Food abundance determines distribution and density of a frugivorous bird across seasons

Abstract

Although food abundance is a principal determinant of distribution and abundance of many animals, most previous studies have not quantitatively assessed its importance relative to other factors that may also determine species distributions. We estimated frugivorous phainopepla Phainopepla nitens occupancy and density, food density, and vegetation structure on transects in fragmented mesquite and acacia woodlands over three years in non‐breeding and breeding seasons. Using an AIC framework and controlling for detection probability, we determined relative impacts of food abundance, vegetation structure, and habitat fragmentation on patch occupancy and density, and concomitant extinction and colonization probabilities of phainopeplas. Initial occupancy in winter 2002 was high (0.87 ± 0.047), and primarily positively correlated with food abundance and woodland area (Akaike weights wi= 0.998 and 0.750 respectively). Woodland area more strongly influenced occupancy where food was scarcer. Phainopepla density in both seasons was strongly positively correlated with food abundance, especially in the 2002 drought when density was higher (wi=1.0 for food and year). Density was higher in acacia than mesquite woodlands (wi= 1.0), and moderately negatively correlated with elevation (wi= 0.789). Extinction probability (patches vacated) was low (0.078 ± 0.040), and principally influenced by phainopepla density (wi= 0.968) and tree height (wi= 0.749). Colonization probability was low (0.15 ± 0.034) and determined by vegetation structure (wi= 1.0). Much recorded colonization was reoccupancy of woodlands previously occupied by single males in winter, then vacated in a breeding season. These results suggest that for an animal occupying a highly fragmented landscape, distributions and densities at the habitat patch scale are driven by food abundance, are moderately affected by habitat fragmentation, and are slightly influenced by vegetation structure.