California Bird’s Puzzling Decline
By Gregory Gough, Scott Sillett and Scott Morrison
Interspersed amongst the sprawling urban development of southern California are remnants of a once-widespread habitat type, coastal sage scrub. This semi-arid habitat is home to nearly 100 species that are either endangered, threatened, or of conservation concern, including the secretive and declining Southern California rufous-crowned Sparrow (Aimophila ruficeps canescens).
Coastal sage scrub habitat, home of the California Rufous-crowned Sparrow.
© Scott Morrison
When housing developments, roads, and shopping centers are built in or near coastal sage scrub, they create “islands” of habitat dispersed in an urban sea. For many birds, such habitat fragmentation creates a serious problem. Small patches of suitable habitat have less food and more predators, such as raccoons and house cats, which infiltrate from adjacent developed areas. Consequently, bird nesting success is often reduced. Over time, bird populations in fragments may disappear entirely.
Rufous-crowned sparrows are much less common in fragmented patches of habitat than in large, intact areas. To find out why rufous-crowned sparrows were declining in smaller patches, Scott Morrison of The Nature Conservancy and Douglas Bolger of Dartmouth College investigated a likely culprit: reduced nesting success in fragmented habitat. But surprisingly, nesting success did not differ between large expanses of coastal sage scrub and habitat fragments adjacent to urban areas.
Morrison and Bolger next teamed up with Scott Sillett of the Smithsonian Migratory Bird Center to examine whether survival of adult sparrows differed between intact and fragmented habitat. They hypothesized that adult sparrows living in the interior of large habitat patches would have a better chance of survival from year to year, compared with sparrows living along the developed edges of habitat reserves.
Rufous-crowned sparrows are non-migratory, permanent residents in coastal southern California, and adults remain on their territories throughout the year, so higher mortality in fragmented habitat could lead to localized declines.
From 1997 to 2000, hundreds of sparrows were captured and each bird was outfitted with a unique combination of colored leg bands. In subsequent years, scientists used binoculars to re-sight the marked birds.
The percentage of adult sparrows surviving from year to year was remarkably high for a small bird—about 70 percent for females and 75 percent for males. Most individuals were re-sighted year after year in the same territory, paired with the same mate. Interestingly, although survival in dry years might have been slightly lower in edge habitat, survival in general did not appear to differ.
This research also raised a cautionary note for scientists investigating survivorship of songbirds. Despite marking hundreds of birds and monitoring for multiple years, scientists were limited in their ability to conclude with statistical certainty fragmentation affected survival. Moreover, they discovered that the ability of researchers to detect this cryptic species depends strongly on annual rainfall; in dry years the birds tend to be much less detectable. These findings illustrate the difficulties that scientists encounter in searching to understand the dynamics of wild populations in a changing world.
So the search continues to find the cause of the rufous-crowned sparrow's decline in southern California. Scientists will be looking closely at young birds in fragmented landscapes, to see how far they disperse from their natal territory and how well they survive their first year of life—two aspects of the avian life cycle that are extremely challenging to study. Teasing apart the effects of development on sensitive species like the rufous-crowned sparrow will help conservationists prevent further declines of biodiversity.
This article summarizes the information in this publication:
Morrison, S. L., Bolger, D. T. and Sillett, Terence Scott 2004. Annual survivorship of the sedentary Rufous-crowned sparrow: no detectable effects of edge or rainfall in southern california. The Auk, 121: 904-916.
The Rufous-crowned Sparrow (Aimophila ruficeps) is a nonmigratory passerine that displays an area-sensitive distribution pattern of abundance in fragmented coastal sagescrub habitat of southern California. To determine if habitat fragmentation negatively affected adult survival, we used Cormack-Jolly-Seber models to compare annual survival probabilities of adult sparrows breeding in habitat adjacent to urban-developed edges to those of birds breeding in the interior of large habitat expanses in San Diego County, 1997-2000. During that period, an El Niño event brought heavy rainfall to the study area, and a La Niña event brought drought. Annual survival probabilities were relatively high for a small passerine (females: 0.69 ± 0.05 SE; males: 0.74 ± 0.04 SE) but, given our data, did not differ between habitat types or with rainfall. Annual resighting probabilities for the birds were strongly associated with variation in rainfall, being high in the wet year and low in the dry year. Mate- and site-fidelity were apparently high, and surveys during the nonbreeding season documented that the sparrows stayed paired and on territories year-round. We hypothesize that the high apparent survivorship of this species is related to its nonmigratory habit and its tendency to curtail reproductive effort during periods of food scarcity. Although our survivorship analysis suggests that the urban-wildland interface does not adversely affect survival of territorial Rufous-crowned Sparrows, our power to detect an effect of habitat edge on survival was low. Thus, we urge caution in concluding that edge effects do not have an ecologically important influence on survival rates in this species.
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