Abstract
Increasing global temperature has led to an interest in plasticity in the timing of annual events; however, little is known about the demographic consequences of changing phenology. Annual reproductive success varies significantly among individuals within a population, and some of that variation has to do with the number of broods attempted by reproducing adults. In birds, female age and the timing of reproduction are often predictors of multiple breeding. We hypothesize that double brooding rates may be affected by spring temperature and that the response may vary with female age. We used a long-term reproductive data set for a migratory songbird, the prothonotary warbler (Protonotaria citrea) to assess which factors influence (a) an individual female's probability of double brooding and (b) the annual variation in population-level double brooding rates. We found that older and earlier nesting birds are more likely to double brood, and that there is no evidence for senescence with regard to this trait such that the oldest females were most likely to double brood. Previous experience with double brooding (i.e., whether the female double brooded in the previous year) significantly increased the probability of doing so again. When assessing annual variation in the double brooding rate, we found an interaction between spring temperature and the proportion of older females in the population. Specifically, older females are more likely to double brood in years with warmer springs, but this relationship was not seen for younger females. Previous studies have shown that warmer temperatures lead to earlier and narrower peaks in resources and we hypothesize that these peaks are more available to older and earlier arriving females, enabling them to successfully raise more than one brood in a season. Understanding how different age classes respond to changing environmental conditions will be imperative to managing declining species.
Highlights
Increasing global temperature has led to an interest in the plasticity of the timing of annual events; little is known about the demographic causes or consequences of changing phenology (Forest and Miller-Rushing 2010)
Ecology and Evolution published by John Wiley & Sons Ltd
In recent years (2009–2012, N = 280 females), body condition calculated as the residuals from a regression of tarsus and mass does not correlate with age (P = 0.166), nest initiation date (P = 0.744), or the number of eggs laid in the first clutch (P = 0.337)
Summary
Increasing global temperature has led to an interest in the plasticity of the timing of annual events; little is known about the demographic causes or consequences of changing phenology (Forest and Miller-Rushing 2010). Studies on birds suggest that warming temperatures have led to a mismatch between the timing of reproduction and resources needed for reproduction (Visser et al 1998; Both et al 2006); studies that consider potential age-specific responses to climate warming are rare (Knudson et al 2011). Scientists have recently called for an increased consideration of life history when assessing variation in phenology in an effort to better understand the demographic effects of our changing global climate (Forest and Miller-Rushing 2010).
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