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Timing of arrival in long-distance migration could have fitness consequences: arrival too early impairs survival chances, whereas arrival too late reduces current reproductive success. Evolution thus may have favoured a phenotype that arrived at the optimal time. However, individuals within populations of longdistance migrant species arrive over a considerable time span, and often show consistency in whether they are early or late. This repeatability in arrival varies between studies, and we hypothesise it to be affected by conditions encountered en route or in winter. Here we report on the spring arrival dates of Pied Flycatchers Ficedula hypoleuca to their Dutch breeding sites during eight consecutive years. Our field estimates of arrival were highly accurate, as validated by geolocator data on 13 individuals. Years differed in mean arrival dates. Within years and sexes, arrival date generally spanned more than two weeks. First-year individuals arrived on average 4–5 days later than older individuals. Using repeated arrival dates of more than 500 individuals we show that (1) the overall arrival repeatabilities were similar for females and males, (2) arrival repeatabilities varied temporally, with individuals in consecutive years having sometimes moderate (R = 0.2) and sometimes rather high (>0.40) repeatabilities, and (3) individual females arrived later in their first than in their second year. In females, repeatabilities of arrival and laying dates were similar. We hypothesize that individual flycatchers have a high individual consistency in their spring migration departure date from the wintering grounds. However, previous studies suggest the expression of this individual schedule to be affected by environmental circumstances at the wintering grounds or by what is encountered en route, determining whether this variation is still present at arrival on the breeding grounds. Sexes seemed to differ in this respect, as yearto-year variation in repeatabilities of timing was explained by individual consistency in females, but not in males. We discuss the relevance of the observed variation for the potential for an evolutionary response when environments change.
Intensive dairy farming has changed the agricultural grassland areas of The Netherlands profoundly, with negative impacts on the reproduction of the shorebirds breeding there. This modern agricultural landscape also forms a staging site for migrating shorebirds, where they moult and replenish fuel stores, but staging performance in these areas has received much less attention. We studied northbound migrating Ruffs Philomachus pugnax staging in the grasslands of southwest Friesland over a ten year period, during which peak numbers declined from 20,000 in 2003 to 3500 birds in 2009 and then stabilized. On the basis of resighting locations of individually marked birds, we describe changes in their day-time foraging distribution from spring 2006 to spring 2013. Ruffs progressively retreated to the centre of the c. 10,000 ha study area, where, among intensive grasslands, established and newly created inland wetlands occurred that served as feeding and/or roosting sites. To quantify the spatial changes, in 2013 we repeated a transect survey of meadow use carried out earlier in 2003. Using similar characteristics of individual meadows in terms of herb richness (a measure of agricultural intensity) and landscape characteristics (distance to the roost, soil type), we show that, during spring 2013, as in 2003, Ruffs foraged preferentially on meadows close to roosting areas. The survey also highlights the preference of Ruffs for the Workumerwaard, a particularly large and open polder with a sandy soil and short vegetation bordered by a traditional roosting area on the shoreline. This study provides some evidence that inland wetlands may increase the attractiveness for migrating Ruffs of landscapes dominated by modern grasslands.
The Brown Booby Sula leucogaster is a seabird with a pantropical distribution across a wide variety of oceanic environments. Sexual size dimorphism in Brown Boobies has been proposed as an explanation for intersexual differences in foraging, but results have been inconsistent. We investigated whether there is context-dependent foraging behaviour driven by local environmental conditions. In this study, we evaluated (1) inter-sex differences in foraging behaviour (by capillary tubes, temperature and depth recorders, and diet) at two colonies in the Gulf of California: Isla San Jorge (ISJ) and Farallón de San Ignacio (FSI) and, (2) intercolonial and interannual differences in foraging behaviour, and (at ISJ) their relationship with local-scale environmental variation, using 5-day composite images of sea surface temperature (SST) and primary productivity (PP) as proxies. Inter-sex differences were few and inconsistent between years, and smaller than overall differences between years and localities. At ISJ, Brown Boobies included more prey species in their diet (27 vs. 19 spp.) and dove shallower (2.3 vs. 3.14 m) than at FSI. At ISJ, Brown Boobies exhibited adjustments in diving depth and prey size as a function of environmental variation: shallower plunge dives and smaller prey items were related with lower SST and higher PP values, whereas deeper dives and larger prey items were related with higher SST and lower PP values. Our results confirmed that the Brown Booby is highly plastic in its foraging ecology, which explains its ability to live in places with large-scale environmental variation (intercolony and interannual), such as tropical areas worldwide.
The population of Sanderlings Calidris alba along the East Atlantic flyway has grown considerably during the last decades. Perhaps reflecting this augmented population size, increasing numbers of Sanderling have been reported to stage in the Wadden Sea during spring and autumn migration. Estimates of the numbers of Sanderlings in the Wadden Sea have previously been based on a limited number of counts that were not corrected for the turnover of individuals. In this study, we accounted for turnover using estimates of the probability that individually colour-ringed Sanderlings are still present two days after a sighting. In combination with daily counts during high tide, we estimated the total number of Sanderlings that used the island Griend and surrounding mudflats, in the western Dutch Wadden Sea, during southward passage in 2013 and 2014. We also estimated minimal staging durations of Sanderlings at Griend. Non-moulting birds were significantly heavier upon capture, which suggests that they were refuelling for long non-stop migratory flights. Winter sightings confirmed that the non-moulting Sanderlings winter in sub-Saharan Africa and that the moulting Sanderlings spent the winter in Europe or northern Africa. With an average minimal stay in the western Dutch Wadden Sea of 9 days in 2013 and 12 in 2014, non-moulting Sanderlings stayed for a much shorter time than moulting Sanderlings, which stayed for 32 days in 2013 and 36 days in 2014. Non-moulting individuals were less likely to be resighted between years. Estimates of minimal staging duration are likely underestimates of the true staging duration, and we propose that moulting Sanderlings probably complete their wing moult in the Wadden Sea. We estimated that the total number of Sanderlings using the western Dutch Wadden Sea before migration to European or African wintering areas were 27,546 (95% Cl 22,739–41,449) in 2013 and 22,574 (95% Cl 16,436–46,114) in 2014. This would amount to 11–14% of a total flyway population of 200,000 individuals, representing an amazing degree of concentration for what is regarded as a rather widely and thinly spread shorebird species.
Hunting is a key source of disturbance which affects geese populations directly through killing and indirectly through shooting disturbance. Movements of individuals in response to hunting disturbance may expose geese to reduced feeding opportunities and higher predation risks, which can have consequences on the population level. Thus geese should use habitats that provide access to food and minimize encounters with hunters. Our study focused on differences in the proportion of hunting-free area used and in the total area used by individuals, size of water bodies used by birds, and flight distance between roosting and feeding areas, during two periods — before and after the start of the hunting season. In south Bohemia (Czech Republic), in 2012, 2014 and 2015, we attached seventeen GPS/GSM-transmitters to moulting Greylag Geese Anser anser. We analysed data from nine transmitters, recording two or twelve logs per day, using minimum convex polygons home range estimates and Wilcoxon matched pairs test. We showed that shortly after the opening of the hunting season, individual Greylag Geese used a significantly larger area and increased their roosting-feeding flight distances. These changes in local movements are likely linked with the start of hunting activity which relates to increased human disturbance in the study area. In the post-breeding season, this may broadly affect the birds' energetic costs and ability to accumulate sufficient reserves for the upcoming autumn migration. However, we did not find a significant difference in the proportion of hunting-free (protected) areas used during our study periods. This result, together with the observed decrease in numbers of Greylag Geese in south Bohemia in the second half of August, may be evidence of a lack of larger disturbance-free refuges in the study area.
I examined the foraging and diet energetics of the Great Egret across an upland landscape to compare relative costs and benefits associated with commensal, non-commensal, solitary and group foraging strategies. Across strategies, Great Egrets used a combination of different foraging modes. Net energetic gains and costs differed across strategies. Foraging costs were lower for solitary egrets, mainly as a result of decreased foraging times and lower aggression and relocation frequencies. Solitary egrets likely suffered unmeasured costs from territoriality, expensive capture efforts and handling larger prey. Group foragers accumulated higher costs due to having longer foraging times, and higher pursuit, relocation and aggression frequencies. Commensal foragers had lower overall costs in comparison to solitary and group foragers. When ranked by energetic cost-benefit ratio, solitary foraging was the most efficient strategy and group foraging strategies were the least efficient. The overall benefits from profitable food sources outweighed the estimated costs of terrestrial foraging. Great Egrets are ecologically flexible and expand their niche breadth into upland habitats where ample energy resources are apparently available. The Great Egret's ability to capitalize on different energy sources during different foraging situations likely improves foraging efficiency and energy intake over time.
While alteration of the migratory habits of birds is widely regarded as one of the most evident ecological effects of climate change, studies reporting shifts in migration phenology for long-lived, long-distance migrants have been few. Using time series of count data collected in southern Spain during autumn migration, we examined the magnitude and direction of phenological shifts for six common species of soaring birds. Many current methods for investigating phenological change rely on continuous data sets; however, these data may be unavailable for a variety of reasons. We used a cross-correlation analysis, which allowed us to compare recent data on the timing of migration from 1999–2011 to a historic data set collected during 1976–1977. The direction of phenological shifts for autumn migration was species-specific. White Storks Ciconia ciconia and Black Kites Milvus migrans appeared to have delayed passage, Black Storks Ciconia nigra and European Honey Buzzards Pernis apivorus have advanced their migratory timing, and we found no clear phenological change for Short-toed Eagles Circaetus gallicus or Booted Eagles Hieraaetus pennatus.
We studied sex-specific differences in prey returned to nests and eaten by parents in Whiskered Terns Chlidonias hybrida. The species is size-dimorphic, with females being smaller than males. The sex of the birds was determined by molecular analysis. Prey consumed and carried to chicks was recorded at short (<300 m) and long distances (>300 m) from nests. Only females carried invertebrates to the chicks and they did so only from short distances. The proportion of vertebrates carried to the chicks by both sexes was greater when feeding at short distances from the nest. Males carried significantly larger vertebrates to chicks than those they consumed themselves. These results demonstrate that reproductive trade-offs differ between the sexes in the Whiskered Tern.