Long-term environmental variation in nest survival for Sialia mexicana (Western Bluebird) and Myiarchus cinerascens (Ash-throated Flycatcher)

Abstract

Measuring nest survival is central to understanding determinants of avian reproduction. We investigated long-term environmental determinants of Sialia mexicana (Western Bluebird) and Myiarchus cinerascens (Ash-throated Flycatcher) nest survival in north-central New Mexico, addressing 3 questions: (1) How does phenology or timing of breeding affect nest survival? (2) How does inter-annual climate variation and the elevation of nesting affect annual nest survival? (3) Were there broad-scale differences in nest survival before (1997–2006) and after (2006–2018) a major tree mortality event? We analyzed 22 years of data from 1,649 nests (1,378 S. mexicana, 271 M. cinerascens) between 1997 and 2018. For both S. mexicana and M. cinerascens, nest survival increased as the nesting season progressed. Survival declined with increasing nest age during incubation and nestling stages. For S. mexicana, nest survival also increased with elevation, but there was no association between M. cinerascens nest survival and elevation. Lastly, there were no differences for nestling survival between early (1997–2006) and late (2007–2018) periods for either species. However, bluebird incubation survival decreased significantly by 11% from 2007–2018. Bluebirds nesting in the upper third of the elevational range had 15% higher incubation and nesting success than did those nesting in the lower two-thirds. These results highlight how elevation and temporal factors differentially affect cavity-nesting species' nest survival, with S. mexicana showing elevational gradients potentially related to climate-driven habitat shifts following tree mortality, whereas M. cinerascens exhibited no such patterns despite sharing similar nest sites.