Does cowbird parasitism increase predation risk to American redstart nests?

Abstract

Brood parasitism and nest predation are major causes of reproductive failure for many bird species nesting in fragmented landscapes. While brood parasites and predators may act independently, they could also interact if brood parasites increase the likelihood that predators detect nests. In this study, we examined the interaction between cowbird parasitism and nest predation in a 10 year study on 466 American redstart Setophaga ruticilla nests in central Alberta, Canada. We used advanced nest survival models to examine the support for three mechanisms that might lead to a positive correlation between brood parasitism and nest predation: 1) the presence of a cowbird nestling might increase the detection of the nest by predators, 2) nests with lower cover are more likely to be detected by both cowbirds and predators, and 3) cowbirds and predators may co‐occur in landscapes of similar structure. Twelve percent of nests were parasitized and those nests had a 16–19% higher rate of failure due to predators compared to unparasitized nests. Daily nest predation rates increased during the nestling stage for both groups, but more strongly for parasitized nests. Loud begging by the cowbird nestling and/or higher parental feeding rates for the cowbird may have increased nest detectability to predators. Brood parasitism and nest predation were also positively related to forest cover, indicating landscape level effects were influential. Most nest predators were forest species and we suspect cowbirds responded positively to forest cover because of the increased abundance of songbird hosts. Nest‐site features had less of an impact on nest predation or brood parasitism, although nests with higher overhead cover were less susceptible to predators. Our study shows how multiple mechanisms, particularly the behavioral effects of the brood parasite nestling and landscape structure, can lead to a positive relationship between nest predation and brood parasitism.

Impacts of nest predators and weather on reproductive success and population limitation in a long‐distance migratory songbird

“…Warmer May temperatures could accelerate vegetation phenology, providing greater nest concealment from predators earlier in the season (Martin , Weidinger ), and both warmer temperature and rainfall might lead to a greater availability of insects during the nesting period (Sherry and Holmes , Sillett et al ). Higher food abundance could lead to shorter and less frequent incubation recesses for females (Rastogi et al ), reducing adult activity at nests that might attract predators (Martin et al , Hannon et al ). Increased temperature and precipitation might also result in a greater abundance of alternative foods (other than birds’ eggs and chicks) for nest predators (Schmidt and Whelan ), such as more larval insects in leaf litter, corms of vernal herbs, or mushrooms.…”

“…Complicating this picture, seasonal carry-over eff ects from winter in redstarts and other species (Marra et al 1998, Calvert et al 2009, Reudink et al 2009, Rockwell et al 2013, Cooper et al 2015) also impact reproduction; and a future task is to partition the eff ects of summer conditions on reproductive success vs winter carry-over eff ects. We also recognize that summer ecosystem processes such as mast fruiting of trees, forest fragmentation, and drought likely vary in their eff ects on migratory bird populations geographically (Hannon et al 2009, Wilson et al 2011, McKellar et al 2014, necessitating comprehensive studies of limiting factors both geographically and throughout the annual cycle. in population growth rate using territories as a proxy for population size.…”

Woodlands as quality breeding habitat for black-capped vireos

“…This study emphasizes the multiple negative effects of brown‐headed cowbirds on a host species. Nest parasitism adversely affects nest survival, perhaps because parasitized nests are more susceptible to predation (McClaren and Sealy , Kosciuch and Sandercock , Hannon et al ). Black‐capped vireos will also abandon parasitized nests (Grzybowski , Pease and Grzybowski ).…”

Effects of parents and Brown‐headed Cowbirds (Molothrus ater) on nest predation risk for a songbird

“…Cowbirds can affect predation in various ways for various reasons. Especially loud-begging cowbird nestlings can attract predators themselves (Dearborn 1999; Hoover and Reetz 2006) or elicit greater parental activity (Dearborn et al 1998; Hannon et al 2009), or parasitism can elicit parental-defense behaviors that attract predators (Tewksbury et al 2002). Cowbirds also depredate nests directly in conjunction with their parasitic activities.…”