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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.

Phenotypic plasticity allows organisms to adjust the timing of life‐history events in response to environmental and demographic conditions. Shifts by individuals in the timing of breeding with respect to variation in age and temperature are well documented in nature, and these changes are known to scale to affect population dynamics. However, relatively little is known about how organisms alter phenology in response to other demographic and environmental factors. We investigated how pre‐breeding temperature, breeding population density, age, and rainfall in the first month of life influenced the timing and plasticity of lay date in a population of Savannah Sparrows (Passerculus sandwichensis) monitored over 33 yr (1987–2019). Females that experienced warmer pre‐breeding temperatures tended to lay eggs earlier, as did older females, but breeding population density had no effect on lay date. Natal precipitation interacted with age to influence lay date plasticity, with females that experienced high precipitation levels as nestlings advancing lay dates more strongly over the course of their lives. We also found evidence for varied pace of life; females that experienced high natal precipitation had shorter lifespans and reduced fecundity, but more nesting attempts over their lifetimes. Rainfall during the nestling period increased through time, while population density and fecundity declined, suggesting that increased precipitation on the breeding grounds may be detrimental to breeding females and ultimately the viability of the population as a whole. Our results suggest that females adjust their laying date in response to pre‐breeding temperature, and as they age, while presenting new evidence that environmental conditions during the natal period can affect phenological plasticity and generate downstream, population‐level effects.