Extensive regional variation in the phenology of insects and their response to temperature across North America.

Climate change models often assume similar responses to temperatures across the range of a species, but local adaptation or phenotypic plasticity can lead plants and animals to respond differently to temperature in different parts of their range. To date, there have been few tests of this assumption at the scale of continents, so it is unclear if this is a large-scale problem. Here, we examined the assumption that insect taxa show similar responses to temperature at 96 sites in grassy habitats across North America. We sampled insects with Malaise traps during 2019-2021 (N = 1041 samples) and examined the biomass of insects in relation to temperature and time of season. Our samples mostly contained Diptera (33%), Lepidoptera (19%), Hymenoptera (18%), and Coleoptera (10%). We found strong regional differences in the phenology of insects and their response to temperature, even within the same taxonomic group, habitat type, and time of season. For example, the biomass of nematoceran flies increased across the season in the central part of the continent, but it only showed a small increase in the Northeast and a seasonal decline in the Southeast and West. At a smaller scale, insect biomass at different traps operating on the same days was correlated up to ~75 km apart. Large-scale geographic and phenological variation in insect biomass and abundance has not been studied well, and it is a major source of controversy in previous analyses of insect declines that have aggregated studies from different locations and time periods. Our study illustrates that large-scale predictions about changes in insect populations, and their causes, will need to incorporate regional and taxonomic differences in the response to temperature.

Interacting effects of cold snaps, rain, and agriculture on the fledging success of a declining aerial insectivore.

Climate change predicts the increased frequency, duration, and intensity of inclement weather periods such as unseasonably low temperatures (i.e., cold snaps) and prolonged precipitation. Many migratory species have advanced the phenology of important life history stages and, as a result, are likely to be exposed to these periods of inclement spring weather more often, therefore risking reduced fitness and population growth. For declining avian species, including aerial insectivores, anthropogenic landscape changes such as agricultural intensification are another driver of population declines. These landscape changes may affect the foraging ability of food provisioning parents and reduce the survival of nestlings exposed to inclement weather through, for example, pesticide exposure impairing thermoregulation and punctual anorexia. Breeding in agro-intensive landscapes may therefore exacerbate the negative effects of inclement weather under climate change. We observed that a significant reduction in the availability of insect prey occurred when daily maximum temperatures fell below 18.3°C, and thereby defined any day when the maximum temperature fell below this value as a day witnessing a cold snap. We then combined daily information on the occurrence of cold snaps and measures of precipitation to assess their impact on the fledging success of Tree Swallows (Tachycineta bicolor) occupying a nest box system placed across a gradient of agricultural intensification. Estimated fledging success of this declining aerial insectivore was 36.2% lower for broods experiencing 4 cold-snap days during the 12 days post-hatching period versus broods experiencing none, and this relationship was worsened when facing more precipitation. We further found that the overall negative effects of a brood experiencing periods of inclement weather was exacerbated in more agro-intensive landscapes. Our results indicate that two of the primary hypothesized drivers of many avian population declines may interact to further increase the rate of declines in certain landscape contexts.

Nonideal nest box selection by tree swallows breeding in farmlands: Evidence for an ecological trap?

Animals are expected to select a breeding habitat using cues that should reflect, directly or not, the fitness outcome of the different habitat options. However, human-induced environmental changes can alter the relationships between habitat characteristics and their fitness consequences, leading to maladaptive habitat choices. The most severe case of such nonideal habitat selection is the ecological trap, which occurs when individuals prefer to settle in poor-quality habitats while better ones are available. Here, we studied the adaptiveness of nest box selection in a tree swallow (Tachycineta bicolor) population breeding over a 10-year period in a network of 400 nest boxes distributed along a gradient of agricultural intensification in southern Québec, Canada. We first examined the effects of multiple environmental and social habitat characteristics on nest box preference to identify potential settlement cues. We then assessed the links between those cues and habitat quality as defined by the reproductive performance of individuals that settled early or late in nest boxes. We found that tree swallows preferred nesting in open habitats with high cover of perennial forage crops, high spring insect biomass, and high density of house sparrows (Passer domesticus), their main competitors for nest sites. They also preferred nesting where the density of breeders and their mean number of fledglings during the previous year were high. However, we detected mismatches between preference and habitat quality for several environmental variables. The density of competitors and conspecific social information showed severe mismatches, as their relationships to preference and breeding success went in opposite direction under certain circumstances. Spring food availability and agricultural landscape context, while related to preferences, were not related to breeding success. Overall, our study emphasizes the complexity of habitat selection behavior and provides evidence that multiple mechanisms may potentially lead to an ecological trap in farmlands.

Assessing pesticides exposure effects on the reproductive performance of a declining aerial insectivore.

In the context of increasing global environmental changes, it has become progressively important to understand the effects of human activity on wildlife populations. Declines in several avian populations have been observed since the 1970s, especially with respect to many farmland and grassland birds, which also include some aerial insectivores. Changes in farming practices referred to as agricultural intensification coincide with these major avian declines. Among those practices, increased pesticide use is hypothesized to be a key driver of avian population declines as it can lead to both toxicological and trophic effects. While numerous laboratory studies report that birds experience acute and chronic effects upon consuming pesticide treated food, little is known about the effects of the exposure to multiple pesticides on wildlife in natural settings. We monitored the breeding activities of Tree Swallows (Tachycineta bicolor) on 40 farms distributed over a gradient of agricultural intensification in southern Québec, Canada, to evaluate the presence of pesticides in their diet and quantify the exposure effects of those compounds on their reproductive performance between 2013 and 2018. We first assessed the presence of 54 active agents (or derivatives) found in pesticides in 2,081 food boluses (insects) delivered to nestlings by parents and documented their spatial distribution within our study area. Second, we assessed the effect of pesticide exposure through food (number of active agents detected and number of contaminated boluses on a given farm for a given year, while controlling for sampling effort) on clutch size as well as hatching and fledging successes and nestling's mass upon fledging. Pesticides were ubiquitous in our study system and nearly half (46%) of food boluses were contaminated by at least one active agent. Yet we found no relationship between our proxies of food contamination by pesticides and Tree Swallow reproductive performance. More studies are needed to better understand the putative role of pesticides in the decline of farmland birds and aerial insectivores as potential sublethal effects of pesticides can carry over to later life stages and impact fitness.

Landscape Enhancements in Apple Orchards: Higher Bumble Bee Queen Species Richness, but No Effect on Apple Quality.

Bumble bees are among the most effective pollinators in orchards during the blooming period, yet they are often threatened by the high levels of pesticide use in apple production. This study aimed to evaluate the influence of landscape enhancements (e.g., hedgerows, flower strips) on bumble bee queens in apple orchards. Bumble bee queens from 12 orchards in southern Québec (Canada) were marked, released, and recaptured in the springs and falls of 2017 to 2019. Half of the 12 orchards had landscape enhancements. Apples were harvested in 2018 and 2019 to compare their quality (weight, diameter, sugar level, and seed number) in sites with and without landscape enhancements. Species richness, as well as the occurrence of three species out of eight, was higher in orchards with landscape enhancements than in orchards without such structures. The occurrence of Bombus ternarius was lower in orchards with high levels of pesticide use. Apples had fewer seeds when collected in orchards with landscape enhancements and were heavier in orchards that used more pesticides. Our work provides additional evidence that landscape enhancements improve bumble bee presence in apple orchards and should therefore be considered as a means to enhance pollination within farms.

Agricultural pesticides and ectoparasites: potential combined effects on the physiology of a declining aerial insectivore.

Agricultural pesticides usage has been increasing globally. These compounds have been developed to disrupt pest species physiology, but because their specificity is limited, they can also have adverse effects on non-target organisms. Recent studies have shown that the damaging toxicological effects of pesticides can be amplified in stressful environments. However, few studies have documented these effects in natural settings where organisms are simultaneously exposed to pesticides and to other environmental stressors such as parasites. In this study, we assessed both pesticide and ectoparasite effects on the physiology of a free-ranging bird. We measured physiological markers including haematocrit, bacteria-killing ability (BKA) and leucocyte counts, as well as exposure to haematophagous Protocalliphora larvae, in tree swallow nestlings (Tachycineta bicolor), a declining aerial insectivore, in southern Québec, Canada, for over 3 years. We found that combined exposure to pesticides and Protocalliphora larvae was negatively related to haematocrit, suggesting possible synergistic effects. However, we found no such relationships with BKA and leucocyte counts, highlighting the complexity of physiological responses to multiple stressors in natural settings. Populations of several aerial insectivores are declining, and although sublethal pesticide effects on physiology are suspected, our results suggest that exposure to other factors, such as parasitism, should also be considered to fully assess these effects, especially because pesticides are increasingly present in the environment.

Landscape composition and local floral resources influence foraging behavior but not the size of Bombus impatiens Cresson (Hymenoptera: Apidae) workers.

Bumble bee communities are strongly disrupted worldwide through the population decline of many species; a phenomenon that has been generally attributed to landscape modification, pesticide use, pathogens, and climate change. The mechanisms by which these causes act on bumble bee colonies are, however, likely to be complex and to involve many levels of organization spanning from the community down to the least understood individual level. Here, we assessed how the morphology, weight and foraging behavior of individual workers are affected by their surrounding landscape. We hypothesized that colonies established in landscapes showing high cover of intensive crops and low cover of flowering crops, as well as low amounts of local floral resources, would produce smaller workers, which would perform fewer foraging trips and collect pollen loads less constant in species composition. We tested these predictions with 80 colonies of commercially reared Bombus impatiens Cresson placed in 20 landscapes spanning a gradient of agricultural intensification in southern Québec, Canada. We estimated weekly rate at which workers entered and exited colonies and captured eight workers per colony over a period of 14 weeks during the spring and summer of 2016. Captured workers had their wing, thorax, head, tibia, and dry weight measured, as well as their pollen load extracted and identified to the lowest possible taxonomic level. We did not detect any effect of landscape habitat composition on worker morphology or body weight, but found that foraging activity decreased with intensive crops. Moreover, higher diversity of local floral resources led to lower pollen constancy in intensively cultivated landscapes. Finally, we found a negative correlation between the size of workers and the diversity of their pollen load. Our results provide additional evidence that conservation actions regarding pollinators in arable landscapes should be made at the landscape rather than at the farm level.

Impacts of environmental heterogeneity on natural selection in a wild bird population.

Natural selection has been studied for several decades, resulting in the computation of thousands of selection estimates. Although the importance of environmental conditions on selection has often been suggested, published estimates rarely take into account the effects of environmental heterogeneity on selection patterns. Here, we estimated linear and nonlinear viability selection gradients on morphological traits of 12-day old nestlings in a wild population of tree swallows (Tachycineta bicolor) across a large-scale heterogeneous study system in southern Québec, Canada. We assessed the environmental drivers of nestling survival and evaluated their effects on strength and direction of selection gradients. Separate analyses of environmental variables showed that high temperatures and heavy rainfall caused stronger positive linear selection on morphological traits. Weaker linear selection was also measured in more extensively cultivated areas. Both strength and shape of nonlinear quadratic and correlational components of selection were modified by environmental variables. Considering all environmental variables revealed that precipitation since hatching affected patterns of linear selection on traits, while temperatures since hatching shaped nonlinear selection patterns. Our study underlines the importance of quantifying linear and nonlinear natural selection under various environmental conditions and how the evolutionary response of traits may be affected by ongoing human-induced environmental changes.

A range-wide domino effect and resetting of the annual cycle in a migratory songbird.

Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographical variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows ( Tachycineta bicolor) originating from 12 North American breeding populations. A swallow's breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at autumn stopover locations and timing of arrival at non-breeding locations. This 'domino effect' between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals reside at non-breeding sites for extended periods of time.

Offspring mass variation in tree swallows: A case of bet-hedging?

The evolution of reproductive strategies is affected by the ability of organisms to deal with future environmental conditions. When environments are temporally unpredictable, however, it is difficult to anticipate optimal offspring phenotype. Diversification of offspring phenotypes, a strategy called diversified bet-hedging, may allow parents to maximize their fitness by reducing between-year variation in reproductive success. The link between diversification of offspring phenotypes and individual reproductive success, however, has rarely been documented empirically. We used an eight-year dataset (1215 broods, 870 females) on individually marked tree swallows (Tachycineta bicolor) to assess whether intra-brood mass variation was compatible with a diversified bet-hedging strategy. Intra-brood mass variation was weakly, but significantly repeatable within females, suggesting consistent individual differences. Greater intra-brood mass variation, however, was not associated with reduced between-year variation in reproductive success or increased female reproductive success. Moreover, contrary to diversified bet-hedging expectations, fledging success of large broods was greater when hatchlings had similar rather than variable masses. Our results suggest that intra-brood mass variation may not result from diversified bet-hedging, but rather from complex interactions between environmental, brood, and maternal characteristics.

Effects of blood parasite infection and innate immune genetic diversity on mating patterns in a passerine bird breeding in contrasted habitats.

Genetic diversity at immune genes and levels of parasitism are known to affect patterns of (dis)assortative mating in several species. Heterozygote advantage and/or good genes should shape mate choice originating from pathogen/parasite-driven selection at immune genes. However, the stability of these associations, and whether they vary with environmental conditions, are still rarely documented. In this study, we describe mating patterns in a wild population of tree swallows (Tachycineta bicolor) over 4 years and assess the effects of haemosporidian parasite infection and immune genetic diversity at ß-defensin genes on those patterns within two habitats of contrasting environmental quality, in southern Québec, Canada. We first show that mating patterns were only very weakly related to individual status of infection by haemosporidian parasites. However, we found a difference between habitats in mating patterns related to infection status, which was likely due to a non-random distribution of individuals, as non-infected mating pairs were more frequent in lower quality habitats. Mating patterns also differed depending on ß-defensin heterozygosity at AvBD2, but only for genetic partners outside of the social couple, with heterozygous individuals pairing together. Our study underlines the importance of considering habitat heterogeneity in studies of sexual selection.

Environmental determinants of haemosporidian parasite prevalence in a declining population of Tree swallows.

The prevalence of vector-borne parasites such as haemosporidian species is influenced by several environmental factors. While the negative effects of parasitism on hosts are well documented, these can also be amplified by interactions with environmental stressors, many of which are anthropogenic. Yet, we know little about the possible effects of anthropogenic perturbations on parasite prevalence. The goals of this study were to assess the prevalence and environmental determinants of haemosporidian parasites in a declining population of Tree swallows (Tachycineta bicolor) living in an agricultural landscape in southern Québec, Canada. Overall, a low prevalence and a moderate lineage diversity were identified in both adults and nestlings, confirming that transmission can occur during the breeding period. Anthropic areas, extensive cultures (hayfields and pastures) and forest cover within 500 km of nest boxes, as well as daily temperature fluctuations, were all related to infection by haemosporidian parasites. These findings suggest that anthropogenic alterations of landscape composition can modulate the prevalence of haemosporidian parasites in Tree swallows. Our results represent a baseline for future comparative studies assessing haemosporidian parasite prevalence in human-modified landscapes.

Assessment of individual and conspecific reproductive success as determinants of breeding dispersal of female tree swallows: A capture-recapture approach.

Breeding dispersal is a key process of population structure and dynamics and is often triggered by an individual's breeding failure. In both colonial and territorial birds, reproductive success of conspecifics (RSc) can also lead individuals to change breeding sites after a failure on a site. Yet, few studies have simultaneously investigated the independent contribution of individual reproductive success (RSi) and of RSc on dispersal decision. Here, we develop a modeling framework to disentangle the effects of RSi and RSc on demographic parameters, while accounting for imperfect individual detection and other confounding factors such as age or dispersal behavior in the previous year. Using a 10-year capture-recapture dataset composed of 1,595 banded tree swallows, we assessed the effects of nonmanipulated RSi and RSc on female breeding dispersal in this semicolonial passerine. Dispersal was strongly driven by RSi, but not by RSc. Unsuccessful females were 9.5-2.5 times more likely to disperse than successful ones, depending if they had dispersed or not in the previous year, respectively. Unsuccessful females were also three times less likely to be detected than successful ones. Contrary to theoretical and empirical studies, RSc did not drive the decision to disperse but influenced the selection of the following breeding site once dispersal had been initiated. Because detection of individuals was driven by RSi, which was positively correlated to RSc, assuming a perfect detection as in previous studies may have lead us to conclude that RSc affected dispersal patterns, yet our approach corrected for this bias. Overall, our results suggest that the value and use of RSc as public information to guide dispersal decisions are likely dictated by multiple ecological determinants, such as landscape structure and extent, if this cue is indeed used.

Agricultural Intensification Is Linked to Constitutive Innate Immune Function in a Wild Bird Population.

Immunity represents an important defense mechanism against pathogens and is intimately linked to fitness. Previous studies have found significant interindividual variation of immune responses in wild populations and have emphasized the importance of ecological factors in explaining this variability. A deterioration of environmental conditions, such as agricultural intensification, can modify resource availability and, as a result, alter immune functions. The aim of this study was to investigate the effect of agricultural intensification on innate immune functions in breeding adult tree swallows (Tachycineta bicolor) over 6 yr. We investigated the relationship between agricultural intensification and bacteria-killing ability (BKA) against a strain of E. coli. We also assessed how the presence and the number of bacteria on the surface of eggshells changed with agricultural intensification. Contrary to our expectations, we found a positive relationship between agricultural intensification and BKA, where individuals had higher BKA in intensive compared to nonintensive farmlands. While this trend was observed through the 6-yr study, we also found a tendency for a reduction of BKA over time. We found no relationships between the presence or number of different types of eggshell bacteria and agricultural intensification or an association between BKA of females and eggshell bacteria of their clutch. Our results suggest that differences in immune function observed between intensive and nonintensive farmlands could reflect genetic differences among individuals and/or trade-offs with other traits expressed among habitats within our study system.

Patterns of Diversity and Spatial Variability of ß-Defensin Innate Immune Genes in a Declining Wild Population of Tree Swallows.

Assessing the genetic variation and distribution of immune genes across heterogeneous environmental conditions in wild species is essential to further our understanding of the role of pathogen pressure and potential resistance or prevalence in hosts. Researchers have recently investigated ß-defensin genes in the wild, because their variability suggests that they may play an important role in innate host defense. This study investigated the variation occurring at 6 innate immune genes of the ß-defensin family in a declining population of tree swallows (Tachycineta bicolor) in southern Québec, Canada (N = 160). We found that all 6 genes showed synonymous and nonsynonymous single nucleotide polymorphisms (SNPs) within the exon coding for the mature peptide. These results indicated that this group of genes was diverse in tree swallows. Our results suggested a potential interaction of this group of genes with fluctuating pathogen diversity, however, we found no sign of positive or negative selection. We assessed whether or not the distribution of genetic diversity of ß-defensin genes in our study population differed between 2 regions that strongly differ in their level of agricultural intensification. Adults are highly philopatric to their breeding sites and their immunological responses differ between these 2 regions. However, we found little evidence that the level and distribution of genetic variability differed between these heterogeneous environmental conditions. Further studies should aim to assess the link between genetic diversity of ß-defensin genes and fitness-related traits in wild populations.

Multidimensional environmental influences on timing of breeding in a tree swallow population facing climate change.

Most phenological traits are extremely sensitive to current climate change, and advances in the timing of important life-history events have been observed in many species. In birds, phenotypic plasticity in response to temperature is thought to be the main mechanism underlying yearly adjustment in the timing of breeding. However, other factors could be important and interact to affect the levels of plastic responses between and/or within-individuals. Here, we use long-term individual-based data on tree swallow (Tachycineta bicolor) to identify the spatial and environmental drivers affecting plasticity in laying date and to assess their importance at both population and individual levels. We found that laying date has advanced by 4.2 days over 10 years, and that it was mainly influenced by latitude and an interaction between spring temperature and breeder density. Analyses of individual plasticity showed that increases in temperature, but not in breeder density, resulted in within-individual advances in laying date. Our results suggest that females can adjust their laying date as a function of temperature, but that this adjustment will be partly constrained in habitats with lower breeder densities. Such potential constraint is especially worrying for the broad array of species already declining as a result of climate change.

Liquid chromatography-tandem mass spectrometry determination for multiclass pesticides from insect samples by microwave-assisted solvent extraction followed by a salt-out effect and micro-dispersion purification.

The effects of phyto-pharmaceutic compounds (PPCs), such as neonicotinoids, on wildlife reproduction and survival are a rising concern. Yet, understanding the biological consequences of PPC use is particularly complex given the large diversity of PPCs and their derivatives to which wildlife can be exposed. Here, we present a simple and sensitive method for the simultaneous detection and quantification of multiclass PPCs (54 molecules) in single insect boluses (<0.05 g dry mass) by ultra-high pressure liquid chromatography coupled to a tandem mass spectrometer (LC-MS/MS). A key part of this new method is the use of a two-step extraction method combining (i) the high efficiency of a microwave-assisted solvent extraction (MAE) for extracting analytes that might be tightly bound to environmental matrices and (ii) the versatility of a salt-out effect adapted from the QuEChERS methodology allowing the extraction and purification of a wide array of analytes. This microwave-assisted salt-out extraction (MASOE) approach was compared to classical extraction methods including matrix solid phase dispersion (MSPD), microwave-assisted extraction (MAE), and the QuEChERS method. Average recoveries for 54 analytes ranged from 49% to 106%, (relative standard deviations <22%). The limits of detection (LODs) and quantification (LOQs) were in the ranges of 0.10-3.00 ng g(-1) and 0.40-7.00 ng g(-1), respectively. We applied this method to analyse 881 insect boluses collected from Tree Swallow (Tachycineta bicolour) nestlings along an agricultural intensification gradient in southern Québec (Canada). We detected 25 PPCs out of the 54 considered. We detected at least one PPC in 30% of samples and were able to quantify at least one of them in 17% of samples. Our study shows that the MASOE method should prove to be a powerful tool for studying the fate and impacts of PPCs on wildlife.

Estimating dispersal among numerous sites using capture-recapture data.

Dispersal affects processes as diverse as habitat selection, population growth, and gene flow. Inference about dispersal and its variation is thus crucial for assessing population and evolutionary dynamics. Two approaches are generally used to estimate dispersal in free-ranging animals. First, multisite capture-recapture models estimate movement rates among sites while accounting for survival and detection probabilities. This approach, however, is limited in the number of sites that can be considered. Second, diffusion models estimate movements within discrete habitat using a diffusion coefficient, resulting in a continuous processing of space. However, this approach has been rarely used because of its mathematical and implementation complexity. Here, we develop a multi-event capture-recapture approach that circumvents the issue of too many sites while being relatively simple to be implemented in existing software. Moreover, this new approach allows the quantifying of memory effects, whereby the decision of dispersing or not on a given year impacts the survival or dispersal likelihood of the following year. We illustrate our approach using a long-term data set on the breeding ecology of a declining passerine in southern Quebec, Canada, the Tree Swallow (Tachycineta bicolor).

Balancing energy budget in a central-place forager: which habitat to select in a heterogeneous environment?

Foraging animals are influenced by the distribution of food resources and predation risk that both vary in space and time. These constraints likely shape trade-offs involving time, energy, nutrition, and predator avoidance leading to a sequence of locations visited by individuals. According to the marginal-value theorem (MVT), a central-place forager must either increase load size or energy content when foraging farther from their central place. Although such a decision rule has the potential to shape movement and habitat selection patterns, few studies have addressed the mechanisms underlying habitat use at the landscape scale. Our objective was therefore to determine how Ring-billed gulls (Larus delawarensis) select their foraging habitats while nesting in a colony located in a heterogeneous landscape. Based on locations obtained by fine-scale GPS tracking, we used resource selection functions (RSFs) and residence time analyses to identify habitats selected by gulls for foraging during the incubation and brood rearing periods. We then combined this information to gull survey data, feeding rates, stomach contents, and calorimetric analyses to assess potential trade-offs. Throughout the breeding season, gulls selected landfills and transhipment sites that provided higher mean energy intake than agricultural lands or riparian habitats. They used landfills located farther from the colony where no deterrence program had been implemented but avoided those located closer where deterrence measures took place. On the other hand, gulls selected intensively cultured lands located relatively close to the colony during incubation. The number of gulls was then greater in fields covered by bare soil and peaked during soil preparation and seed sowing, which greatly increase food availability. Breeding Ring-billed gulls thus select habitats according to both their foraging profitability and distance from their nest while accounting for predation risk. This supports the predictions of the MVT for central-place foraging over large spatial scales.

Severe recent decrease of adult body mass in a declining insectivorous bird population.

Migratory bird species that feed on air-borne insects are experiencing widespread regional declines, but these remain poorly understood. Agricultural intensification in the breeding range is often regarded as one of the main drivers of these declines. Here, we tested the hypothesis that body mass in breeding individuals should reflect habitat quality in an aerial insectivore, the tree swallow (Tachycineta bicolor), along a gradient of agricultural intensity. Our dataset was collected over 7 years (2005-2011) and included 2918 swallow captures and 1483 broods. Analyses revealed a substantial decline of the population over the course of the study (-19% occupancy rate), mirrored by decreasing body mass. This trend was especially severe in females, representing a total loss of 8% of their mass. Reproductive success was negatively influenced by intensive agriculture, but did not decrease over time. Interestingly, variation in body mass was independent of breeding habitat quality, leading us to suggest that this decline in body mass may result from carry-over effects from non-breeding areas and affect population dynamics through reduced survival. This work contributes to the growing body of evidence suggesting that declines in migratory aerial insectivores are driven by multiple, complex factors requiring better knowledge of year-round habitat use.