Comparative Foraging Dynamics of Chestnut-Backed and Mountain Chickadees in the Western Sierra Nevada

Abstract

Knowledge of where and how wild animals forage for food resources is essential for effective wildlife management and conservation. Therefore, we quantified foraging behaviors and patterns of habitat use by chestnut-backed chickadees (Poecile rufescens) and mountain chickadees (1? gambeli) in the western Sierra Nevada from May 1986 through February 1989. Chestnut-backed chickadees have invaded mixed-conifer forest habitats in the Sierra Nevada during the past 40 to 50 yr, and now occupy areas where mountain chickadees were the only previous resident member of the family Paridae. This recent range expansion provided a unique opportunity to study comparative aspects of foraging and habitat use by 2 closely related, and recently sympatric, species. Foraging and habitat use data were collected on a monthly basis for 34 mo. During the study, chestnut-backed chickadees foraged on California black oak (Quercus kelloggii) and Douglas-fir (Pseudotsuga menziesii) significantly more than mountain chickadees throughout their annual cycle; whereas, mountain chickadees foraged on ponderosa pine (Pinus ponderosa), sugar pine (1? lambertiana), and white fir (Abies concolor) significantly more than chestnut-backed chickadees. Both chickadees significantly increased the amount of time they foraged on white fir during the breeding season, sugar pine during late summer (July to August), and incense cedar (Calocedrus decurrens) during winter (October to March). Shifts in the use of 2 of these tree species were attributed to the seasonal presence of 2 abundant arthropods: larvae of a bud-mining sawfly (Pleuroneura sp., Hymenoptera, Xyelidae) on white fir and a margaroid scale (Xylococulus macrocarpae, Homoptera, Margaroididae) on incense cedar. When the autocorrelation structures of the data were assessed using time series analyses of monthly time lags, certain aspects of the foraging behaviors (gleaning, hanging, and flaking bark chips) of both species showed evidence of periodic annual cycles. Our data indicated that all 6 of the dominant forest tree species in the mixed forest zone of the western Sierra Nevada provide foraging habitat and associated food resources for these birds during different parts of their annual cycle. Commercial forest management actions in the western Sierra Nevada are simplifying forest structure by concentrating regeneration on ponderosa pine and Douglas-fir at the expense of white fir, sugar pine, incense cedar, and black oak. How such management will affect populations of chickadees and other species of insectivorous birds is currently unknown and should be tested with manipulative field experiments.