Mixed consequences of divorce on reproductive success of songbirds nesting in agricultural hayfields

Abstract

Agricultural management, particularly haying, can cause synchronous nest failure of ground-nesting songbirds. As a consequence, these birds may subsequently renest and choose a new social mate (divorce). This study (1) quantified within-year and between-year divorce rates of grassland songbirds, and (2) determined if divorce rates differed after haying or predation-caused nest failure, and if so, whether divorce influenced reproductive success. From 2002 to 2017, we monitored 121 Bobolink (Dolichonyx oryzivorus) pairs and 436 Savannah Sparrow (Passerculus sandwichensis) pairs in an agricultural region of Vermont, USA. Within-year and between-year divorce rates were 0–84.9% for Bobolinks and 17–69.1% for Savannah Sparrows. Between years, Bobolinks, but not Savannah Sparrows, were more likely to divorce after nest failure, but haying did not influence divorce rates. Within years, Savannah Sparrows were more likely to divorce after nest failure, but as with Bobolinks, divorce rates in Savannah Sparrows were not different between nests that failed due to haying or predation. Across all Savannah Sparrow renests, divorce had no influence on the number of young fledged per female. However, between years, female Bobolinks that divorced fledged more young in their first attempt while those that did not divorce fledged more young in their second attempt. This study showed that pairing decisions were not differentially affected by cause of nest failure. Further, we identified no reproductive benefit to divorce. Our results indicate no adaptive benefit and potentially a significant reproductive cost to divorce, and that these effects vary between species.