Climate change consequences for differential adult survival and the mating system of a temperate breeding shorebird

Managing for the effects of climate change on species whose populations are currently imperiled requires detailed knowledge of the relationship between their demographic rates and climate variables. We sought this information for the West Coast breeding population of snowy plover (Charadrius nivosus), which was federally listed as threatened in 1993 due to substantial declines in the numbers of plovers breeding along the coast and in the number of sites occupied for breeding. Snowy plovers employ a serially polygamous breeding system in which the male typically tends chicks to independence. This unusual breeding system is favored by male-biased sex ratios in local populations. As part of a multispecies study of the effect of climate change on population growth, we used mark-capture models to examine climate drivers of adult survival for 1219 snowy plovers banded at Monterey Bay over 38 years and known to overwinter on the surrounding north-central California outer coast. Nonclimate variables, including sex and unmeasured annual mortality risks (e.g., predator abundance), were the primary factors affecting adult survival. However, there is evidence that cold weather, particularly extended cold snaps with daily low temperatures below 2 degrees C and daily high temperatures below 10 degrees C, decreases overwinter survival. Exceptionally cold winters had a particularly strong effect on adult female plovers, contributing to the male-biased adult sex ratios. Future winter climate on the north-central California coast is projected to be generally warmer with fewer and shorter cold snaps. Reduced mortality from cold winter weather may mitigate other threats faced by plovers, such as anthropogenically enhanced predator populations, habitat loss, and accelerated sea level rise, while altering the adult sex ratio and potentially shifting the evolutionary landscape maintaining the plover's unusual breeding system.