The effect of insect food availability on songbird reproductive success and chick body condition: Evidence from a systematic review and meta-analysis

Differences in urban arthropod communities may not limit the nestling diet of a generalist songbird

Anthropogenic land use, including urbanization, has caused population declines across diverse taxa including arthropods and songbirds. Declines in one taxa can impact other groups based on its role in a community. In particular, declines in lower trophic level taxa, such as arthropods, could have negative impacts on higher trophic level species. Here, we examined how urban arthropod communities compare to rural ones and how these differences may impact song sparrow (Melospiza melodia) nestlings living in urban and rural habitats. We quantified differences in the abundance, biomass, and diversity of arthropod communities between replicate urban and rural sites. At the same sites, we also compared the stomach contents of nestlings because song sparrows rely upon arthropod prey during development. We found that the arthropod community in urban habitats had lower average abundance, average biomass, and Simpson's diversity compared to rural habitats. The arthropod communities also significantly differed in the relative abundance of some higher trophic level taxa, such as spiders. However, we found no difference in the total stomach content mass, nor the mass of invertebrate food items in the stomachs of urban and rural nestlings. Thus, though urban habitats had lower availability of arthropods, possibly driven by the simplification of urban habitats, there was no evidence of a negative impact on the quantity of food provided to urban song sparrow nestlings.

Differences in phenology across three trophic levels between two Afrotropical sites separated by four degrees latitude

Birds time their life cycle events to favourable windows in environmental conditions. In tropical environments, where photoperiod variation is small, birds show high variability in the timing of life cycle stages, yet these species have been severely underrepresented in phenology research. Here, we investigated temporal patterns in bird life cycles and resource availability in two sites in tropical Africa: Weppa (Nigeria, 7° N) and Elat (Cameroon, 3° N). In these sites we captured common bulbuls (Pycnonotus barbatus), a widespread generalist, and recorded breeding and moult over a 12-month period. Simultaneously, we surveyed fruiting tree and arthropod abundance. Our aim was to quantify seasonal patterns in moult and breeding in bulbuls at both sites, and link them to fluctuations in local fruit and arthropod abundance and precipitation. Moult was more seasonal than breeding in both sites, and seasonality of both life cycle events was stronger in Nigeria than Cameroon. The peak timing for moult was 1.5 months earlier in Nigeria than Cameroon. Seasonal variation in abundance of fruiting trees and arthropods was different between sites, as were the associations with breeding and moulting. In Nigeria, we found a positive association between moult and arthropod abundance, and a negative one with fruiting tree abundance. In contrast, in Cameroon moult was associated with higher precipitation, while breeding occurred at times with higher fruit abundance. Our results provide evidence that, even in similar habitats separated by four degrees in latitude, seasonal patterns across three trophic levels are variable. Understanding links between environmental conditions and life cycle events can reveal potential vulnerabilities of tropical species, and guide conservation efforts.

Individual quality and environmental factors interact to shape reproduction and survival in a resident bird of prey

Investigating among-individual differences in reproductive success and survival is essential for understanding eco-evolutionary processes. We used 5 years of demographic data from 556 breeding barn owls (Tyto alba) to estimate associations between intrinsic and extrinsic covariates on survival and reproduction throughout the annual cycle. As males and females have distinct roles in reproduction, environmental conditions and individual quality may be differentially linked to their fitness at different time points. Males breeding early and inhabiting prey-rich areas experienced higher reproductive success but faced greater reproductive costs. Indeed, the number of offspring a male cared for was negatively associated with his body condition and survival. However, our results indicate that these influences can be mitigated in males experiencing favourable post-breeding environmental conditions. For female owls, early breeding and high food availability during the breeding period were linked with increased reproductive success. Prey availability during incubation and higher reproductive output were associated with higher survival into the next breeding period in females. Unlike males, females did not exhibit obvious trade-offs between reproductive success and survival. Our research demonstrates trade-offs between fecundity and survival, and that females paired with males able to provide sufficient food experience higher survival and reproduction.

The impact of urbanization on health depends on the health metric, life stage and level of urbanization: a global meta-analysis on avian species

Stressors associated with urban habitats have been linked to poor wildlife health but whether a general negative relationship between urbanization and animal health can be affirmed is unclear. We conducted a meta-analysis of avian literature to test whether health biomarkers differed on average between urban and non-urban environments, and whether there are systematic differences across species, biomarkers, life stages and species traits. Our dataset included 644 effect sizes derived from 112 articles published between 1989 and 2022, on 51 bird species. First, we showed that there was no clear impact of urbanization on health when we categorized the sampling locations as urban or non-urban. However, we did find a small negative effect of urbanization on health when this dichotomous variable was replaced by a quantitative variable representing the degree of urbanization at each location. Second, we showed that the effect of urbanization on avian health was dependent on the type of health biomarker measured as well as the individual life stage, with young individuals being more negatively affected. Our comprehensive analysis calls for future studies to disentangle specific urban-related drivers of health that might be obscured in categorical urban versus non-urban comparisons.

Food availability aligns with contrasting demographics in populations of an at-risk songbird

Golden-winged Warblers (Vermivora chrysoptera) have become rare across much of their historic breeding range and response to conservation efforts is variable. Evidence from several recent studies suggests that breeding output is a primary driver explaining responses to conservation and it is hypothesized that differences in food availability may be driving breeding output disparity between two subpopulations of the warbler's Appalachian breeding range. Herein, we studied two subpopulations: central Pennsylvania ("central subpopulation"), where breeding productivity is relatively low, and eastern Pennsylvania ("eastern subpopulation"), where breeding productivity is relatively high. To test the food-availability hypothesis in this system, we measured density of caterpillars, plasma lipid metabolites (triglycerides [TRIG; fat deposition] and glycerol [GLYC; fat breakdown]), body mass of adults males, and acquired body mass data for fledglings at 38 sites managed for nesting habitat. Consistent with our prediction, leaf-roller caterpillar density, the group upon which Golden-winged Warblers specialize, was 45× lower in the central subpopulation than the eastern subpopulation. TRIG concentrations were highest within the eastern subpopulation during breeding grounds arrival. The change in TRIG concentrations from the breeding-grounds-arrival stage to the nestling-rearing stage was subpopulation dependent: TRIG decreased in the eastern subpopulation and was constant in the central subpopulation, resulting in similar concentrations during the nestling-rearing stage. Furthermore, GLYC concentrations were higher in the eastern subpopulation, which suggests greater energy demands in this region. Despite this, adult male warblers in the eastern subpopulation maintained a higher average body mass. Finally, fledgling body mass was 16% greater in the eastern subpopulation than the central subpopulation before and after fledging. Collectively, our results suggest that poor breeding success of Golden-winged Warblers in the central subpopulation could be driven by lower availability of primary prey during the breeding season (leaf-roller caterpillars), and this, in turn, limits their response to conservation efforts.

Insecticides, more than herbicides, land use, and climate, are associated with declines in butterfly species richness and abundance in the American Midwest

Mounting evidence shows overall insect abundances are in decline globally. Habitat loss, climate change, and pesticides have all been implicated, but their relative effects have never been evaluated in a comprehensive large-scale study. We harmonized 17 years of land use, climate, multiple classes of pesticides, and butterfly survey data across 81 counties in five states in the US Midwest. We find community-wide declines in total butterfly abundance and species richness to be most strongly associated with insecticides in general, and for butterfly species richness the use of neonicotinoid-treated seeds in particular. This included the abundance of the migratory monarch (Danaus plexippus), whose decline is the focus of intensive debate and public concern. Insect declines cannot be understood without comprehensive data on all putative drivers, and the 2015 cessation of neonicotinoid data releases in the US will impede future research.

Comparison of Reproductive Strategies between Two Sympatric Copsychus Passerines

Reproduction plays a crucial role in determining the development, fate, and dynamics of bird populations. However, reproductive strategies vary among species and populations. In this study, we investigated the reproductive strategies of the Oriental Magpie Robin (Copsychus saularis) and White-rumped Shama (C. malabarica), which are closely related passerines that reproduce in sympatric areas. We found that although these two species were both cavity nesting, their nest-site selection differed; the Shama preferred nesting close to trees and forests, whereas the Magpie Robin nested close to human residential areas. Furthermore, their egg incubation patterns differed; the Shama increased daily incubation frequency with incubation time, but the Magpie Robin maintained its daily incubation time regardless of changes in incubation frequency. However, the nestling heating patterns of these two species were similar, indicating a critical demand for regulating hatchling body temperature during this crucial stage. The feeding frequencies of male parents were strongly correlated with those of females in both species, suggesting equal contribution and good synchronization between the sexes. Nestling feeding frequency was also correlated with nest cleaning frequency, implying coordination between feeding and defecation by parents and offspring, respectively. This research explored the divergence and convergence of reproductive strategies between these two sympatric species, providing valuable insights into the niche differentiation theory.

Rising minimum temperatures contribute to 50 years of occupancy decline among cold-adapted Arctic and boreal butterflies in North America

Global climate change has been identified as a potential driver of observed insect declines, yet in many regions, there are critical data gaps that make it difficult to assess how communities are responding to climate change. Poleward regions are of particular interest because warming is most rapid while biodiversity data are most sparse. Building on recent advances in occupancy modeling of presence-only data, we reconstructed 50 years (1970-2019) of butterfly occupancy trends in response to rising minimum temperatures in one of the most under-sampled regions of North America. Among 90 modeled species, we found that cold-adapted species are far more often in decline compared with their warm-adapted, more southernly distributed counterparts. Furthermore, in a post hoc analysis using species' traits, we find that species' range-wide average annual temperature is the only consistent predictor of occupancy changes. Species with warmer ranges were most likely to be increasing in occupancy. This trend results in the majority of butterflies increasing in occupancy probability over the last 50 years. Our results provide the first look at macroscale butterfly biodiversity shifts in high-latitude North America. These results highlight the potential of leveraging the wealth of presence-only data, the most abundant source of biodiversity data, for inferring changes in species distributions.

Population links between an insectivorous bird and moths disentangled through national-scale monitoring data

Insects are key components of food chains, and monitoring data provides new opportunities to identify trophic relationships at broad spatial and temporal scales. Here, combining two monitoring datasets from Great Britain, we reveal how the population dynamics of the blue tit Cyanistes caeruleus are influenced by the abundance of moths - a core component of their breeding diet. We find that years with increased population growth for blue tits correlate strongly with high moth abundance, but population growth in moths and birds is less well correlated; suggesting moth abundance directly affects bird population change. Next, we identify moths that are important components of blue tit diet, recovering associations to species previously identified as key food sources such as the winter moth Operoptera brumata. Our work provides new evidence that insect abundance impacts bird population dynamics in natural communities and provides insight into spatial diet turnover at a national-scale.

Inconsistent shifts in warming and temperature variability are linked to reduced avian fitness

As the climate has warmed, many birds have advanced their breeding timing. However, as climate change also changes temperature distributions, breeding earlier might increase nestling exposure to either extreme heat or cold. Here, we combine >300,000 breeding records from 24 North American birds with historical temperature data to understand how exposure to extreme temperatures has changed. Average spring temperature increased since 1950 but change in timing of extremes was inconsistent in direction and magnitude; thus, populations could not track both average and extreme temperatures. Relative fitness was reduced following heatwaves and cold snaps in 11 and 16 of 24 species, respectively. Latitudinal variation in sensitivity in three widespread species suggests that vulnerability to extremes at range limits may contribute to range shifts. Our results add to evidence demonstrating that understanding individual sensitivity and its links to population level processes is critical for predicting vulnerability to changing climates.