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1. Visitation rate, but not foraging range, responds to brood size manipulation in an aerial insectivore

   https://doi.org/10.1007/s00265-022-03244-z  

   Madden, Sage A. ; McDermott, Molly T. ; Safran, Rebecca J. ( September 2022 , Behavioral Ecology and Sociobiology)

   Life history theory predicts that increased investment in current offspring decreases future fecundity or survival. Avian parental investment decisions have been studied either via brood size manipulation or direct manipulation of parental energetic costs (also known as handicapping). However, we have limited experimental data on the potential interactive effects of these manipulations on parent behavior. Additionally, we know little about how these manipulations affect spatial foraging behavior away from the nest. We simultaneously manipulated brood size and parental costs (via added weight in the form of a GPS tag) in wild female barn swallows (Hirundo rustica). We measured multiple aspects of parent behavior at and away from the nest while controlling for measures of weather conditions. We found no significant interactive effects of manipulated brood size and parental costs. Both sexes increased their visitation rate with brood size, but nestlings in enlarged broods grew significantly less post-brood size manipulation than those in reduced broods. Foraging range area was highly variable among GPS-tagged females but was unaffected by brood size. As such, increased visitation rate in response to brood size may be more energetically costly for far-ranging females. GPS-tagged females did not alter their visitation rate relative to un-tagged birds, but their mates had higher visitation rates. This suggests that GPS tagging may affect some unmeasured aspect of female behavior, such as prey delivery. Our findings indicate that investigation of foraging tactics alongside visitation rate is critical to understanding parental investment and the benefits and costs of reproduction.

   Avian parental investment decisions have been studied by either brood size manipulation or direct manipulation of parental costs, but rarely both simultaneously. We simultaneously manipulated brood size and parental costs (via addition of a GPS tag) in a wild avian system, allowing us to examine interactive effects of these manipulations. Additionally, studies of parental investment often examine behaviors at the nest, but measurements of parental care behavior away from the nest are rare. Our study is unique in that we measured multiple aspects of parental care, including spatial foraging behavior tracked with GPS tags. We found no interactive effects of manipulated brood size and parental costs on visitation rate or nestling growth, and spatial foraging behavior of females was individually variable. Documenting foraging tactics alongside visitation rate is critical to understanding parental investment because the same visitation rate might be more costly for far-ranging females.

    

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2. Proximate and evolutionary sources of variation in offspring energy expenditure in songbirds

   https://doi.org/10.1111/geb.13467  

   Mitchell, Adam E. ; Wolf, Blair O. ; Martin, Thomas E. ; Sheard, ed., Catherine ( February 2022 , Global Ecology and Biogeography)

   Understanding variation in offspring energy expenditure is important because energy is critical for growth and development. Weather may exert proximate effects on offspring energy expenditure, but in altricial species these might be masked by parental care and huddling with siblings. Such effects are particularly important to understand given changing global weather patterns, yet studies of wild offspring in the presence of parental care are lacking. Offspring energy expenditure may also vary among species due to evolved responses to environmental selection pressures, requiring studies at both proximate and ultimate levels.

   USA, South Africa, Malaysia.

   2016–2019.

   Songbirds.

   We used the doubly‐labelled water technique to estimate nestling daily energy expenditure of 54 songbird species across three continents. We used Bayesian phylogenetic mixed models to test proximate and evolutionary causes of variation in offspring energy expenditure while accounting for phylogeny and phylogenetic uncertainty.

   Offspring energy expenditure increased with more rainfall and colder air temperatures, but decreased among offspring in broods with more siblings. Across species, nestling and adult mortality, but not growth rate, were positively associated with offspring energy use.

   Weather had clear proximate effects on offspring energy expenditure and parents were either unable or unwilling to fully offset these effects. However, the decrease in offspring energy use when huddling with more siblings demonstrated a modulating effect of life history traits. For example, high nest predation rates favour reduced parental care and can force offspring to spend more energy coping with environmental conditions. Furthermore, reduced energy expenditure is thought to facilitate increased longevity, which is increasingly realized with lower extrinsic mortality rates, providing an explanation for the positive association between adult mortality and offspring energy expenditure. Ultimately, both proximate and evolutionary influences need to be considered to better understand causes of offspring energetics.

    

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3. No apparent trade-off between the quality of nest-grown feathers and time spent in the nest in an aerial insectivore, the Tree Swallow

   https://doi.org/10.1093/ornithology/ukac017  

   Taff, Conor C. ; Johnson, Brianna A. ; Anker, Allison T. ; Rodriguez, Alyssa M. ; Houtz, Jennifer L. ; Uehling, Jennifer J. ; Vitousek, Maren N. ( April 2022 , Ornithology)

   Life history theory provides a framework for understanding how trade-offs generate negative trait associations. Among nestling birds, time spent in the nest, risk of predation, and lifespan covary, but some associations are only found within species while others are only observed between species. A recent comparative study suggests that allocation trade-offs may be alleviated by disinvestment in ephemeral traits, such as nest-grown feathers, that are quickly replaced. However, direct resource allocation trade-offs cannot be inferred from interspecific trait associations without complementary intraspecific studies. Here, we asked whether there is evidence for a within-species allocation trade-off between feather quality and time spent in the nest in Tree Swallows (Tachycineta bicolor). Consistent with the idea that ephemeral traits are deprioritized, nest-grown feathers had lower barb density than adult feathers. However, despite substantial variation in fledging age among nestlings, there was no evidence for a negative association between time in the nest and feather quality. Furthermore, accounting for differences in resource availability by considering provisioning rate and a nest predation treatment did not reveal a trade-off that was masked by variation in resources. Our results are most consistent with the idea that the interspecific association between development and feather quality arises from adaptive specialization, rather than from a direct allocation trade-off.

    

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4. Effects of brood and group size on nestling provisioning and resource allocation in a communal bird

   https://doi.org/10.1093/beheco/arad020  

   Savagian, Amanda ; Riehl, Christina ; Komdeur, ed., Jan ( April 2023 , Behavioral Ecology)

   Resource limitations, either due to environmental conditions or constraints on parental provisioning effort, can drive intense competition among offspring. In communal groups, resource availability may increase if parents receive assistance from other group members; however, if those caregivers also produce young, offspring demand may increase at the same time. It is possible, therefore, that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. We tested the relationships between group size, brood size, and provisioning rates in the greater ani (Crotophaga major), a communally nesting cuckoo in which multiple breeding pairs and nonreproductive helpers cooperatively raise a shared brood. Crucially, brood and group size can vary independently in this species, allowing us to test changes in each variable separately. Using video footage of 2255 prey deliveries across 10 nests, we found that an increase in the number of adult caregivers within a group did not sufficiently offset a corresponding increase in the number of dependent young within a brood: prey availability per average nestling decreased with brood size, regardless of group size. In larger broods, last-hatched nestlings received significantly less prey than their broodmates, in part due to greater hatching asynchrony that exacerbated competitive asymmetries and facilitated inequality in food allocation. Our results indicate that last-hatched ani nestlings suffer a “double cost” in large broods: they must compete with more nestmates, and suffer disproportionately from asynchronous hatching. These costs may contribute to increased parent–offspring conflict and may constrain group size in communal breeders.

    

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5. Data and code from "No evidence of sex ratio manipulation by black-throated blue warblers in response to food availability" Kaiser et al. 2023 Behavioral Ecology and Sociobiology

   https://doi.org/10.6073/pasta/37bcca63dea73a0b1386c7a134dedc6b  

   Kaiser, Sara A ; Grabenstein, Kathryn C ; Sillett, T Scott ; Webster, Michael S ( January 2023 , Environmental Data Initiative)

   This dataset is published in support of "No evidence of sex ratio manipulation by black-throated blue warblers in response to food availability" by Kaiser et al. 2023 in Behavioral Ecology and Sociobiology. Data and code to test the assumptions and key predictions of the Trivers-Willard hypothesis, which proposes that females produce more sons or daughters depending on food availability, in the black-throated blue warbler at the Hubbard Brook Experimental Forest, NH, 2007-2012. Datasets support analyses of sex ratio bias at both the nest and nestling levels. Data tables support the comparison of the ratio of variances in the scaled pre-fledging mass of male and female nestlings using an F test and reproduction of Figures 2a and 2b. Figures are those used in the published manuscript. Code supports the calculation of offspring sex ratio bias at the population level, and considering separately both low- and high-quality habitats, using the Neuhäuser test, statistical models testing the assumptions of the Trivers-Willard hypothesis, effects of food availability and parental provisioning on offspring sex ratio, and effects of food availability on pre-fledging nestling mass of sons and daughters, and a power analysis to determine the power to detect an effect of food supplementation on sex ratio. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the US Forest Service, Northern Research Station. 

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