Two decades of change in a coastal scrub community: songbird responses to plant succession
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Coastal scrub habitats in California are threatened by habitat loss, fragmentation, and degradation. Local and statewide declines have been observed in several birds that breed in coastal scrub, most notably the California Gnatcatcher (Polioptila californica; Atwood 1993), but also include more common species such as the Whitecrowned Sparrow (Zonotrichia leucophrys; Trail and Baptista 1993; Sauer et al. 2001; Point Reyes Bird Observatory [PRBO], unpubl. data). While Breeding Bird Survey data can be used to track population trends over larger scales (Sauer et al. 2001), they are not always adequate to monitor trends of distinct breeding populations. Such populations include the nuttalli subspecies of the White-crowned Sparrow, which breeds only in the narrow, humid, coastal strip of central and northern California, and the Song Sparrows (Melospiza melodia) that breed in northern California coastal scrub habitat. On the other hand, while local monitoring programs have the advantage of being population- and habitat-specific, it is difficult to tell whether trends demonstrated in local study areas are associated with landscape-scale changes or are occurring primarily in response to local changes in habitat characteristics (Holmes and Sherry 2001). We monitored plant succession and associated changes in bird population density in a single study site in northern California coastal scrub habitat. Our objectives were to (1) describe plant succession in coastal scrub over 22 years, and (2) evaluate evidence for population responses to plant succession in three coastal scrub resident birds. Here, we report preliminary results concerning the changes in abundance of three species of coastal scrub-breeding birds.