LACK OF AN URBAN EDGE EFFECT ON REPRODUCTION IN A FRAGMENTATION-SENSITIVE SPARROW

Abstract

Many studies have documented degradation of nesting habitat quality for songbirds breeding in fragmented forests. Few studies, however, have focused on the effects of urban fragmentation on arid region avifauna, even though a unique suite of ecological changes may occur along the urban–arid wildland interface. The Rufous-crowned Sparrow (RCSP; Aimophila ruficeps) is a ground-nesting passerine that resides in coastal sage scrub habitat of southern California, USA. Its spatial pattern of abundance in urban-fragmented habitat in San Diego suggests that it is sensitive to changes in habitat configuration or quality that occur with fragmentation. We investigated whether RCSP were subject to adverse edge effects on reproduction by testing hypotheses that nest success and total reproductive output were lower in habitat adjacent to urban edges. We monitored annual fecundity of breeding pairs occupying territories near edges of residential development and in habitat interior areas from 1997 to 1999. Daily nest predation rates were high but did not differ between edge and interior areas. Even within edge plots there was no relationship between edge proximity and predation rate; nests within 75 m of the edge were depredated at the same rate as nests farther from the edge. Using video surveillance, we documented 10 nest predation events; nine identified predators were snakes. The lack of an edge effect on nest predation rate appeared to be due to a lack of edge sensitivity of snakes; snake encounter rates did not differ between edge and interior areas. Cowbird brood parasitism did not affect RCSP reproduction; no brood parasitism was observed in 266 nests. Reproductive output displayed substantial annual variation, apparently due to high interannual variation in rainfall. Total reproductive output did not differ between edge and interior in any year of the study, in part because more frequent renesting by RCSP in edges compensated for slightly lower brood sizes in some years. The lack of a difference in reproductive output between edge and interior habitat suggests that processes other than those affecting reproductive success in edges generate the fragmentation-sensitive pattern in this species. These results caution against generalizing across habitat types and species when predicting fragmentation effects.