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A subspecies is a collection of populations within a biological species that are diagnosably distinct from other such collections of populations. That infraspecific designation has motivated a litany of spirited debates over the past half-century, from impassioned pleas for its retention to heated outcries for its abolition. We believe that the vast majority of attacks on the subspecies concept have resulted from displeasure with its improper application, not from serious flaws in the concept itself. The recognition of diagnosable subspecies allows one to address many questions not easily answered otherwise, ranging from dispersal and migration to local selection and adaptation and biogeographic affinities, yet that goal was lost for many years. Many taxonomists in the late nineteenth century and first half of the twentieth century named subspecies on the basis of average differences between populations under study, a procedure at odds with identification of diagnosable populations. To resolve that dilemma, we make explicit the established 75% rule for subspecies recognition, including formalizing the rule and developing a simple statistic to test whether diagnosability is met. The equations can be adapted readily to any level of diagnosability. We apply the concept and the statistic to a revision of the subspecies of the Sage Sparrow (Amphispiza belli). Rather than the seven named subspecies or the five that are generally considered valid, we show that only three aggregates of populations are diagnosable, and thus only three subspecies should be recognized: (1) A. b. belli in chaparral and sage scrub of coastal California, northwestern Baja California, and San Clemente Island; (2) A. b. cinerea in desert scrub of west-central Baja California; and (3) A. b. nevadensis in sagebrush and saltbush of the Great Basin and interior California. Consistent application of the 75% rule will result in fewer trinomials and a more biologically meaningful and taxonomically useful subspecies concept.
We present a method for estimating density of nesting birds based on double sampling. The approach involves surveying a large sample of plots using a rapid method such as uncorrected point counts, variable circular plot counts, or the recently suggested double-observer method. A subsample of those plots is also surveyed using intensive methods to determine actual density. The ratio of the mean count on those plots (using the rapid method) to the mean actual density (as determined by the intensive searches) is used to adjust results from the rapid method. The approach works well when results from the rapid method are highly correlated with actual density. We illustrate the method with three years of shorebird surveys from the tundra in northern Alaska. In the rapid method, surveyors covered ∼10 ha h–1 and surveyed each plot a single time. The intensive surveys involved three thorough searches, required ∼3 h ha–1, and took 20% of the study effort. Surveyors using the rapid method detected an average of 79% of birds present. That detection ratio was used to convert the index obtained in the rapid method into an essentially unbiased estimate of density. Trends estimated from several years of data would also be essentially unbiased. Other advantages of double sampling are that (1) the rapid method can be changed as new methods become available, (2) domains can be compared even if detection rates differ, (3) total population size can be estimated, and (4) valuable ancillary information (e.g. nest success) can be obtained on intensive plots with little additional effort. We suggest that double sampling be used to test the assumption that rapid methods, such as variable circular plot and double-observer methods, yield density estimates that are essentially unbiased. The feasibility of implementing double sampling in a range of habitats needs to be evaluated.
Counting techniques are widely used to study and monitor terrestrial birds. To assess current applications of counting techniques, we reviewed landbird studies published 1989–1998 in nine major journals and one symposium. Commonly used techniques fell into two groups: procedures that used counts of bird detections as an index to abundance (index counts), and procedures that used empirical models of detectability to estimate density. Index counts rely upon assumptions concerning detectability that are difficult or impossible to meet in most field studies, but nonetheless remain the technique of choice among ornithologists; 95% of studies we reviewed relied upon point counts, strip transects, or other index procedures. Detectability-based density estimates were rarely used and deserve wider application in landbird studies. Distance sampling is a comprehensive extension of earlier detectability-based procedures (variable-width transects, variable circular plots) and a viable alternative to index counts. We provide a conceptual overview of distance sampling, specific recommendations for applying this technique to studies of landbirds, and an introduction to analysis of distance sampling data using the program DISTANCE.
In uniparental intermittent incubators, incubating parents must simultaneously regulate both the temperature of the clutch and their own energy level. To examine energetic consequences of providing different thermal environments for clutches of different sizes, a dynamic model was constructed in which energy level of an incubating European Starling (Sturnus vulgaris) and temperature of its clutch were simultaneously described. Adult energy balance after a day of incubation decreased as mean clutch temperature increased, such that the debt accrued while maintaining a clutch within the optimal developmental temperature range (36–39°C) was predicted to be prohibitively high, despite the prediction that the mean basal metabolic rate (BMR) required to maintain this temperature was <2 BMR. Thus, this model can explain our observation that starlings maintained their eggs at 32–33°C, well below the developmental optimum. Consistent with empirical studies, our model predicted that the metabolic demand of incubation increases with clutch size. That increase was predicted to affect adult energy balance and hence cost of incubation when starlings maintained their clutches at optimal temperatures, but not at the lower temperatures actually observed. Hence, clutch-size-dependent variation in incubation demands may be unlikely to influence optimal number of eggs that a starling should lay. However, exact relationship between clutch size and adult energy debt depended on the nature of the relationship between clutch size and clutch thermal properties and on mean incubation temperature. Thus, consequences of clutch size for the cost of incubation are not clearly predictable, and caution may be required when using experimental clutch enlargements to manipulate reproductive costs.
In several bird species, offspring from larger eggs survive better than birds from smaller eggs, but mechanisms responsible for that pattern have not been clearly identified. Studies of waterfowl have found relationships between egg size, body composition of hatchlings, and duckling growth. Therefore, body composition and growth rate of newly hatched King Eider (Somateria spectabilis) ducklings were measured to assess if traits consistent with higher probability of survival early in duckling varied in relation to egg size. Forty-one King Eider eggs were collected in June and July 1998 from two lakes in the central Canadian Arctic, and artificially incubated, of which 34 hatched. Body composition of 15 hatchlings (<1 day old) was determined, and 19 ducklings were raised in captivity to measure growth rate. Larger eggs produced larger ducklings with absolutely more lipid and protein reserve; absolutely larger breast and leg muscles; higher functional maturity for whole body, leg, and breast muscles; and higher tarsal growth rates than ducklings from small eggs. Such patterns of hatchling composition and growth in relation to increasing egg size may improve likelihood of early survival by improving thermogenesis, reducing time spent as optimal prey size for avian predators, and as shown elsewhere, by enhancing motor performance of ducklings through improvements in foraging efficiency and predator evasion.
Reversing decades of fire exclusion by hardwood midstory reduction is now used to recover populations of the federally endangered Red-cockaded Woodpecker (Picoides borealis) in longleaf pine (Pinus palustris) forest ecosystems. The effects of Red-cockaded Woodpecker management on winter birds in longleaf pine sandhill forests are largely unknown. Examining habitat use of winter migrants, some of which are declining, may influence the selection of habitat management techniques used for Red-cockaded Woodpeckers to benefit overwintering migrants. During the winters (December–February) of 1997–1998 and 1998–1999, we tested experimentally the effects of hardwood reduction treatments applied in 1995 on winter birds at Eglin Air Force Base in fire-excluded northwest Florida longleaf pine sandhills. Treatments were (1) prescribed spring burning, (2) herbicide application, (3) mechanical felling and girdling, and (4) a control where decades of fire exclusion was maintained. We also sampled winter bird flocks in frequently burned, nonexperimental reference plots to measure management success. Hardwood reduction techniques had no effect on flock species richness, which averaged 7.9 and 7.2, respectively, during 1997–1998 and 1998–1999. Larger flocks in felling and girdling and in herbicide plots were primarily due to significantly higher numbers of overwintering Chipping Sparrows (Spizella passerina), as well as resident Red-cockaded Woodpeckers and an influx of temperate migrant Pine Warblers (Dendroica pinus). In contrast, flocks in control plots were smaller (flock size and species composition in spring burn plots were intermediate) and composed of hardwood-associated species, such as Tufted Titmouse (Baeolophus bicolor) and Carolina Chickadee (Poecile carolinensis). The relative uses of longleaf pines and hardwoods by Red-cockaded Woodpeckers, Pine Warblers, and Brown-headed Nuthatches (Sitta pusilla) during both winters best explained that winter birds present in herbicide, felling and girdling, and reference plots were more likely to forage on the same tree species and substrates than birds in spring-burned plots, and least likely to forage on the same species and substrates as birds in the control plots. Those differences corresponded to the following increasing order of hardwood stem mortality among treatments: control, spring burn (41%), felling and girdling (62%), and herbicide (92%). Repeated burning is recommended to restore the reference foraging condition because it was eight times less expensive than other techniques, which favored mostly Chipping Sparrows.
The growing use of comparative methods to address evolutionary questions has generated an increased need for robust hypotheses of evolutionary relationships for a wide range of organisms. Where a phylogeny exists for a group, often more than one phylogeny will exist for that group, and it is uncommon that the same taxa are in each of the existing trees. The types of data used to generate evolutionary trees can also vary greatly, and thus combining data sets is often difficult or impossible. To address comparative questions for groups where multiple phylogenetic hypotheses already exist, we need to combine different hypotheses in a way that provides the best estimate of the phylogeny for that group. Here, we combine seven seabird phylogenies (based on behavioral, DNA–DNA hybridization, isozyme, life history, morphological, and sequence data) to generate a comprehensive supertree for the Procellariiformes using matrix representation with parsimony. This phylogeny contains 122 taxa and represents a conservative estimate of combined relationships presented in the original seven source trees. We compared the supertree with results of a combined sequence data supermatrix for 103 seabird taxa. Results of the two approaches are broadly concordant, but matrix representation with parsimony provides a more comprehensive and more conservative estimate of the phylogeny of the group because it is less influenced by the largest of the source studies (which uses a single, relatively quickly evolving gene). Genetic data sets that can be combined in a supermatrix approach are currently less likely to be available than phylogenies that can be combined using some form of supertree approach. Although there are limitations to both of those approaches, both would be simpler if all phylogenetic studies made both their data sets and trees they generate available through databases such as TREEBASE.
We monitored adult and juvenile breeding-season movements and habitat use of radio-tagged Wood Thrushes (Hylocichla mustelina) at the Piedmont National Wildlife Refuge, central Georgia, USA. We investigated the effects that management for Red-cockaded Woodpeckers (Picoides borealis), thinning and burning >30 year old loblolly pine (Pinus taeda) habitat, had on Wood Thrushes, a ground-foraging and midstory-nesting species. Adult Wood Thrush pairs regularly moved long distances between nesting attempts (range 1 to 17,388 m). The only experimental effect we found on adult movements was a decrease in weekly emigration rates (Ψ) from thinned and burned compartments after silvicultural management. Adult males preferred riparian hardwoods with sparse to moderate cover and those preferences increased following management. Juveniles remained near their nest site (x̄ = 177 m, SE = 113) for an average 24 days (SE = 6.3), and then dispersed a mean 2,189 m (SE = 342). Before dispersal, juveniles preferred upland hardwood–pine mixed habitat (P < 0.05) with moderate overstory cover (P < 0.05). We found no management effects on dispersal distances or predispersal habitat use. However, juveniles from thinned and burned compartments dispersed to hardwood habitats with dense cover, whereas birds from control compartments dispersed to pine-dominated habitats with sparse cover. All juveniles dispersed to areas with habitat similar to what they used before dispersal. Small-scale thinning and burning for Red-cockaded Woodpeckers may have had little effect on Wood Thrush habitat use and movements because typical movements were often larger than the scale (stand or compartment) targeted for management.
Parents are expected to vary the sex ratio of their offspring in relation to the sex-specific fitness benefits. However, benefits of producing sex-biased broods may be dependent on condition of the female. For example, mothers in good condition could achieve greater fitness if they produced high-quality sons, whereas, mothers in poor condition would gain more by producing daughters rather than poor-quality sons. As a consequence, we would expect to see a relationship between female condition and sex ratio of offspring. We examined effect of maternal condition on nestling condition and sex ratio in the House Wren (Troglodytes aedon). Overall sex ratio of nestlings in the population was not biased, but females in better condition produced relatively more sons. Overall positive relationship between female condition and proportion of male offspring was due to second broods, which were significantly male-biased and more likely to be produced by females in good condition. Females in better condition also tended to provision young more often and produced both male and female nestlings in better condition. Polygyny and extrapair mating are common in House Wrens. If males in good condition are more likely to be successful breeders as adults, then it may benefit mothers in good condition to produce more sons.
We used capture and recapture data (1985 to 1994) to examine seasonal variation in habitat use, movements within and among habitats, and survival rates of manakins (Manacus candei, Corapipo altera, Pipra mentalis, P. pipra) in northeastern Costa Rica. Manakins were captured in young and old second-growth woodlands in the lowlands and in old-growth forest at approximately 50, 500, and 1,000 m. Manakin species differed in their use of habitats, with old-growth forest species showing large and predictable seasonal variation in capture rates. Corapipo capture rates in lowland (nonbreeding) habitats were greater during the wet season than during the dry season and were greater in old-growth forest than in second growth. Capture rates at 500 m were higher in the dry season. Pipra mentalis capture rates were high in second growth and old growth. Capture rates were higher in the wet season and were correlated with capture rates of Corapipo, indicating that at least some individual P. mentalis migrate along the elevational gradient. P. pipra capture rates were highest at 1,000 m; few individuals descended to lowlands in the wet season. Manacus capture rates were highest in young second growth and did not vary between wet and dry seasons. Use of second-growth habitats by species typically associated with old-growth forests illustrates the value of maintaining a mosaic of habitats to accommodate seasonal changes in use of habitats. Contrary to expectations based on lek mating systems, there was little evidence that movements within habitats (i.e. recapture distances) varied between sexes. Yet, recapture percentages were higher in all species for adult females than males. Adult survival rates were ∼0.75 for Manacus in young second growth, 0.62 for Corapipo in old-growth forest at 50 m and 0.66 at 500 m, and 0.70 for Pipra mentalis in lowland old-growth forest. Results support the suggestion that geographic variation in survival rates may be common in the tropics and illustrate the need for examining survival rates separately by age and sex.
The demographic consequences of migration have important implications for both evolutionary ecology and conservation biology. We investigated local survival rates for six populations of sparrows at a wintering site. Recent developments in mark–recapture statistics were applied to a 13 year dataset with large numbers of marked individuals (n = 1,632 to 4,394). The study taxa were closely related, and included one resident species (Song Sparrow [Melospiza melodia gouldii]), one short-distance migrant (“Puget Sound” White-crowned Sparrow [Zonotrichia leucophrys pugetensis]), two moderate-distance migrants (Lincoln's [Melospiza lincolnii] and Fox [Passerella iliaca] sparrow), and two long-distance migrants (“Gambel's” White-crowned [Zonotrichia leucophrys gambelii] and Golden-crowned [Zonotrichia atricapilla] sparrow). A literature review demonstrated a cline in fecundity among these sparrows: resident and short-distance migrants laid multiple clutches of few eggs, whereas long-distance migrants tended to produce one large clutch. Annual rates of local survival were low in the interval after first capture (<0.35), possibly because of variation in true survival, site-fidelity, presence of transients and heterogeneity of capture. Estimates of local survival among birds that returned at least once were more robust and were comparable among Song (0.558 ± 0.054 SE), Puget Sound White-crowned (0.461 ± 0.026), Lincoln's (0.456 ± 0.066), Fox (0.352 ± 0.0), Golden-crowned (0.422 ± 0.023) and Gambel's White-crowned (0.432 ± 0.0) sparrows. Estimates of survivorship for Lincoln's and Fox sparrows are among the first values available for those species. Local survival was not higher among resident than migratory taxa, nor did it covary with migration distance among migratory species. These results did not support the time-allocation hypothesis of Greenberg (1980), but are consistent with aspects of bet-hedging theory. While these analyses have potential implications for conservation of migratory birds, further work is required to establish whether these patterns are applicable to Neotropical migrants.
The Hawaii Akepa (Loxops coccineus coccineus) is an endangered bird that has declined dramatically in the last 100 years, and is now rare or absent from many areas that appear to support suitable habitat. Food availability may play a role in these distribution patterns, but differences in food between sites may arise from different sources. I compared prey availability between a site supporting a large, stable Hawaii Akepa population, and a site from which Hawaii Akepa have declined in the last 100 years for unknown reasons. I used three spatial scales to compare food between sites to explore the basis of differences in food between sites. At a scale appropriate for comparing prey population dynamics (scale 1), I found that prey population densities are similar between sites, suggesting that introduced (or native) predators or parasitoids have not affected prey populations differently between sites. At two larger scales incorporating habitat structure, I found that food availability is much lower at the site of Hawaii Akepa declines. Differences in canopy density per square meter (scale 2), and in canopy cover per square kilometer (scale 3), result in lower food availability that may have effects on individual foraging birds as well as on potential Hawaii Akepa population density. These findings illustrate the importance of explicitly incorporating spatial scale into inquiries about food for Hawaii Akepa, and suggest that the site of population declines may not be suitable habitat with respect to food for this species.
Frugivorous birds may be able to reduce the cost of processing seeds by discarding seeds, selecting fruits that have a high pulp-to-seed ratio, or by choosing fruit in which seeds are packaged in a way that the frugivore's gut can void them more rapidly. A preference for fruit based on pulp-to-seed ratio or seed composition within a fruit is likely to have important implications for plants and evolution of seed size. We tested whether captive Silvereyes (Zosterops lateralis) discriminate among artificial fruit on the basis of seed presence by presenting birds with artificial fruit with or without a seed. In the first experiment, fruit were translucent so that birds could see which fruit contained a seed. In the second experiment, the visual cue was removed. When Silvereyes were presented with a choice between translucent, artificial fruit with or without a seed, they showed a strong preference for fruit that did not contain a seed. However, when the visual cue to seed presence was removed, preference for seedless fruit was still significant, but markedly reduced. We also tested seed-size preference of Silvereyes in the field in Victoria, Australia. Seeds from a fruit commonly consumed by Silvereyes, fragrant saltbush (Rhagodia parabolica), were recovered from Silvereye faecal samples and their volumes measured. Comparisons were made between seed volumes of fruit consumed by Silvereyes and those within fruit available on the plant. Silvereyes consumed significantly smaller seeds than the mean size available on saltbush plants. When Silvereyes were presented with a cereal-based diet containing artificial seeds (designed to mimic large fruit containing many small seeds), they avoided seed ingestion and were able to consume proportionally more cereal than seeds, even when on a high seed-load diet (30%). Seed dispersal by Silvereyes may be inefficient for plant species with large fruit containing many small seeds, because Silvereyes in this experiment were able to avoid ingesting seeds.
In a captive-feeding study using Herring Gulls (Larus argentatus), plasma amino-acid concentrations increased in response to an increase in dietary protein. Plasma amino-acid concentrations were also measured in wild Herring Gulls captured during incubation at eight Laurentian Great Lakes colonies. Those concentrations were used as an indicator of protein availability at those locations. Significant differences in amino acid concentrations were observed among colonies. Lower amino acid levels, particularly of the essential amino acids, were measured in gulls nesting on Lake Superior, whereas values in gulls captured on Lake Ontario and Lake Erie were greater. Those geographic differences in protein availability likely reflected spatial differences in availability of high quality prey (e.g. fish). Geographic differences in prey availability probably affected diet composition. Comparison of amino-acid levels in wild birds to reference values obtained through the captive feeding study indicated that gulls nesting on Lake Superior may have been protein limited. Colony-wide estimates of adult female body condition, intraclutch variation in egg size, and productivity were correlated with an index of plasma amino-acid concentrations.
We studied the early morning cage orientation of nocturnally migrating Swainson's Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainson's Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.
The need restore energetic reserves at stopover sites constrains avian migration ecology. To describe that constraint, we examined relationships among mass gained by Wilson's Warblers (Wilsonia pusilla) during stopover, abundance of Wilson's Warblers (i.e. capture rate), and arthropod abundance during autumn migration. We found that amount of mass gained by Wilson's Warblers during stopover declined as a function of their abundance. We found no relationship between mass gained by Wilson's Warblers and arthropod abundance. Abundance of Wilson's Warblers was positively associated with arthropod abundance. These patterns are consistent with the hypothesis that mass gain during stopover is density dependent. They also suggest that Wilson's Warblers respond numerically to abundance of food at that stopover site. This numerical response may override any functional response and thereby account for lack of a relationship between mass change and arthropod abundance.
Parasites may affect host behavior in a number of ways, including their locomotory performance. We investigated whether the number of holes produced by the feather louse (Myrsidea rustica) affected flight behavior in adult male Barn Swallows (Hirundo rustica) by video-taping flight performance of individuals during escape and level flight. Percentage of time spent flapping during foraging flight was positively related to number of holes, but not to other flight parameters such as wingbeat frequency. These results suggest indirect effects of feather lice on host performance that must be considered together with effects of thermoregulation and feather breakage. This is the first report of an effect of parasite load on flight behavior.
Diets of three warbler species were analyzed during a spruce budworm outbreak in the boreal forest of northern Ontario. Beetles constituted a large portion of the food items consumed by Cape May (Dendroica tigrina), Bay-breasted (Dendroica castanea), and Tennessee (Vermivora peregrina) warblers early in the breeding season (7–11 June), and caterpillars were the most frequently used food category shortly later (18–24 June). Differences in diet served to differentiate the warbler species in the earlier period when Bay-breasted Warblers consumed more beetles, Tennessee Warblers consumed more caterpillars, and Cape May Warblers consumed more flies than the other species. Only Bay-breasted Warblers' continuing preference for beetles differentiated the warblers' diets in the later period. Food-niche overlaps increased for two of the three warbler species pairs between the two periods in June, but there was no change in the overlap between Bay-breasted and Cape May warbler diets.
Neonate, gosling, and adult Canada Geese (Branta canadensis interior) and Lesser Snow Geese (Chen caerulescens caerulescens) were collected to evaluate if growth rates and developmental patterns differed interspecifically and to determine if such differences were better explained by physiology of the growth process or by ecological conditions historically experienced by those two species. Patterns of growth and development of Canada and Lesser Snow goose goslings were similar to those reported for other Arctic geese, but differences in relative growth rates and developmental patterns of external structures, digestive organs, and skeletal muscles were observed between these two species. As compared to Canada Geese, body parts associated with locomotion and acquisition or processing of food generally increased at relatively faster rates and were more developed relative to adult size in Lesser Snow Geese. Relative rates of increase for carcass protein and body mass in these two species did not support a physiological constraint on growth. Rates and patterns of growth and development were better explained as adaptations to ecological factors, such as growing season and nesting or brood rearing conditions, historically experienced by these two species.
Extrapair copulations and fertilizations are common among birds, especially in passerines. So far, however, few studies have examined genetic mating systems in socially monogamous shorebirds. Here, we examine parentage in the Western Sandpiper (Calidris mauri). Given that Western Sandpipers nest at high densities on the Arctic tundra, have separate nesting and feeding areas, and show high divorce rates between years, we expected extrapair paternity to be more common in this species compared to other monogamous shorebirds. However, DNA fingerprinting of 98 chicks from 40 families revealed that only 8% of broods contained young sired by extrapair males, and that 5% of all chicks were extrapair. All chicks were the genetic offspring of their social mothers. We found that males followed females more often than the reverse. Also, cuckolded males were separated from their mates for longer than those that did not lose paternity. Although these results suggest a role for male mate guarding, we propose that high potential costs in terms of reduced paternal care likely constrain female Western Sandpipers from seeking extrapair copulations.
We evaluated distribution and size of Magellanic Penguin (Spheniscus magellanicus) colonies in northern San Jorge Gulf, Argentina, characterizing nesting habitat at 14 islands with colonies (IC) and 28 islands without nesting penguins (INC). Digital analysis of a Landsat TM satellite image and field measurements were used to assess vegetation and substrate. The 14 breeding colonies ranged in size from 13 to 96,300 nests, and number of breeding pairs was estimated at 218,460. Colonies were only located on islands. Digital analysis of the satellite image showed that the proportion of area with shrub–steppe vegetation was significantly higher at IC than at INC (38.6 vs. 4.9%), whereas percentage of rocky substrate was lower at IC (33.6 vs. 64.8%). Percentage of vegetation cover was positively correlated with island area and with number of breeding pairs. Most nests (98.6%) were located under bushes, whereas the rest were burrows dug into the substrate. Mean size of bushes used for nesting was higher at IC (4.47 vs. 0.99 m2) as was the percentage of silt–clay substrate (47.31 vs. 1.98%). Coast minimum slopes at INC were steeper than the slopes of entrance routes at IC (27.6 vs. 5.7°). Probability of occupation of islands increased with amount of shrub-type vegetation cover and silt–clay substrate, and decreased with increasing slopes and amount of sandy substrate. Magellanic Penguins selected islands with shrub-type vegetation and soil that allow building nests with adequate cover. These results are consistent with other studies showing vegetation cover is an important nesting requirement for Magellanic Penguins.
Forest fragmentation in North America concerns many biologists because of its effects on wildlife populations. One group that has demonstrated particular sensitivity is Neotropical migrant birds. We studied Ovenbirds (Seiurus aurocapillus) in forest fragments in a suburban landscape in eastern Massachusetts to determine effects of cowbird parasitism on Ovenbird reproductive success. Our three large (120–312 ha) and six small (10–59 ha) forests were all smaller than most of those studied by other researchers, and they were surrounded by wooded suburban lots rather than agricultural land. Twenty-nine percent of nests found were parasitized by cowbirds; that frequency is lower than other investigators have reported for small, isolated fragments. The number of Ovenbirds fledged in successful parasitized (x̄ = 2.4) and unparasitized (x̄ = 3.8) nests was similar to other studies. Cowbirds were found to remain in nests for one to two days after their Ovenbird nestmates fledged. Differences in parasitism rate between this and other Ovenbird studies may be related to landscape characteristics. Ovenbirds nesting in small fragments in relatively wooded landscapes, as was the case in this study, have higher reproductive success than do Ovenbirds nesting in similar-sized fragments within an agricultural landscape. Using Ovenbirds as a model for neotropical migrants, we suggest that small fragments in a landscape with relatively wooded connections between forested areas may allow migrants to attain higher reproductive success than similar sized forests surrounded by agricultural land.
We analyzed patterns of egg laying and breeding frequency of Humboldt Penguins (Spheniscus humboldti) between 1993 and 1997 at Punta San Juan, Peru. Egg-laying extended from mid-March to the first week of December, showing two well-defined peaks in April and August–September. The extended breeding period of these birds was the result of individuals having a second clutch. About half of the females (n =189) had two clutches per year, most of which were double broods (73%). The date of completion and outcome of reproduction, or whether a change of mates occurred from the previous year, did not affect timing of egg laying. The majority of first clutches (62%) were laid in April each year. Two-clutch breeders that started laying eggs early in April had a higher breeding success than those starting in late April, and double brooders had greater success than single brooders. Two-clutch breeders started to lay eggs earlier than single-clutch breeders. Taking into account that a penguin breeding cycle (from egg laying to fledging) lasts ∼4 months, laying eggs early in April increases the chance of rearing two successful broods per year. During three consecutive years, females tended to have two clutches instead of only one clutch and an average breeding success of 4.54 fledglings over 3 years. Having as many clutches as possible when conditions are favorable appears to be a strategy used by Humboldt Penguins to maximize their lifetime reproductive success within a productive but unpredictable environment.
Heterothermy in birds occurs in species that are generally small and whose diet fluctuates. This study of thermoregulation of Malachite Sunbirds (Nectarina famosa) showed that they have circadian fluctuations in Tb and VO2, as in most birds. Of special importance was the high degree of rest-phase hypothermy and exhibition of torpor at ambient temperatures of 10°C and lower. These patterns are significant because they have not been described in detail for a passerine species. Surgically implanted minimitters were used to measure Tb continuously and without disturbing the birds. Minimum VO2 during rest phase was 1.70 (mL O2 g–1 h–1) at 25°C. As ambient temperature decreased, VO2 minimum during the rest phase did not increase to maintain Tb. No birds remained normothermic during scotophase. At 5°C, that resulted in torpor in Malachite Sunbirds with a decrease of 15°C in Tb. Birds increased Tb to active-phase levels with the onset of light. It was difficult to define limits and ranges of physiological parameters associated with observed heterothermy. Individuals showed similar responses; however, those differed with ambient temperature. Malachite Sunbirds conserved energy nocturnally by reducing metabolic rate and, concomitantly, Tb. This plasticity in Tb shows that daily variations in Tb of homeotherms are biologically important. Furthermore, this heterothermy (particularly nocturnal hypothermia and torpor) in a small avian species would be important in an unpredictable environment where food resources fluctuate to prevent an energy deficit.
We assessed nutritional constraints on clutch size in Mallards (Anas platyrhynchos) by observing incidence and consequences of continuous laying—the sequential production of eggs in two or more nest bowls. Continuous laying behavior was detected in 278 of 3,064 radiotracked Mallards (9.1%). Continuous laying females produced an average of 12.12 total eggs (SD = 2.70, range 5–18, n = 69), versus 8.90 eggs for normal nesting females (SD = 1.67, range 4–14, n = 587). On average, continuous laying females were 25 g heavier than noncontinuous laying females, and body mass was positively correlated with egg production among continuous laying females. Nest success was not affected by continuous laying, but continuous laying females that abandoned their nests were more likely to be young or to have laid a greater number of eggs. A large component of the breeding Mallard population can lay more eggs than they typically do, and there appear to be minimal consequences of that behavior. These observations appear inconsistent with the egg-formation hypothesis.
In Procellariiformes, parents guard the chick for some time after it has attained homeothermy. Such a strategy may have evolved to protect the chick from predation or inclement weather, but it is costly because only one parent can forage at a time. Therefore, the decision to leave the chick seems to be a trade-off between the chick's ability to care for itself, body condition of the parent present at the nest, and ability of the bird out foraging to return to the nest before its mate's body condition has degraded. We studied chick growth and survival together with number of days Snow Petrel (Pagodroma nivea) chicks were guarded before being left alone for the first time in relation to the parents body condition and ability to return to the nest in time. Parents in good body condition were more likely to produce a chick that survived the guard stage. They also guarded their chick for a longer period (range 2–8 days, x̄ = 4.5) and finally left it alone with a higher body mass than those in poor body condition. However, whether the foraging bird was able to return to the nest in time to relieve its mate was also strongly related to number of days the chick was guarded and its body mass. The chicks' survival from when they were left alone and until day 10 posthatch was positively related both to number of days they were guarded and their body condition (body mass corrected for age).