Abstract
How environmental variation affects the strength of selection is important for understanding phenotypic variation within a population. An indirect method to evaluate differences in selection pressures is to compare the age composition of a breeding population. A larger adult/first-year breeding ratio, due to higher juvenile mortality, may indicate stronger selection. We investigated the adult/first-year breeding ratios of food-caching mountain chickadees inhabiting different elevations. Chickadees at high elevations experience harsher winter conditions and cache more food items and have better spatial cognition to recover caches compared with their low-elevation counterparts. Cache recovery and spatial cognition are critical for overwinter survival and individuals with better spatial cognitive abilities are more likely to survive their first winter; therefore, these differences are likely a result of stronger selection pressures on spatial cognition and caching behavior at higher elevations. In support of this hypothesis, we found a significantly larger adult/first-year breeding ratio at high elevations compared with low elevations. This may suggest higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations, consistent with elevation-related phenotypic differences.
Significance statement
The strength of selection on survival-related traits varies across heterogenous environments. One way to assess the strength of selection is to measure the traits of individuals that are able to survive a particular selection event. Here we assessed the adult/first-year breeding ratios of resident, food-caching birds experiencing differentially harsh winter climates. These birds rely on spatial cognition to retrieve food caches and survive the winter; the largest annual selection event. At the individual level, those with better cognitive ability have demonstrated higher overwinter survival. However, at the population level, it remains unclear whether or not the strength of selection differs between harsh and mild winter climates. By assessing the age of breeders, we found a significantly larger adult/first-year breeding ratio in birds inhabiting harsher, high elevations compared with milder, low elevations. This suggests higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations.
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Data availability
Data is available through Google Drive https://drive.google.com/open?id = 1spGAReKrWBDkR03cabSuer96QCQb37R5
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Acknowledgments
Thank you to Jeff Brown and staff of Sagehen Experimental Forest field station (University of California Berkeley) for invaluable assistance at our field site. We would also like to thank two anonymous reviewers for helpful comments that improved the manuscript.
Funding
This research was financially supported by the National Science Foundation grant (IOS1351295) to VVP and by NSF Doctoral Dissertation Improvement Grant (IOS1600845) to CLB.
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CLB and VVP designed the experiment. CLB, AMP, DYK, BRS, and LMB collected data on breeding across three years. CLB and VVP conducted analyses and wrote the manuscript.
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To the best of our knowledge, no birds were harmed by the collection of this data. All procedures were approved by the UNR IACUC ethics committee and in accordance with the UNR IACUC protocol (00046), under California Department of Fish and Wildlife Permit SC-5210 (DocID: D-0019571790-9).
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The authors declare that they have no conflict of interest.
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Communicated by N. Clayton
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Branch, C.L., Pitera, A.M., Kozlovsky, D.Y. et al. Elevation-related differences in the age structure of breeding birds suggest stronger selection at harsher elevations. Behav Ecol Sociobiol 73, 143 (2019). https://doi.org/10.1007/s00265-019-2750-4
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DOI: https://doi.org/10.1007/s00265-019-2750-4