FACTORS INFLUENCING PREDATION ON OVENBIRD (SEIURUS AUROCAPILLA) NESTS IN NORTHERN HARDWOODS: INTERACTIONS ACROSS SPATIAL SCALES

Auk, The, Jan 2006 by Mattsson, Brady J, Niemi, Gerald J

ABSTRACT.-

Microsite, patch, and landscape conditions may interact to influence nest predation. In northern Minnesota, silvicultural and agricultural practices may be involved in recent increases in nest predators and regional declines in several ground-nesting songbirds. To examine this problem, we evaluated 17 hierarchical models of predation on Ovenbird (Seiurus aurocapilla) nests that included microsite variables, distances to edges, and amount of core forest within a 2-km radius surrounding six study plots. During 2000 and 2001, 157 Ovenbird nests were monitored to estimate nest predation rates. A model that included the main effects of litter depth and core forest area and an interaction term between the two best described variation in predation on Ovenbird nests (AIC^sub c^ weight = 0.83). The nest predation rate from this model was 0.51 ± 0.01 (mean ± SE), assuming mean values of litter depth and amount of core forest. Shallow litter was associated with higher nest predation in three plots surrounded by less core habitat (40-60 ha), whereas there was no relationship in three plots surrounded by more core area (100-150 ha). Management that promotes deep leaf litter and the maintenance of large, intact forest tracts will likely benefit Ovenbirds and other forest songbirds. Received 7 March 2004, accepted 29 May 2005.

Key words: ground-nest predation, information-theoretic approach, Minnesota, multi-scale, Neotropical migratory songbirds, Ovenbird, Seiurus aurocapilla, silviculture.

Factores que Influencian la Depredación de Nidos de Seiurus aurocapilla en los Bosques del Norte: Interacciones a través de las Escalas Espaciales

RESUMEN. - Las condiciones del micro-sitio, del parche y del paisaje pueden interactuar para influenciar la depredación de nidos. En el norte de Minnesota, las prácticas silviculturales y agrícolas pueden estar involucradas en el incremento reciente de los depredadores de nidos y en la disminución regional de varias aves canoras que nidifican en el suelo. Para examinar este problema, evaluamos 17 modelos jerárquicos de depredación de nidos de Seiurus aurocapilla. Los modelos incluyeron variables de micro-sitio, distancia a los bordes y cantidad de área núcleo de bosque en un radio de 2 km alrededor de seis parcelas de estudio. Durante 2000 y 2001, estudiamos 157 nidos para estimar las tasas de depredación de nidos. Un modelo que incluyó los efectos principales de la profundidad de la hojarasca, el área núcleo de bosque y la interacción entre estos dos factures fue el que mejor describió la variación en la depredación de nidos de S. aurocapilla (peso del Criterio de Información de Akaike^sub c^ = 0.83). La tasa de depredación de nidos de este modelo fue de 0.51 ± 0.01 (media ± EE), basado en los valores medios de profundidad de la hojarasca y de cantidad de área núcleo de bosque. La hojarasca poco profunda se asoció con uria mayor depredación de nidos en tres parcelas rodeadas por menos área núcleo de bosque (40-60 ha), mientras que no hubo una relación en tres parcelas rodeadas de un área núcleo mayor (100-150 ha). Las estrategias de manejo que promueven una hojarasca profunda y el mantenimiento de sectores de bosque grandes e intactos probablemente beneficiarán a S. aurocapilla y a otras aves canoras.

MANY FACTORS CAN determine patterns of nest predation in a population of ground-nesting migratory songbirds, and these patterns exist across at least three spatial scales. Predation risk may increase for nests that lack appropriate amounts of surrounding vegetation for concealment (Martin and Roper 1988). Food availability within a breeding territory can also influence nest predation. Field experiments have shown that food supplementation allows breeding pairs to spend more time defending the nest (e.g. Pearse et al. 2004), and this can lead to lower nest predation rates (Arcese and Smith 1988). Nests near edges (e.g. Flaspohler et al. 2001) or nests in landscapes with high predator densities (e.g. Burke and NoI 2000) may also be at higher risk of predation. Inexperienced breeders may have nest sites that are at high risk of predation because of factors at any of these scales (Holmes et al. 1996). Many studies have considered factors at one or more of these scales as individual or additive effects (e.g. Hanski et al. 1996), but few have evaluated how factors interact across scales to influence nest predation (Donovan et al. 1997, Tarvin and Garvin 2002, Zanette et al. 2003).

Elevated nest predation is among the primary suspected causes for declines in many insectivorous songbird populations that breed in fragmented forests of eastern North America and winter in the Neotropics (Böhning-Gaese et al. 1993, Donovan and Flather 2002). Songbirds breeding in large, unfragmented forests of northern Minnesota and northern Wisconsin could be source populations, whereas those in fragmented oak-hickory forests of the midwestern United States are likely sinks (Robinson et al. 1995). These source populations are important for maintaining stability in their respective metapopulations (Pulliam 1988). Thus, understanding effects of land-use practices on songbirds in contiguous forests is critical for their conservation.