T. Scott Sillett’s research while affiliated with National Park Service and other places

Interactions with competitors and predators can generate strong selection and favour the evolution of novel strategies for mitigating fitness costs. Adaptations to mitigate competition and predation risk often involve evolution of traits which directly reduce costs. Simultaneously, the evolution of behaviours that co‐opt the functional traits of non‐interacting, third‐party species have also been observed but remain poorly studied (e.g. anointing behaviour, tri‐trophic interactions). Here we investigate if the novel behavioural co‐option of conifer resin in a cavity‐nesting bird can ameliorate interactions with nest predators and nest site competitors. Red‐breasted nuthatches (Sitta canadensis) collect resin from live trees and apply it around the entrances of their nest cavities in dead trees. Using paired nest boxes, we mimicked the nuthatch behaviour by apply resin around the hole of one and not the paired box in two field experiments to test a priori hypotheses about the evolution and adaptive value of this behaviour. The first experiment baited the paired boxes with food to attract mammalian predators (e.g. red squirrels Tamiasciurus hudsonicus). The second experiment left boxes available for breeding by cavity‐nesting birds that compete with nuthatches for nest sites (e.g. house wrens Troglodytes aedon). We also used ancestral trait reconstruction to test whether resin use is a non‐adaptive derivation of mud‐plastering behaviour observed in numerous congeners. Experimental nest boxes with resin applied to the entrance were less likely to have bait removed and less likely to have nests built or initiated than paired nest boxes without resin. Ancestral trait reconstruction suggested that resin use was more likely (>77%) to have evolved in ancestors that did not exhibit mud‐plastering behaviour. Our results do not support a major role of phylogenetic inertia and provide experimental evidence that application of conifer resin to nest cavities ameliorates interference competition from other cavity‐nesting species and reduces nest predation risk. Overall, our results showcase the ecological function of behavioural co‐option and its important consequences for fitness in nature. Read the free Plain Language Summary for this article on the Journal blog.

Changes in leaf phenology from warming spring and autumn temperatures have lengthened the temperate zone growing “green” season and breeding window for migratory birds in North America. However, the fitness benefits of an extended breeding season will depend, in part, on whether species have sufficient dietary flexibility to accommodate seasonal changes in prey availability. We used fecal DNA metabarcoding to test the hypothesis that seasonal changes in the diets of the insectivorous, migratory black‐throated blue warbler (Setophaga caerulescens) track changes in the availability of arthropod prey at the Hubbard Brook Experimental Forest, New Hampshire, USA. We examined changes across the breeding season and along an elevation gradient encompassing a 2‐week difference in green season length. From 98 fecal samples, we identified 395 taxa from 17 arthropod orders; 242 were identified to species, with Cecrita guttivitta (saddled prominent moth), Theridion frondeum (eastern long‐legged cobweaver), and Philodromus rufus (white‐striped running crab spider) occurring at the highest frequency. We found significant differences in diet composition between survey periods and weak differences among elevation zones. Variance in diet composition was highest late in the season, and diet richness and diversity were highest early in the season. Diet composition was associated with changes in prey availability surveyed over the green season. However, several taxa occurred in diets more or less than expected relative to their frequency of occurrence from survey data, suggesting that prey selection or avoidance sometimes accompanies opportunistic foraging. This study demonstrates that black‐throated blue warblers exhibit diet flexibility and track seasonal changes in prey availability, which has implications for migratory bird responses to climate‐induced changes in insect communities with longer green seasons.

Inferring the environmental selection pressures responsible for phenotypic variation is a challenge in adaptation studies as traits often have multiple functions and are shaped by complex selection regimes. We provide indirect evidence that morphology of the multifunctional avian bill is primarily shaped by climate and thermoregulatory ability in Melospiza melodia (Song Sparrow) on the California Channel Islands. Our research builds on a study in M. melodia museum specimens that demonstrated a positive correlation between bill surface area and maximum temperature, suggesting a greater demand for dry heat dissipation in hotter, xeric environments. We sampled contemporary sparrow populations across 3 climatically distinct islands to test the hypotheses that bill morphology is influenced by habitat differences with functional consequences for foraging efficiency and is related to maximum temperature and, consequently, important for thermoregulation. Measurements of >500 live individuals indicated a significant, positive relationship between maximum temperature and bill surface area when correcting for body size. In contrast, maximum bite force, seed extraction time, and vegetation on breeding territories (a proxy for food resources) were not significantly associated with bill dimensions. While we cannot exclude the influence of foraging ability and diet on bill morphology, our results are consistent with the hypothesis that variation in M. melodia need for thermoregulatory capacity across the northern Channel Islands selects for divergence in bill surface area.

Predicting how the range dynamics of migratory species will respond to climate change requires a mechanistic understanding of the factors that operate across the annual cycle to control the distribution and abundance of a species. Here, we use multiple lines of evidence to reveal that environmental conditions during the nonbreeding season influence range dynamics across the life cycle of a migratory songbird, the American redstart ( Setophaga ruticilla ). Using long-term data from the nonbreeding grounds and breeding origins estimated from stable hydrogen isotopes in tail feathers, we found that the relationship between annual survival and migration distance is mediated by precipitation, but only during dry years. A long-term drying trend throughout the Caribbean is associated with higher mortality for individuals from the northern portion of the species’ breeding range, resulting in an approximate 500 km southward shift in breeding origins of this Jamaican population over the past 30 y. This shift in connectivity is mirrored by changes in the redstart’s breeding distribution and abundance. These results demonstrate that the climatic effects on demographic processes originating during the tropical nonbreeding season are actively shaping range dynamics in a migratory bird.

Sex allocation theory predicts that females should bias their offspring sex ratios when the fitness benefits of producing sons or daughters differ depending on rearing environment. The Trivers-Willard hypothesis proposes that whether females produce more sons or daughters depends on food availability via both intrinsic maternal condition and differing reproductive potential (typically from mating system structure) for sons versus daughters. However, tests of its key predictions are often based on untested, implicit assumptions that are difficult to quantify, especially in migratory animals. In a 5-year study, we manipulated food availability in low- and high-elevation forest to test the Trivers-Willard hypothesis in the migratory black-throated blue warbler (Setophaga caerulescens). We found that the population-wide offspring sex ratio was significantly male-biased (population mean: 0.58), which was driven by an overproduction of sons in high-elevation forest (high-quality habitat mean: 0.59). Yet, we found no effect of food availability on offspring sex ratio from either natural variation or supplemental feeding. Sex-specific developmental costs did not differ for sons and daughters reared under low and high food availability. These results suggest that female black-throated blue warblers do not manipulate offspring sex ratios in response to food availability and are not consistent with the predictions of the Trivers-Willard hypothesis. This study highlights challenges of examining mechanisms driving patterns in offspring sex allocation in migratory species for which both the costs of rearing and relative fitness benefits of sons and daughters cannot be tracked into adulthood.

Striking examples of local adaptation at fine geographic scales are increasingly being documented in natural populations. However, the relative contributions made by natural selection, phenotype-dependent dispersal (when individuals disperse with respect to a habitat preference), and mate preference in generating and maintaining microgeographic adaptation and divergence are not well studied. Here, we develop quantitative genetics models and individual-based simulations (IBSs) to uncover the evolutionary forces that possibly drive microgeographic divergence. We also perform Bayesian estimation of the parameters in our IBS using empirical data on habitat-specific variation in bill morphology in the island scrub-jay (Aphelocoma insularis) to apply our models to a natural system. We find that natural selection and phenotype-dependent dispersal can generate the patterns of divergence we observe in the island scrub-jay. However, mate preference for a mate with similar bill morphology, even though observed in the species, does not play a significant role in driving divergence. Our modeling approach provides insights into phenotypic evolution occurring over small spatial scales relative to dispersal ranges, suggesting that adaptive divergence at microgeographic scales may be common across a wider range of taxa than previously thought. Our quantitative genetic models help to inform future theoretical and empirical work to determine how selection, habitat preference, and mate preference contribute to local adaptation and microgeographic divergence.

As migratory species move through the stages of their annual cycle, individuals often display variation in the degree to which they remain in proximity to one another, a phenomenon called migratory connectivity. We show scale dependence in the strength of migratory connectivity in Painted Buntings (Passerina ciris), a North American passerine with disjunct eastern and interior breeding populations. Based on light-level geolocator data from 112 individuals at 11 breeding sites, migratory connectivity between breeding and wintering grounds was strong at the range-wide scale, with interior and eastern Painted Buntings remaining separated throughout the annual cycle. Conversely, migratory connectivity within the eastern and interior populations was weak, with individuals from different breeding areas mixing extensively on winter quarters. We found weak migratory connectivity within populations as birds moved from the breeding grounds to the wintering grounds (breeding-to-winter), with individuals from different regions of each population mixing extensively on the wintering grounds. The interior population, however, displayed strong migratory connectivity as birds moved from the breeding grounds to the intermediate molting grounds (breeding-to-molting), with birds from different breeding sites showing contrasting migratory strategies during the molting period. Our results suggest that spatial scale dependence of migratory connectivity is likely to be a pervasive phenomenon, given that migratory routes and the likelihood of molt migration often differ among populations. When possible, researchers should be deliberate about the spatial design of tracking studies to reduce potential biases that can result from spatial scale-dependent migratory connectivity.