Nestling Savannah Sparrows and Tree Swallows differ in their sensitivity to weather

AmericanOrnithology.org Volume 139, 2022, pp. 1–14 https://doi.org/10.1093/ornithology/ukac032 RESEARCH ARTICLE Nestling Savannah Sparrows and Tree Swallows differ in their sensitivity to weather Nathaniel T. Wheelwright,1,*, Corey R. Freeman-Gallant,2 and Robert A. Mauck3 Department of Biology, Bowdoin College, Brunswick, Maine, USA Department of Biology, Skidmore College, Saratoga Springs, New York, USA 3 Department of Biology, Kenyon College, Gambier, Ohio, USA * Corresponding author: 1 2 ABSTRACT Savannah Sparrows (Passerculus sandwichensis) and Tree Swallows (Tachycineta bicolor) breed and forage in the same habitat on Kent Island, a boreal island in the Bay of Fundy, New Brunswick, but respond differently to the same weather conditions. The 2 passerines are similar in body size but because Tree Swallows depend upon small flying insects captured on the wing, they may be more sensitive to weather than Savannah Sparrows, which forage on insects and seeds on the ground and in shrubs and trees. To compare how reproductive success in the 2 species was affected by weather conditions, we took advantage of an 18-year dataset and used a model-building approach that controlled for year, adult sex and age, and field where they nested. We focused on 3 measures of reproductive success (hatching success, fledging success, and nestling condition) and different time periods (3- to 18-day time windows) before hatching or fledging. The responses of the 2 species differed in magnitude and direction. In Tree Swallows, adding weather variables to the basic model increased the explanatory power of fixed effects by 19.1%, illustrating the swallows’ sensitivity to weather. In contrast, in Savannah Sparrows, the addition of weather variables only increased the model’s explanatory power by 0.4% and the proportion of variation attributed to fixed factors by only 1.5%, which reflected the species’ hardiness in the face of inclement weather. Our results suggest that how a bird species forages and the nature of its prey may influence its sensitivity to weather and indicate that increased rainfall, strong winds and other events associated with climate change may affect Tree Swallows and other aerial insectivores more than ground-foraging birds such as Savannah Sparrows. Keywords: foraging behavior, nestlings, Passerculus sandwichensis, reproductive success, Tachycineta bicolor, weather LAY SUMMARY • Nestling Savannah Sparrows and Tree Swallows, which occur in the same habitat on Kent Island, New Brunswick, Canada, were affected differently by the same weather conditions, likely because of differences in the way their parents forage for food. • Nestling Tree Swallows, whose parents depend on capturing small insects in flight, were highly sensitive to rain and wind, whereas weather had relatively little effect on nestling Savannah Sparrows, which forage mainly on the ground. • As a consequence, Tree Swallows and other aerial insectivores may be especially vulnerable to climate change. Passerculus sandwichensis y Tachycineta bicolor difieren en su sensibilidad al clima RESUMEN Passerculus sandwichensis y Tachycineta bicolor se reproducen y se alimentan en el mismo hábitat en la Isla Kent, una isla boreal en la Bahía de Fundy, New Brunswick, pero responden de manera diferente a las mismas condiciones climáticas. Los dos paseriformes son similares en tamaño corporal, pero debido a que T. bicolor depende de pequeños insectos voladores capturados en el vuelo, pueden ser más sensibles al clima que P. sandwichensis, que se alimentan de insectos y semillas en el suelo y en los árboles y arbustos. Para comparar cómo el éxito reproductivo en las dos especies se vio afectado por las condiciones climáticas, aprovechamos una base de datos de 18 años y utilizamos un enfoque de construcción de modelo que controlaba por año, sexo y edad de los adultos, y por el campo donde anidaban. Nos enfocamos en tres medidas de éxito reproductivo (éxito de eclosión, éxito de emplumamiento y condición del polluelo) y en diferentes períodos de tiempo (ventanas de tiempo de 3 a 18 días) antes de la eclosión o del emplumamiento. Las respuestas de las dos especies difirieron en magnitud y dirección. En T. bicolor, agregar variables climáticas al modelo Copyright © American Ornithological Society 2022. All rights reserved. For permissions, e-mail: . Submission Date: April 7, 2022; Editorial Acceptance Date: June 24, 2022; Published August 2, 2022 2 N. T. Wheelwright, C. R. Freeman-Gallant, and R. A. Mauck Sensitivity to weather in nestling sparrows and swallows básico aumentó el poder explicativo de los efectos fijos en un 19,1%, lo que ilustra la sensibilidad de las golondrinas al clima. En contraste, en P. sandwichensis, la adición de variables climáticas solo aumentó el poder explicativo del modelo en un 0,4% y la proporción de variación atribuida a factores fijos en solo un 1,5%, lo que refleja la resistencia de la especie frente a las inclemencias del clima. Nuestros resultados sugieren que la forma en que una especie de ave se alimenta y la naturaleza de sus presas pueden influir su sensibilidad al clima e indican que el aumento de las precipitaciones, los vientos fuertes y otros eventos asociados con el cambio climático pueden afectar a T. bicolor y a otros insectívoros aéreos más que a las aves que se alimentan en el suelo, como P. sandwichensis. Palabras clave: clima, comportamiento de forrajeo, éxito reproductivo, Passerculus sandwichensis, polluelos, Tachycineta bicolor INTRODUCTION Ornithology 139:1–14 © 2022 American Ornithological Society METHODS Study Site and Species Study site. We studied Savannah Sparrows and Tree Swallows from 1987 until 2005 on Kent Island, New Brunswick, Canada, a 100-ha island in the Bay of Fundy (because of changes in field methods after 2005, subsequent data were not directly comparable). The study site consists of 3 open fields (total = 10 ha), where both species Severe weather has long been known to reduce avian hatching and fledging success, nestling growth rate, and size at fledging (Bryant 1978, Webb 1987, Siikamaäki 1996). Cold temperatures, strong winds, and heavy precipitation can act directly on nestlings by elevating metabolic costs of thermoregulation or compromising immune function (Peréz et al. 2008, Ardia 2013). Such conditions can negatively affect nestlings indirectly by elevating parental foraging costs, reducing the availability of nestling food, extending the time between incubation shifts or nestling feedings, or compromising other components of parental care (Nooker et al. 2005, Low et al. 2008, Ardia et al. 2010, MacDonald et al. 2013, de Bruijn and Romero 2013, Winkler et al. 2013). Weather conditions and their effect on nest microclimate can have longer term carry-over consequences as well (Ardia et al. 2010, Öberg et al. 2014). For example, body size at fledging, which is influenced by weather conditions, can determine a bird’s body condit (...truncated)