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ABSTRACT Many songbirds begin active incubation after laying their penultimate egg, resulting in synchronous hatching of the clutch except for a last‐hatched individual (“runt”) that hatches with a size deficit and competitive disadvantage to siblings when begging for food. However, climate change may elevate temperatures and cause environmental incubation as eggs are laid, resulting in asynchronous hatching and larger size hierarchies among siblings. Although previous work demonstrated that asynchronous hatching reduces nestling growth and survival relative to synchrony, the physiological mechanisms underlying these effects are unclear. To test the effects of asynchronous hatching on runt growth, survival, physiology, and compensatory growth‐related tradeoffs, we manipulated incubation temperature in nest boxes of European starlings (Sturnus vulgaris) to increase asynchronous hatching and collected nestling morphological measurements and blood samples to assess physiology and development. Independent of heating treatment, runts from asynchronously hatched nests had lower survival than runts from more synchronous nests. Surviving runts from asynchronous nests were smaller and had reduced stress‐induced corticosterone concentrations and reduced circulating glucose compared with runts from synchronous nests. Despite persistent size and energy deficits, runts from asynchronous nests did not have significant deficits in immunity or telomere length when compared with runts from synchronous nests, suggesting no trade‐off between investment in immune development or telomere maintenance with growth. Overall, these results suggest that increased asynchrony due to climate change could reduce clutch survival for altricial songbirds, especially for the smallest chicks in a clutch, and that the negative effects of asynchrony may be driven by persistent energetic deficits. 

The non-breeding season is an understudied, yet likely critical, period for many species. Understanding species’ resource requirements, and determining when limited resources and increased densities may lead to intraspecific competition and demographic partitioning, may aid species conservation efforts. Monitoring species’ resource requirements during the non-breeding season may be more important in highly modified ecosystems, such as intensive agricultural landscapes, where anthropogenic pressures may further limit resources. The Loggerhead Shrike (Lanius ludovicianus) is a rapidly declining avian species that winters in agricultural areas in the southeastern United States, but little is known about their ecology or potential demographic partitioning in this context. To fill these knowledge gaps, we compared multi-scale habitat selection, survival, and space use across age and sex classes of shrikes inhabiting an agricultural landscape in Arkansas, USA. We found that habitat selection differed among demographic classes. Specifically, females preferred areas with more fallow cover, utility wires, and anthropogenic perches, whereas males preferred areas with more agricultural fields and ditches and less soybean cover. However, overall, shrikes exhibited numerous similarities in habitat selection, generally preferring areas with greater developed land cover (within a predominantly agricultural landscape), greater water availability, and taller perches. Despite the observed variability in habitat selection, no differences in apparent seasonal and annual survival rates or home range size existed among groups. However, non-breeding dispersal distance between years differed by age class, with older individuals being more site faithful than younger individuals. We suggest that the demographic habitat partitioning we detected may reflect adaptive differential life history strategies associated with age and sex classes, but further study of habitat selection by Loggerhead Shrikes across seasons and habitat types will help clarify the variation, importance, and potential carry-over effects of non-breeding habitat partitioning. 

Abstract Migrating animals are known to play an important role in nutrient transfer over short distances; however, this phenomenon has not been well studied for long-distance migrants. In this preliminary study, we focused on nitrogen (N) transfer by 44 bird species that migrate from Eurasia to two regions in sub-Saharan Africa that fall into the lowest 10% quantile of global N-deposition (mean annual deposition ≤ 10.44 mg/m 2 /year). We estimated the number of birds that die during the non-breeding season in these areas and then used N content and species-specific mass values to calculate annual N-deposition rates. For these two areas of low N-deposition, we found that bird mortality contributed 0.2 – 1.1% of total nitrogen deposition, which is a relatively small proportion. Therefore, we conclude that nitrogen transfer by long-distance bird migrants using the East Atlantic Flyway and the West Asian-East African Flyway currently has limited impact on the sub-Saharan nitrogen cycle. However, it is worth noting that this impact may have been more important in the past due to larger bird populations and lower background N-deposition (i.e., less anthropogenic impact).