Large-scale source-sink dynamics in Tachycineta bicolor breeding in nest-boxes across eastern Canada's farmlands

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.

We manipulated brood sizes of Tree Swallows (Tachycineta bicolor) in 1996 and 1997 to test for the existence of intra- and intergenerational costs of reproduction. Modal clutch size was six eggs, but experimental brood sizes ranged from two to nine young. Nestling starvation was higher in 1996 (and dependent on brood size) than in 1997, but in both years enlargement of brood size resulted in increased productivity. Nestling mass near fledging was negatively correlated with brood size, but tarsus length and wing chord were not. Food deliveries by parents increased steadily between broods of two to six young but then remained constant between broods of six to nine young. The loss of female mass between incubation and the end of the nestling period was positively related to the pair's total feeding effort, and female mass near fledging declined with increasing brood size. The latter decline disappeared, however, when broods of nine were omitted. Adult return rate (1996 to 1997) was highest among birds that raised enlarged broods. Our results, and a review of other studies of Tree Swallows, suggest that broods of seven or eight young can be raised without costs to the parents or young, and it appears that costs associated with feeding young have not influenced annual fecundity of Tree Swallows. Rather, egg production is most likely limited by energy availability to laying females. A major cost of reproduction for Tree Swallows probably arises from nest-site competition in that early arrival in spring to obtain nest sites exposes adults to high risks of death from starvation.

The relative importance of predators and resources (i.e. food) for the dynamics of migratory bird populations is poorly known. Resource availability may be more likely in resource poor environments, but given that nest failure in most systems is due mainly to predation, predator effects may predominate. We document a rapid decline of an isolated eastern kingbird Tyrannus tyrannus population breeding in the Great Basin Desert of eastern Oregon, USA, and evaluate whether it was driven by limited food resources (water availability ~ food), nest predation, or first‐year or adult return rate (RR J and RR A , respectively) that reflect nonbreeding season events. Most nests failed (~68% of nests) due mainly to nest predation (> 90% of failures); nestling starvation was rare. Bioyear precipitation (October–April), breeding season precipitation, and river flow all varied widely but none could account for annual variation in either nest success (NS) or fledging success of successful nest (FSSN). Neither RR J nor RR A varied with year, any measure of water resources, or reproductive success. Annual population growth rate (ln[N t+1 /N t ]) was independent of year, all measures of water availability, RR J and RR A , and NS of replacement nests, but was high following years of high NS of initial nests; FSSN was also higher in years of high NS. High rates of nest predation, due likely to the expansion of the local American crow Corvus brachyrhynchos population, appeared to drive the population decline. Empirical data and population simulations indicated that replacement nests contributed little to λ, while simulations revealed that, given site‐specific vital rates, the decline would have been even greater without immigration (λ = 0.812) than the observed (λ = 0.914). Long distance dispersal may thus be critical for persistence of kingbirds in the naturally highly fragmented habitat that it occupies in the Great Basin Desert.

Several studies concerned with mate choice, reproductive performance, and life history strategies have been conducted with secondary cavity nesting birds breeding in nest boxes. Although the need for comparative studies has been recognized, populations breeding in nest boxes often have not been compared with those breeding in natural cavities. We compared the ecology of Tree Swallows breeding in nest boxes and natural cavities to determine if nest box populations of Tree Swallows are accurate models of natural populations. Two nest site characteristics, nest site dispersion and cavity height, were similar for birds in both nesting environments. Greater cavity entrance area at natural cavities resulted in increased interspecific competition in natural populations, involving larger competitors, more species, and a greater abundance of each species. Clutch size was smaller in natural cavities compared with nest boxes, likely because floor area was smaller in natural cavities. Fledging success did not differ between populations. Disproportionately more after-second-year females bred in nest boxes, and more second-year females bred in natural cavities, as estimated by a model of Tree Swallow survivorship. Tree Swallows settle at nest boxes before natural cavities in our study area, perhaps as a result of the greater potential for reproductive success and reduced interspecific competition in the nest boxes as opposed to natural cavities. For some aspects of the ecology of secondary cavity nesters, nest boxes do not provide an accurate representation of natural populations. Therefore, evolutionary interpretations of nest box studies should be compared with observations of birds in natural environments.

Latitudinal differences in life histories are believed to be underlain by dif- ferences in trade-offs between current and future reproduction. I report differences in trade- offs between parent and offspring across the range of a widespread avian species, the Tree Swallow (Tachycineta bicolor). I manipulated parental effort and found that in Alaska, where yearly adult return rates are low, breeding females increase their reproductive in- vestment to maintain offspring quality, whereas in Tennessee, where yearly adult return rates are high, breeding females are either unable or unwilling to increase reproductive investment and, consequently, raise offspring of lower quality. I further investigated a critical mechanism of self-maintenance that may underlie differences in survival among sites: immunocompetence. Females breeding in Alaska mounted weaker immune responses when raising enlarged broods, whereas females in Tennessee did not, further suggesting a strategy in Alaskan females to incur costs (i.e., reduce future reproduction) in order to maintain offspring quality. Insect availability increased as the breeding season progressed in both sites, but more food was available in Alaska than in Tennessee. This is among the first studies to report geographic differences in immune function consistent with life history theory, which predicts that individuals with higher survival probabilities should invest more in self-maintenance.

Assessing the role of local populations in a landscape context has become increasingly important in the fields of conservation biology and ecology. A growing number of studies attempt to determine the source-sink status of local populations. As the source-sink concept is commonly used for management decisions in nature conservation, accurate assessment approaches are crucial. Based on a systematic literature review of studies published between 2002 and 2013, we evaluated a priori predictions on methodological and biological factors that may influence the occurrence of source or sink populations. The review yielded 90 assessments from 73 publications that included qualitative and quantitative evidence for either source or sink population(s) for one or multiple species. Overall, sink populations tended to occur more often than source populations. Moreover, the occurrence of source or sink populations differed among taxonomic classes. Sinks were more often found than sources in mammals, while there was a non-significant trend for the opposite to be true for amphibians. Univariate and multivariate analyses showed that the occurrence of sources was positively related to connectivity of local populations. Our review furthermore highlights that more than 25 years after Pulliam's widely cited publication on 'sources, sinks, and population regulation', in-depth assessments of the source-sink status of populations based on combined consideration of demographic parameters such as fecundity, survival, emigration and immigration are still scarce. To increase our understanding of source-sink systems from ecological, evolutionary and conservation-related perspectives, we recommend that forthcoming studies on source-sink dynamics should pay more attention to the study design (i.e. connectivity of study populations) and that the assessment of the source-sink status of local populations is based on λ values calculated from demographic rates.

The number of visits parents make to their nest during chick rearing is a commonly used measure of the amount of food delivered to nestlings and an index of the quality of parental care. Use of the number of visits for these purposes assumes that parents feed nestlings when they visit and that there are no systematic differences in the amount of food delivered on each visit. These assumptions were tested in Tree Swallows (Tach- ycineta bicolor) breeding in nest boxes near Ithaca, New York. Video observations of parents inside their nest boxes showed that parents feed nestlings on 95-98% of visits to the nest. An average visit delivered 18.1 insects with a total dry mass of 24.1 mg, usually to a single nestling. Although females visited more frequently, the load they carried on each visit was not different than that carried by males. Load size did not differ with nestling age, brood size or the date of the feeding. Overall, there were few systematic differences in load size among nests. Therefore, the number of visits to the nest is a good measure of food delivery and parental care in Tree Swallows.

Nest predation is the primary cause of nest failure in passerines. In order to contribute to our understanding of how nest predators shape avian nesting ecology and life history traits, we report nest predator identity and nest predation rates for 3 species of passerines in the Central Andes of south temperate Argentina. We used video cameras and opportunistic observations with photographic documentation to identify nest predators of Grass Wrens (Cistothorus platensis) breeding in a riparian grassland, as well as House Wrens (Troglodytes aedon) and House Sparrows (Passer domesticus) breeding in nest boxes in a tree plantation. From 13 nest predation events we were able to identify 3 nest predator species: mousehole snake (Philodryas trilineata), South American gray fox (Lycalopex griseus), and American Kestrel (Falco sparverius). Field observations also suggested fire ants (genera Solenopsis) as a possible nest predator. Mousehole snakes were identified at both grassland and forest plantation, representing 76.9% of the identified predation events. House Sparrows had the highest nest predation rate (43.1%), followed by Grass Wrens (30.8%) and House Wrens (29.3%). Egg predation was more frequent for House Wrens (64.2%) and House Sparrows (50.0%) than for Grass Wrens (22.1%). In contrast, nestling predation was considerably higher for Grass Wrens (77.9%) than for House Wrens and House Sparrows (34.7% and 50.0%, respectively). Knowledge of the nest predator community and information of nest predation rates of different species in temperate South America will contribute to understand nest predation effects on patterns and processes of nesting success, life history traits, and future management decisions in this region.

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.

Many songbirds are under increasing pressure owing to habitat loss, land-use changes, and rapidly changing climatic conditions. Using citizen science data collected from 1980 to 2014, we asked how local weather and regional climate influenced the breeding dynamics of Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)) and Tree Swallows (Tachycineta bicolor (Vieillot, 1808)). Mountain Bluebird reproduction was strongly associated with local weather: number of nestlings and fledglings both decreased in years of high rainfall. Clutch size and number of fledglings also declined over the study period. Abundance of Mountain Bluebirds was higher in years of lower early-season snowfall and warmer local temperatures, as well as more negative Southern Oscillation Index (SOI) values, indicating a positive influence of El Niño conditions. Tree Swallow reproduction (clutch size, number of nestlings, and number of fledglings) was negatively associated with SOI values, and the number of Tree Swallow nestlings decreased in years of higher rainfall and warmer temperatures. Tree Swallows also showed a marked decline in abundance over the period of the study, consistent with recent range-wide declines. Together, our results demonstrate that local weather and regional climate differentially affect the reproductive dynamics of Mountain Bluebirds and Tree Swallows and highlight the importance of long-term citizen science data sets.

Population increase during the mid-twentieth century as mortality declines in many developing nations were not matched with reductions in fertility, resulting in unprecedented growth rates. Concern with environmental change has come to the forefront primarily since 1970, with discernible levels of environmental degradation fueling public concern with the scope of contemporary environmental transformations and the advent of satellite imagery aiding environmental research. Rapid population growth and global environmental change are two issues that have received important public attention over the past several decades. It has become increasingly clear that human populations have a powerful effect on the environment and become a global public policy issue. Yet the exact relationship between population dynamics and the environment is complex. This paper reviews the environmental significances of population dynamics, based on current knowledge of the relationships between population factors and various aspects of the natural environment. This research is important for several reasons. • Compared with human-induced environmental changes of centuries past, the geographic scope of contemporary human-induced change is much larger. The rate of change is also much faster. The Earth's surface air temperature has increased by between 0.3 degrees and 0.6 degrees. Scientists have discovered that human activities as land-use change and fossil fuel use appear linked o increased concentrations of several atmospheric gases which warm the Earth's atmosphere and surface temperature.' As a result, humans appear partially responsible for environmental changes at the global scale-large-scale changes that have taken place in less than 200 years. • Some human-induced environmental changes have irreversible consequences. Ii is estimated that nearly 12 percent of mammals and 11 percent of bird species are currently threatened with extinction. In fact, the currently confronted episode of biodiversity loss is greater than the orld has experienced for the past 65 million years. Although some human activities, such as conservation programs, aim to preserve or enhance biodiversity, human-induced habitat destruction remains the primary cause of species decline. • The effects of contemporary environmental changes on humans are also increasing in scope. Global estimates suggest there may be as many as 25 million ”environmental refugees” -individuals who have migrated because they can no longer secure a livelihood from the land because of deforestation, desertification, soil erosion, and other environmental problems. • Human population continues to grow. In the face of rapid, large-scale, human-induced environmental change, human numbers increase each year by approximately 80 million. Those areas with continuing high fertility and resulting population growth are typically those with the least environmental resilience and those facing the greatest resource constraints. Although Earth already faces severe air pollution, water contamination, and other environmental ills, more than 90 percent of future population growth is projected to take place in these areas. This paper discusses the relationship between population and natural environmental change, the forces that mediate this relationship, and how population dynamics specifically affect climate change and land-use change and like to outline both sides of the policy debate that have emerged-those who would limit fertility and population growth in the developing world and those who need reduced consumption in the developed world. I also like to call for policymakers to pay increased attention to the role of population and forsake the search for one-step solutions to environmental problems.

Host-parasite interactions increasingly are influenced by human-induced rapid environmental change (HIREC), and the fitness effects of parasitism may be compounded or exacerbated by host traits and/or exposure to additional extrinsic stressors associated with HIREC. Potential interactions between parasitism and different stressors associated with environmental change, however, remain poorly understood for most systems. We examined how parasitism by bird blow flies (Trypocalliphora braueri), ambient weather conditions and habitat disturbance jointly affected offspring traits and juvenile mortality for two declining species of sagebrush songbirds (Brewer's Sparrow, Spizella breweri; and Sage Thrasher, Oreoscoptes montanus) in Wyoming, USA. We evaluated two alternative hypotheses: that parasitism could act (i) in an independent and additive manner with temperature and habitat alteration (Multiple Stressors Hypothesis) or (ii) synergistically to exacerbate the effects of temperature and habitat alteration (Parasitism-HIREC Interaction Hypothesis) on offspring traits and juvenile mortality. We assessed morphometric traits of nestlings and survival of fledglings in relation to parasite loads, temperature and habitat disturbance associated with natural gas development to test these hypotheses. Higher parasite loads and colder temperatures were associated with different effects for nestlings of each host species, reducing tarsus and wing chord length for Brewer's Sparrow and increasing mass for Sage Thrasher. Despite differences in the effect of parasitism on nestling traits, post-fledging mortality risk for both species increased with higher parasite loads. The effects of parasitism and temperature mainly were additive, with limited evidence that weather exacerbated the effects of parasitism. Habitat disturbance had a weak positive effect on nestling tarsus length and post-fledging survival probability for Brewer's Sparrow. Although parasitism rarely results in direct mortality of hosts, parasites can nonetheless exert considerable fitness consequences, especially when combined with extrinsic stressors associated with human-induced environmental changes.

We manipulated brood sizes of Tree Swallows (Tachycineta bicolor) in 1996 and 1997 to test for the existence of intra- and intergenerational costs of reproduction. Modal clutch size was six eggs, but experimental brood sizes ranged from two to nine young. Nestling starvation was higher in 1996 (and dependent on brood size) than in 1997, but in both years enlargement of brood size resulted in increased productivity. Nestling mass near fledging was negatively correlated with brood size, but tarsus length and wing chord were not. Food deliveries by parents increased steadily between broods of two to six young but then remained constant between broods of six to nine young. The loss of female mass between incubation and the end of the nestling period was positively related to the pair's total feeding effort, and female mass near fledging declined with increasing brood size. The latter decline disappeared, however, when broods of nine were omitted. Adult return rate (1996 to 1997) was highest among bir...

Birds breeding in agricultural landscapes contend with potential reductions in prey availability that may alter behavior and incur physiological costs. Individuals may need to increase foraging intensity, producing elevated reactive oxygen species involved in oxidative damage. Food scarcity or low food quality may also reduce levels of diet‐derived antioxidants. Here, we tested whether short‐term changes in insect biomass on agricultural sites produce changes in the oxidative status of tree swallows (Tachycineta bicolor), with the potential to influence return rates. We sampled 368 adult and 374 nestling swallows at five agricultural cropland and grassland‐dominated sites over three years from 2012 to 2014. Blood plasma was assessed for antioxidant capacity (OXY) and reactive oxygen metabolites (ROMs), a marker for oxidative damage. Overall, males appeared to have higher oxidative damage and therefore oxidative stress (calculated as the ratio between ROMs and OXY) than females. Consistent with our hypothesis, aerial insect biomass was a strong predictor of oxidative status in swallows. Adult plasma antioxidants were higher with greater insect biomass, while nestling plasma antioxidants, oxidative damage, and oxidative stress exhibited negative relationships with insect biomass. Annual return rates of adults were predicted by site type, age, fledging success, and oxidative status. In 2013, adult return rates were higher at grassland sites (41%) than at cropland sites (25%) and birds with lower plasma antioxidants the previous year were more likely to return. In 2014, adults were more likely to return if they had fledged more nestlings the previous season and if they had lower levels of oxidative damage the previous year. We conclude that even subtle changes in insect prey availability in agricultural landscapes caused by pesticides, intense cropping, and natural habitat loss can play a key role in swallow oxidative status, with subsequent effects on local return rates.

Secondary cavity-nesting birds depend on tree cavities for nesting and roosting, but many studies of these birds are conducted using nest boxes. Implementation of effective conservation strategies for cavity-nesting species such as nest-site supplementation requires careful comparisons of fecundity and other vital rates for birds using both natural and artificial nest site types. We compared breeding phenology, clutch and brood sizes, and fledging success of Tree Swallows (Tachycineta bicolor) nesting in tree cavities and nest boxes during 2001–2003 in British Columbia, Canada. Swallows using nest boxes initiated egg-laying and hatched young at approximately the same time as those in tree cavities (2 June, 23 June, respectively). Female Tree Swallows in boxes laid larger clutches (5.9 ± 0.9 eggs, N = 76) than those in tree cavities (4.2 ± 1.6 eggs, N = 67). The mean number of nestlings hatched was greater in nest boxes (5.2 ± 1.1 nestlings, N = 67) than in tree cavities (2.6 ± 2.0 nestlings, N = 58). Pairs in boxes were over twice as successful in producing fledglings (93.4%; 57 of 61 pairs fledged > 1 young) than those in tree cavities (35.8%; 19 of 53 pairs). Of those successful nests, pairs nesting in boxes fledged 5.1 ± 1.1 young (N = 57), whereas those in tree cavities fledged 3.5 ± 1.2 young (N = 18). Because cavities in nest boxes averaged 60% larger in volume and 1.8 cm wider internally than tree cavities, we suggest that increased reproductive output was correlated with boxes enabling a larger clutch size. In previous research, we found that Tree Swallows were a poor competitor with other cavity-nesting passerines for tree cavities. The addition of nest boxes may serve as an effective way to supplement local reproduction for secondary cavity-nesting bird populations by reducing competition for limited nest sites. This is especially true in regions where the availability of natural nesting sites is highly variable, and where species compete with many other cavity-nesting passerines using a similar ecological niche and nesting cavities.