Incidental Nest Predation in Songbirds: Behavioral Indicators Detect Ecological Scales and Processes

Abstract

Incidental predation occurs when secondary prey items are encountered and subsequently consumed, not through directed search for such prey, but through their consequential encounter by a predator engaged in search for primary prey. We developed a mathematical model that examines the relationships between the abundance of primary prey, patch exploitation (i.e., quitting harvest rates), and the rate of incidental predation on secondary prey items. The model's predictions are dependent upon the spatial scale over which a forager integrates foraging costs and thus determines its quitting harvest rate (QHR). At local (i.e., foraging) spatial scales, we predicted that incidental predation should increase with local food abundance. Also at the foraging scale, local food abundance should not influence QHRs, but local predation risk (from higher trophic levels) should increase QHRs. Therefore, we predicted that incidental predation rates should be negatively correlated with QHRs. Over large (i.e., landscape) spatial scales, greater food abundance and predation risk increase QHRs, and we predicted that predation rates should vary inversely with QHR through two complementary mechanisms: foragers use a greater proportion of space and spend more time foraging as quitting harvest rates decrease.We experimentally tested the qualitative predictions of the theory in the field using artificial Veery (Catharus fuscescens) nests depredated by white-footed mice, Peromyscus leucopus, across three spatial scales. We used the technique of giving-up densities to measure QHRs and to determine the scale at which mice integrate different foraging costs. In accord with our predictions, nest predation was positively influenced by the local abundance of food at the foraging scale, and local predation risk to mice and perhaps interference competition from chipmunks resulted in higher giving-up densities and lower nest predation. At the landscape scale, there was an inverse relationship between giving-up densities and nest predation, which was probably the result of large-scale differences in resource abundance between plots. Our study demonstrates how linking theoretical development to the use of empirical behavioral indicators can help determine the relevant ecological scales and processes necessary for understanding predator-prey interactions.

Foraging behaviour of nest predators at open-cup nests of woodland passerines

“…However, the study of behaviour of individual nest predators is inherently difficult and only a few studies have tried to differentiate among alternative mechanisms (e.g. Schmidt et al 2001;Vigallon and Marzluff 2005). Progress in this field is hindered, among others, by the lack of basic knowledge of predator foraging behaviour.…”

Genetic structure of Greater Sage-Grouse (Centrocercus urophasianus) in a declining, peripheral population

“…Synanthropic predators can have detrimental effects on populations of threatened or endangered species because these predators are subsidized by anthropogenic resources and, in contrast to predators that do not utilize these resources, will prey on wildlife populations even if they occur in very low numbers (for review see Kristan III and Boarman 2003). The addition of anthropogenic features induces numerical and functional responses in synanthropic predator populations Boarman 2003, Kristan III et al 2004), thereby exposing wildlife populations to higher rates of incidental predation (Schmidt et al 2001), and facilitating the expansion of these predators into sagebrush habitats where they are, in the absence of anthropogenic features, either found only at low densities or are absent (Restani et al 2001, Kristan III andBoarman 2003). For sagebrush wildlife populations in general, we modeled the spatial distribution of house cats (Felis cattus) and domestic dogs (Canis familiaris).…”

Linking edge effects and patch size effects: Importance of matrix nest predators

“…We also assume that all matrix nest predators belong to the same species, hence they have basically the same foraging behavior and a diet composed fundamentally of a single prey species inhabiting the matrix habitat. In consequence, forest patches are secondary foraging habitats for matrix predators and thus nest predation within patches is incidental (Vickery et al, 1992;Schmidt et al, 2001). As a consequence of these assumptions, the model considers that bird density does not affect the predator population dynamics, which is determined by ecological processes in the matrix (e.g.…”

“…Andrén, 1992;Kays et al, 2008; see also Chalfoun et al, 2002). Predator density can vary significantly over time, such as populations of small mammals (Ostfeld et al, 1996;Schmidt et al, 2001). Accordingly, if N remains constant, then P is directly proportional to the perimeter/area ratio for a circle.…”