds, and arthropods. In winter, 2 samples were taken, one each in November, 2006, and January, 2007, and the results were averaged to get single values for the wintering season. A mating season sample was collected in April, 2007, and a postmating season sample was collected in July, 2007.
Plant and seed availabilities (kg ha−1) were estimated by randomly throwing a 25 × 25 cm metal square 20 times per sampling field, which is adequate to get reliable information about plant composition (Ponce et al. 2011). In each square, plants were identified and sorted into 3 functional groups (cultivated cereals, legumes, and weeds), and their biomasses (kg) were calculated from estimated cover values through allometric equations (Smith and Brand 1983). The mean plant biomass per substrate type was then estimated for each Great Bustard area and season. The total surfaces of the different substrate types varied among Great Bustard areas, and therefore the mean plant biomass (kg) was multiplied by the relative surface area of each substrate type.
Seeds were counted and sorted by size into 4 groups (<1 mm; 1–5 mm; 5–10 mm; >10 mm) within each sample square. The mean weight of each group was obtained in the laboratory and the total seed biomass availability (kg ha−1) was then calculated for each Great Bustard area and season by multiplying the relative surface area of each substrate type by the mean seed biomass in each substrate type and Great Bustard area.
Fruit availability (kg ha−1) was estimated for olives, grapes, and black nightshades. The mean dry weight of each fruit was calculated in the laboratory (n = 20 fruits of each species). The total fruit biomass availability (kg ha−1) was then calculated for each Great Bustard area and season. In the case of olives and grapes, the number of fruits was counted on a sample of olive trees and grapevines (n = 20) and multiplied by the total number of olive trees and grapevines in the Great Bustard area, which was estimated from aerial photographs using SIGPAC software ( http://sigpac.mapa.es/fega/visor/). For black nightshade, the number of Solanum plants was estimated by randomly throwing a 25 × 25 cm metal square 20 times per field in each substrate and counting nightshade fruits within each sampling square. Mean fruit biomass was estimated for each substrate type per Great Bustard area and multiplied by the relative surface area of each substrate type in the Great Bustard area.
Arthropod availability (kg ha−1) was estimated in transects (30 m long × 1 m wide) by walking slowly and counting and identifying all invertebrates observed to the family level (Lane et al. 1999, Ponce et al. 2014). We collected a random sample of 7,515 arthropods (12% of the total detected). To estimate arthropod biomass we followed the protocol described in Hódar (1996), in which length–weight equations are provided for adult arthropods of the Mediterranean region. Maximum body length excluding all appendages (legs, wings, antennae, and ovipositors) was measured for all collected adult arthropods with a pair of digital calipers (0.01 mm precision). Estimates of weights were calculated by means of linear regressions of body weight on body length for each family (order in some cases) for each transect and, subsequently, for each sampled field (Clere and Bretagnolle 2001, Jiguet 2002). For each season, mean arthropod biomass was estimated for each substrate type per Great Bustard area and multiplied by the relative surface area of each substrate type in the Great Bustard area (Ponce et al. 2014).
We used the relative availabilities of the main food types, season, and sex as potential predictors of the composition and diversity of Great Bustard diets. Variables that significantly explained diet composition and diversity were selected with linear mixed-effects models (McCullagh and Nelder 1989). We assumed a binomial error distribution and a logit-link function in predictive models of the percentages of legumes, weeds, cultivated cereals, arthropods, seeds, and fruits in droppings (dependent variables). The normal distribution was selected to calculate the Shannon index (H′) as a measure of dietary diversity. Sex and season (wintering, mating, and postmating) were defined as independent variables. We included the availability of legumes, weeds, cultivated cereals, arthropods, seeds, and fruits as covariates. As diet composition and food availability may vary among areas, Great Bustard area was included as a random effect in the models. Following Zuur et al. (2007), we built ‘beyond optimal' models (the most complex models, with all factors and their plausible interactions) with different random error structures using the restricted maximum likelihood estimation procedure. Once the random structure had been defined, we defined the fixed effects structure using the maximum likelihood ratio test. Models were evaluated with Akaike's Information Criterion to determine the best predictive model. A model was defined as the best model when it had the lowest AICc value with a difference (ΔAICc) >2 compared with the second-best model. Model-averaging was performed when more than 1 model had ΔAICc < 2. To evaluate the amount of variance explained, we calculated the R2 values of the models using the methods of Nakagawa and Schielzeth (2013). We calculated the marginal R2 (), which describes the variance explained by the fixed effects, and the conditional R2 (), which is concerned with the variance explained by both the fixed and random effects (Nakagawa and Schielzeth 2013). We verified the normal distribution of model residuals by visually checking the normal probability plots and also with a Shapiro-Wilk's test. The homogeneity of variance and goodness-of-fit was checked with plots of residuals vs. fitted values.
The diversity of dietary items in the droppings was calculated using the Shannon diversity index: H' = −∑(pi·ln(pi)), where pi is the proportion of taxon i per dropping. We evaluated overlap in the diet between the sexes with the simplified Morisita index (Horn 1966), which ranges from 0 (no overlap) to 1 (complete overlap). This index has relatively low bias and is appropriate for studies in which the selection of resources is reported as a proportion of the total resources used by the animal (Krebs 1989). Firstly, we calculated an overlap index for plants, arthropods, seeds (including fruits), and total diet for each season and Great Bustard area using the proportions of identified taxa (family or species level). Secondly, we analyzed seasonal changes in the overlap index using Kruskal-Wallis tests, where the overlap index was the dependent variable and season was a fixed factor. We used the FSA package (Ogle 2013) in R 2.15.1 (R Development Core Team 2012) to calculate the overlap index.
Diet selection was examined using a compositional analysis (Aebischer et al. 1993) in the Excel Macro version Compositional Analysis 6.1 plus (Smith 2010), which examines food use in relation to availability, taking the log-ratio differences of used and available foods. The proportional availabilities of all plants (weeds, cultivated cereals, and legumes), arthropods, seeds, and fruits were compared with their respective proportions in the droppings. Null proportions were replaced with 0.01 (Aebischer et al. 1993). The value of t, which measures the significance of the deviation in the use of each food category relative to random, was also obtained for each food category (Aebischer et al. 1993). This was determined by randomization with 1,000 iterations, as recommended by Aebischer et al. (1993). The positive log-ratio differences calculated with the compositional analyses were ranked, the most positive log-ratio difference receiving the highest rank as the most preferred food type. From the log-ratios matrix, the sexual effect on rank preference was analyzed with a generalized linear model (GLM), in which the dependent variable was the log-ratios matrix and sex was defined as a fixed factor. This analysis was performed in SPSS 19.0 (IBM Corporation, Armonk, New York, USA).
A total of 1,528 prey items from 179 mating and postmating droppings was measured to explore differences in prey size of male vs. female Great Bustards. Average prey body size was sorted into 5 size categories (≤10.0, 10.1–20.0, 20.1–30.0, 30.1–40.0, >40.1 mm), and the relative frequency of each size class was calculated for each dropping. Sexual differences were analyzed with a chi-square test. These analyses were carried out in R 2.15.1 (R Development Core Team 2012).
Differences in the weight, volume, and density of droppings between the sexes and among seasons were assessed with generalized linear mixed models (GLMMs). The weight, volume, and density of droppings could depend on diet composition. Therefore, dry weight percentages of plants, arthropods, seeds, and fruits were included as covariates. Sex and season were defined as fixed effects, and Great Bustard area was defined as a random effect. We utilized a Poisson distribution and a log-link function for the models. GLMMs were run as explained above. Firstly, we compared ‘beyond optimal' models with different random error structures. Once the random structure had been defined, we defined the fixed effects structure using maximum likelihood. Models were ranked with Akaike's Information Criterion to determine the best predictive model. Marginal R2 and conditional R2 were also calculated as explained above. Normal distribution was checked as explained above. All GLMMs were run in R 2.15.1 (R Development Core Team 2012).
The dry weight fractions of the whole sample of droppings (n = 619) were 79.6 ± 29.5% green plant material, 9.8 ± 23.0% arthropods, 6.5 ± 18.2% fruits, and 4.1 ± 11.0% seeds (mean ± SD). Up to 67 green plant species were identified, although most of these plants were recorded only occasionally (Appendix Table 6). The most frequently recorded functional groups were weeds (45.9 ± 31.1% SD dry weight) and legumes (25.1 ± 28.1% SD dry weight; Figure 3). The best models included season and sex as significant factors influencing the percentages of the 6 food types considered (legumes, weeds, cultivated cereal plants, arthropods, seeds, and fruits; Table 1, Appendix Table 9). The best model for legume consumption also included legume availability (Table 1). Legume consumption increased with legume availability (0.03 ± 0.01, estimate ± SE; Table 1), reaching its highest value during the mating season for both sexes (Figure 3A). Legume consumption was higher for females during the wintering season, and weed consumption was higher for males in all seasons (Table 1, Figures 3A, 3B). Males consumed more cultivated cereal plants than females in winter, with both sexes showing much lower consumption during the mating season (Figure 3C).
Final generalized linear mixed models (GLMMs) for the most frequently found food types in droppings of Great Bustards in central Spain: green vegetables (legumes, weeds, and cereal plants), arthropods, seeds, and fruits.
A total of 4,612 individuals of 7 arthropod orders was identified in the droppings (Appendix Table 2). The most frequently observed orders were Coleoptera (found in 36.5% of droppings), Hymenoptera (15.7%), Hemiptera (12.1%), and Orthoptera (5.5%). The best model explaining arthropod consumption included season, sex, and arthropod availability as significant factors (Table 1, Appendix Table 9). Arthropod consumption was higher during the postmating period (3.21 ± 0.07, estimate ± SE; Table 1) than in the other 2 seasons (Figure 3D), and was positively influenced by arthropod availability (0.20 ± 0.06, estimate ± SE; Table 1, Figure 3D). Females consumed more arthropods than males during the postmating season (Figure 3D).
Final linear mixed model (LMM) for the dietary diversity of Great Bustards in central Spain.
The seeds of 11 plant families were identified in Great Bustard droppings (Appendix Table 3), with the highest frequencies coming from cereal species, such as common wheat (Triticum aestivum) and common barley (Hordeum vulgare), which were in 39.9% of all droppings. Model selection for seed consumption resulted in 2 models with ΔAICc < 2. Model-averaging resulted in a GLMM that included season and seed availability as significant factors (Table 1, Appendix Table 9). Seed consumption was higher during the wintering and postmating seasons than in the mating season (Table 1, Figure 3E). Females tended to consume more seeds than males in all seasons, although this difference was not significant (−12.17 ± 28.67, estimate ± SE; P = 0.67; Table 1).
Overlap in the 3 main dietary components and the whole diet between male and female Great Bustards in central Spain. The Morisita index of overlap (mean ± SD) is shown for the 3 seasons (wintering, mating, and postmating). The number of Great Bustard areas available to calculate the index varied among seasons and food types (n = 5–9 Great Bustard areas).
The best model describing fruit consumption included season, sex, and fruit availability as significant factors (Table 1, Appendix Table 9). Olive consumption was highest in winter (8.7% of droppings; Appendix Table 3), while grape and black nightshade consumption was highest during the postmating season (1.2% and 6.8% of droppings, respectively; Appendix Table 8). Females consumed more fruits than males in all seasons (Figure 3F). Fruit consumption was positively influenced by fruit availability (0.12 ± 0.01, estimate ± SE; Table 1, Figure 3F).
The best model for predicting dietary diversity included sex, season, and legume availability as significant factors (Table 2, Appendix Table 10). Dietary diversity was higher for males than for females (0.15 ± 0.07, estimate ± SE; Table 2). There was a significant interaction between sex and legume availability; female dietary diversity was highest when legume availability was lowest (during the postmating season), while the opposite was true for males (Table 2, Figure 4). The dietary diversity of males increased with legume availability (0.03 ± 0.01, estimate ± SE; Table 2).
Dietary Overlap between the Sexes
Dietary overlap between males and females was 0.73 ± 0.22 across the whole year (mean ± SD). The lowest overlap value was found during the postmating season (0.65 ± 0.06; Table 3). Splitting the sample by food classes, the overlap index between males and females did not show significant seasonal changes (Table 3). The highest overlap index for a single food class was that of plants in the mating season (0.84 ± 0.22).
There was a significant effect of season on diet selection, and this effect differed between the sexes. Great Bustards preferentially selected legumes in the wintering and mating seasons, but not in the postmating season, when arthropods were the most preferred food (Table 4). There were significant sexual differences in food selection values in the wintering and postmating seasons, but not in the mating season (Table 4). In winter, males selected weeds and legumes equally, preferring these foods to cereals and seeds, whereas females significantly preferred legumes to seeds. During the mating season, both sexes preferentially selected legumes, but females selected weeds and seeds equally, while males significantly selected weeds over seeds. Finally, both sexes selected arthropods during the postmating season, but females preferred fruit as a second food type, whereas males preferred weeds (Table 4). In summary, females showed a more marked preference than males for legumes in winter.
Compositional analysis matrix based on the comparison between available food types and those used by Great Bustards in central Spain.
Average prey length was 13.1 ± 10.1 mm (range: 2–83 mm). Splitting prey length by taxon, Acrididae and Meloidae were the largest prey, at 32.9 ± 7.0 mm (n = 39) and 31.3 ± 7.2 mm (n = 85), respectively, and Chrysomelidae and Cerambycidae were the smallest, at 6.9 ± 1.2 mm (n = 86) and 7.4 ± 3.0 mm (n = 11), respectively. There were significant differences in the length of prey eaten by male and female Great Bustards (χ2 = 9.82, P = 0.04), with males consuming larger individuals than females (Figure 5). Within taxa, there were differences in the sizes of Meloidae and Tenebrionidae eaten by males vs. females (χ2 = 6.2, P = 0.04 and χ2 = 8.0, P = 0.005, respectively), but not other taxa (Acrididae, Scarabaeidae, Carabidae, Curculionidae, Cerambicidae, Formicidae, and Hemiptera; all P > 0.05).
Weight, Volume, and Density of Droppings
The weight, volume, and density of droppings varied between the sexes, among seasons, and by dropping composition (Table 5). The droppings of males were significantly heavier and more voluminous (3.6 ± 3.1 g and 7.5 ± 5.4 cm3) than the droppings of females (1.5 ± 1.2 g and 3.5 ± 2.9 cm3; Table 5). The weight of droppings also depended upon diet composition, increasing with the percentage of fruit ingested (Table 5). The 3 best models for dropping volume showed similar AICc values (Appendix Table 11). Model-averaging resulted in a GLMM that included sex and season as significant factors (Table 5). The volume of droppings did not depend significantly on dropping composition (percentage of plants, arthropod, seeds, and fruits; Table 5).
Final generalized linear mixed model (GLMM) for the weight, volume, and density of Great Bustard droppings in central Spain.
The average density of droppings was 0.54 ± 0.49 g per cm3 (females: 0.49 ± 0.26 g per cm3; males: 0.58 ± 0.63 g per cm3). Model selection for the density of droppings resulted in 4 models with ΔAICc < 2 (Appendix Table 11). Model-averaging of the top models resulted in a GLMM that included sex and season as significant factors (Table 5). Dropping density was significantly higher for males than for females in the postmating season (0.17 ± 0.05, estimate ± SE; Table 5).
As expected, our best models included sex as a main factor explaining differences in the diet composition and diversity of Great Bustards throughout the annual cycle. Males and females shared a similar dietary spectrum, but consumed some of the available food resources in different proportions, and males showed higher dietary diversity than females, except during the postmating season.
The proportions of green plants, arthropods, seeds, and fruits in the diets of both sexes varied seasonally in relation to the availability of these elements. Being omnivorous, Great Bustards behaved as opportunistic foragers and showed ecological plasticity in their feeding behaviors. Similar seasonal changes in the Great Bustard diet have been reported in other areas of Spain (Lucio 1985, Lane et al. 1999) and in Portugal (Rocha et al. 2005). In spite of their relatively opportunistic and generalist foraging habits (note that their diet includes 103 species), Great Bustards have a clear preference for a few digestible and nutritious plants, with 5 species contributing ~40% to their year-round diet. These most-consumed species were Convolvulus arvensis, Anacyclus clavatus, Papaver rhoeas, Raphanus raphanistrum, and Vicia sativa (Appendix Table 1), showing the importance of weeds and legumes in the bustards' diet. The prevalent consumption of weeds and the strong selection of legumes are surely important for supporting the high food requirements of large-bodied birds such as Great Bustards. Weeds are generally abundant in their habitat and legumes are one of the best protein sources among green plants. Despite the observed seasonal variation in diet, green plants were so prevalent that Great Bustards could be described as mainly herbivorous. Females were more selective than males, with higher legume consumption and, consequently, lower dietary diversity values. They followed a primary prediction of optimal foraging theory, which proposes that when the preferred resource is abundant, foragers concentrate on this resource and have narrow food niches. As the preferred resource becomes scarce, lower-quality resources are added to the diet and individuals become more opportunistic (MacArthur and Pianka 1966, Araújo et al. 2008). In contrast to females, the dietary diversity of males did not decrease with increasing legume availability, in accordance with their lower preference for legumes and their less selective diet. Our compositional analysis corroborated the higher selection of legumes by females, particularly during winter (Table 4).
A less specialized diet when alternative highly nutritious resources are available seems paradoxical, but it could be a consequence of sexual dimorphism. For example, in Red Knots (Calidris canutus), individuals with smaller gizzards selected higher-quality prey rather than larger prey, due to digestive constraints (van Gils et al. 2005a, 2005b). The larger sex could meet its nutritional needs by feeding on a wide range of prey items due to its bigger body and gut size, but the smaller sex had to exploit the opportunity provided by the abundance of the most nutritious resource. Our results agree with those of studies of sexually dimorphic herbivorous mammals (Demment and Van Soest 1985, Bowyer 2004), where larger-bodied individuals were found to be less selective than smaller-bodied individuals.
Because females showed a greater proportion of legumes in their diet and lower dietary diversity than males, the dietary overlap between the sexes was relatively low in all seasons. Although males and females fed on the same spectrum of available food items, their dietary overlap was ca. 0.73, one of the smallest overlap values among birds (C. Bravo personal observation). The highest dietary overlap was in the plants component of the diet and was observed in the mating season (0.84), when both sexes met at leks and considerably reduced their spatial segregation. In contrast, their dietary overlap was lowest during the postmating season, when females carry out chick rearing alone and males usually abandon the leks to recover from their high investment in mating (Alonso et al. 2009b, Palacín et al. 2009). This suggests that sexual differences in the diet are not only direct consequences of the bustards' SSD, but also mediated by the distinct reproductive roles of males and females, which may be another cause of dietary divergence (Fairbairn 1997). A similar pattern of seasonal diet partitioning between the sexes as a consequence of their different reproductive roles has been reported for Greater Prairie-Chickens (Blanco-Fontao et al. 2013). In addition, females consumed more arthropods and seeds than males, particularly during the postmating season. Arthropods and seeds are rich in crude protein, calcium, and phosphorus, and probably help females to restore their calcium levels after the egg-laying period. This result also suggests the importance of the reproductive role as a cause of the dietary divergence between the sexes.
Males consumed fewer arthropods, but of significantly larger size, than females. These sexual differences in the average size of prey eaten were also found in young Great Bustards (Bravo et al. 2012) and in Australian Bustards (Ardeotis australis; Ziembicki 2010). In addition, flying or highly mobile arthropods such as grasshoppers were more frequent in the female than in the male diet (Appendix Table 7), suggesting that females, being smaller and thus more agile than males, have a higher ability to capture mobile prey. Additionally, the female role as food provider to offspring could determine the selection of specific arthropods, as well as the most effective prey size (Andersson 1994). These differences likely contribute to reducing intersexual competition for food resources in size-dimorphic species (Selander 1966, Shine 1989, Fairbairn 1997).
The feces of males were 2.4 times heavier than the feces of females, as expected from sexual differences in body weight (males are ~2.4 times heavier than females; Alonso et al. 2009a). The sexual difference in average dropping weight could represent a proxy for the sexual difference in food intake, assuming a similar defecation rate for both sexes. Defecation rate currently cannot be sampled for Great Bustards, as they defecate mainly in roosts at night and at resting sites at midday (Bautista et al. 2013). A similar defecation rate could be assumed following published studies of avian herbivory in which a sex effect in defecation rate has not been reported (van Gils and Tijsen 2007, van Gils et al. 2007). It is noteworthy that the size and density of Great Bustard droppings changed among seasons. This seasonal change in dropping size could be related to a seasonal change in gut morphology determined by food quality and quantity, as shown for other bird species (Leopold 1953, Pendergast and Boag 1973, Geluso and Hayes 1999, Williamson et al. 2014). A sexual difference in dropping density is compatible with a sexual difference in digestive ability, and it would be worth investigating whether such a sexual difference does exist. Our results should be confirmed with further research on sexual differences in gut performance, gut allometry, and digestive efficiency in Great Bustards.
In summary, the results of this study confirm that the diets of adult male and adult female Great Bustards are different. The divergent nutritional and energetic requirements of males and females and their sexual differences in diet and foraging behavior could be explained in part by sexual segregation as a primary proximate factor. Sexual segregation may in turn be a consequence of the distinct reproductive roles and the extreme SSD of both sexes, both of which are ultimately determined by the strong sexual selection operating in this species. Sexual differences in microhabitat selection should be further investigated in this species to confirm these hypotheses and to establish the most plausible causal relationships.
We thank A. Lucas for his assistance with food availability sampling, and M. Gallardo, A. Lucas, N. Panizo, L. Hernández, A. Alcalde, M. de Simón, P. Mas-Peinado, I. Noguerón, A. Mora, J. Morente, B. Casares, and M. Barreiro for their help in the laboratory. We thank the farmers, landowners, and the Consejería de Medio Ambiente y Ordenación del Territorio of the Community of Madrid for permission to work on their private properties and SPAs, respectively. The comments of C. Corbacho, Daphne Fairbairn and two anonymous referees contributed to improving the manuscript.
Funding statement: This study was financially supported by the Spanish Ministry of Science and Innovation and the Spanish Ministry of Economy and Competitiveness through projects CGL2005-04893, CGL2008-02567, and CGL2012-36345. None of the funders had any influence on the content of the submitted or published manuscript. None of the funders required approval of the final manuscript to be published. C.B. and C.P. were supported by contract HENARSA-CSIC awarded to J.C.A.
Ethics statement: Our research was carried out on private lands with landowner permission and on SPA 139 (corresponding to L1–L4) with permission from the Consejería de Medio Ambiente y Ordenación del Territorio de la Comunidad de Madrid. Sampling protocols were approved by National R and D Agencies (Ministry of Economy and Competitiveness), and the experiments and fieldwork complied with the current laws of Spain.
Author contributions: C.B., C.P., L.M.B., and J.C.A. conceived the idea, design, and experiment; C.B., C.P., and L.M.B. performed the experiments; C.B. and L.M.B analyzed the data; and C.B., C.P., L.M.B., and J.C.A. wrote the paper. All authors discussed the experiments and results and their interpretation.
TABLE 6. Green plant species identified in Great Bustard droppings by season and sex. The occurrence of each plant species is shown as the percentage of droppings containing the species, and dry weight is shown as the percentage of the total dry weight of the droppings.
TABLE 7. Arthropod orders and families identified in Great Bustard droppings by season and sex. The occurrence of each arthropod taxon is shown as the percentage of droppings containing each taxon, and dry weight is shown as the percentage of the total dry weight of the droppings.
TABLE 8. Seed and fruit species identified in Great Bustard droppings by season and sex. The occurrence of each taxon is shown as the percentage of droppings containing each taxon, and dry weight is shown as the percentage of the total dry weight of the droppings.
TABLE 9. Best generalized linear mixed models (GLMMs) for legume, weed, cereal, seed, and arthropod consumption by Great Bustards in central Spain. Model selection was based on Akaike's Information Criterion corrected for small sample sizes (AICc). Delta (ΔAICc) and weight values (wAICc) for each AICc are also shown. Marginal R2 (R2m; proportion of variance explained by the fixed factors alone) and conditional R2 (R2c; proportion of variance explained by both the fixed and random factors) were computed for each model using methods described by Nakagawa and Schielzeth (2013).
TABLE 10. Best linear mixed models (LMMs) for the dietary diversity of Great Bustards in central Spain. Model selection was based on Akaike's Information Criterion corrected for small sample sizes (AICc). Delta (ΔAICc) and weight values (wAICc) for each AICc are also shown. Marginal R2 (R2m; proportion of variance explained by the fixed factors alone) and conditional R2 (R2c; proportion of variance explained by both the fixed and random factors) were computed for each model using methods described by Nakagawa and Schielzeth (2013).
TABLE 11. The 4 best generalized linear mixed models (GLMMs) for weight, volume, and density of Great Bustard droppings in central Spain. Model selection was based on Akaike's Information Criterion corrected for small sample sizes (AICc). Delta (ΔAICc) and weight values (wAICc) for each AICc are also shown. Marginal R2 (R2m; proportion of variance explained by the fixed factors alone) and conditional R2 (R2c; proportion of variance explained by both the fixed and random factors) were computed for each model using methods described by Nakagawa and Schielzeth (2013).