Warmer April Temperatures on Breeding Grounds Promote Earlier Nesting in a Long-Distance Migratory Bird, the Prothonotary Warbler

Jeffrey P Hoover,Wendy M Schelsky

doi:10.3389/fevo.2020.580725

Abstract

Global climate change and warming are altering hemispheric and local weather patterns. Altered weather patterns have great potential to affect the phenology of life history events, such as the initiation of breeding in organisms that reproduce seasonally. Long-distance migratory birds may be particularly challenged by changes in local weather on breeding grounds because they arrive from distant locations and must commence breeding when conditions are appropriate. Here we explore the effects of local temperature on first egg dates and annual productivity in a long-distance Neotropical migratory songbird, the prothonotary warblerProtonotaria citrea. We present results from a 20-year (1994 to 2013) study documenting the detailed nesting activities of a color-marked population (average of 155 individual females each year) of warblers in southern Illinois, United States. The warblers typically arrive in April and start breeding in late April and May in our study system. We tested for an effect of local average April daily temperature and female age on first egg dates, total number of offspring produced per female, and the probability of fledging two broods. We found that warmer April temperatures promoted earlier first egg dates and higher average annual productivity in the warblers. On average, older females had earlier first egg dates than 1-year-old females, but both age groups responded similarly to local April temperatures. The reproductive gains associated with earlier first egg dates in warmer years stemmed from an increased probability of successfully fledging two broods, suggesting that earlier first egg dates do not currently create a mismatch with food (insect) resources. Earliest arrival dates of warblers to the region of our study system were not affected by local April temperatures, suggesting that females vary their first egg date based on conditions they experience/assess after their arrival. Whereas these birds currently adjust the timing of their breeding and actually produce more offspring in warmer years, continued global warming may eventually upset the current balance between arrival dates, food resources, and the commencement of nesting.

Highlights

Natural selection favors timing of reproduction that maximizes fitness
During the period 1994 to 2013 we obtained 3,136 first egg dates, annual productivity values, and double-brooding statuses, from 2,017 individual female prothonotary warblers nesting in our study system
Across the spectrum of local April temperatures occurring during our study, mean first egg dates varied by approximately 9 days, from older females in years with warmest April temperatures to young females in years that were coldest (Figure 2)

Summary

Introduction

Natural selection favors timing of reproduction that maximizes fitness. Favorable environmental conditions, availability of mates, and pulses in necessary food resources are some of the main factors shaping where and when to reproduce (Perrins, 1970; Bronson, 1985; Daan and Tinbergen, 1997; Verhulst and Nilsson, 2008). Bird migrations cover thousands of kilometers, with many species moving between equatorial latitudes where they spend time on non-breeding grounds, and northern and southern temperate latitudes where they breed during hemispheric summers (Berthold, 2001; Newton, 2010). These temperate breeding seasons in migratory birds occur annually during pulses and peaks in food resources (often insects) that are enough to support both resident and migratory species as they attempt to reproduce (Alerstam, 1990; Greenberg and Marra, 2005; Thorup et al, 2017). Long-distance migrants are constrained in their ability to adjust their departure dates from non-breeding grounds based on what conditions are like on their breeding grounds (Both and Visser, 2001; Miller-Rushing et al, 2008; Fraser et al, 2013)

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Conclusion