Environmental drivers of female reproductive investment in egg quantity and quality in a grassland sparrow

Abstract

Female investment in reproduction is a critical component of life history that influences both the fitness of the female and her offspring. Oviparous females can allocate energy into egg size and/or clutch size, and this allocation can vary over lifetimes, within populations and among species. However, we know little about whether investment decisions shift within breeding seasons and the factors that influence female reproductive strategies. We studied endogenous and exogenous factors associated with variation in female reproductive investment by measuring 860 eggs from 240 clutches laid by Grasshopper Sparrows Ammodramus savannarum between 2014 and 2019 at the Konza Prairie Biological Station in Manhattan, Kansas, USA. As the breeding season progressed, females laid smaller clutches of heavier eggs, indicating a shift in investment towards fewer but higher quality young. Females that were heavier than expected given their body size laid heavier and larger eggs, but maternal body condition was not related to clutch size. Brood parasitism by Brown-headed Cowbirds Molothrus ater reduced the number of Grasshopper Sparrow eggs in nests, but was unrelated to host egg size or the total number of eggs that a female incubated, implying that sparrows do not adjust investment in response to parasitism risk. Pre-laying ambient temperature and precipitation were not associated with egg size, yet females tended to lay more eggs under rainier conditions. Overall, we provide evidence that both endogenous and exogenous factors shape female reproductive investment. Understanding the flexibility of reproductive strategies in response to multiple sources of selection, and how this mediates key trade-offs that influence demographic rates, can be used in forecasting population growth of bird species affected by anthropogenic changes to breeding areas.