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

• Published in: Ecology and evolution Vol. 2; no. 12; pp. 3079 - 3097
• Main Authors: Latif, Quresh S., Heath, Sacha K., Rotenberry, John T.
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.12.2012; Blackwell Publishing Ltd
• Subjects: Age; Animal behavior; Birds; Breeding; Density; Ecological monitoring; Ecology; Fitness; Habitats; Lake basins; Microenvironments; Microhabitats; Molothrus; Molothrus ater; Mono Lake; nest parasitism; nest predation; nest survival; Nests; Original Research; Parasitism; Parental behavior; population ecology; Population growth; Predation; Predators; predator–prey interactions; Scholarships & fellowships; Setophaga; Setophaga petechia; Site selection; Songbirds; Territory; Yellow Warbler; United States > US; Mono Lake; USA, California, Mono L; Setophaga petechia; predator–prey interactions; nest predation; Mono Lake; parental behavior; nest parasitism; nest survival; Yellow Warbler; population ecology
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.411

Nest predation limits avian fitness, so ornithologists study nest predation, but they often only document patterns of predation rates without substantively investigating underlying mechanisms. Parental behavior and predator ecology are two fundamental drivers of predation rates and patterns, but the role of parents is less certain, particularly for songbirds. Previous work reproduced microhabitat‐predation patterns experienced by Yellow Warblers (Setophaga petechia) in the Mono Lake basin at experimental nests without parents, suggesting that these patterns were driven by predator ecology rather than predator interactions with parents. In this study, we further explored effects of post‐initiation parental behavior (nest defense and attendance) on predation risk by comparing natural versus experimental patterns related to territory density, seasonal timing of nest initiation, and nest age. Rates of parasitism by Brown‐headed Cowbirds (Molothrus ater) were high in this system (49% nests parasitized), so we also examined parasitism‐predation relationships. Natural nest predation rates (NPR) correlated negatively with breeding territory density and nonlinearly (U‐shaped relationship) with nest‐initiation timing, but experimental nests recorded no such patterns. After adjusting natural‐nest data to control for these differences from experimental nests other than the presence of parents (e.g., defining nest failure similarly and excluding nestling‐period data), we obtained similar results. Thus, parents were necessary to produce observed patterns. Lower natural NPR compared with experimental NPR suggested that parents reduced predation rates via nest defense, so this parental behavior or its consequences were likely correlated with density or seasonal timing. In contrast, daily predation rates decreased with nest age for both nest types, indicating this pattern did not involve parents. Parasitized nests suffered higher rates of partial predation but lower rates of complete predation, suggesting direct predation by cowbirds. Explicit behavioral research on parents, predators (including cowbirds), and their interactions would further illuminate mechanisms underlying the density, seasonal, and nest age patterns we observed. Mechanisms underlying nest predation and thus influencing songbird fitness are poorly understood. We used differences between natural and experimental nests to examine the influence of parents on nest predation rates and patterns. We found differences in patterns with respect to date and territory density, suggesting a role of spatiotemporal variation in parental defense in producing date‐ and territory‐related patterns.