Drought Affects Dusky Orange-crowned Warbler
By Gregory Gough
The winter of 2006-2007 was a brutal one in southern California. Little rain fell and the Channel Islands were particularly hard hit. The dusky orange-crowned warbler, a subspecies of the orange-crowned warbler that only nests on Catalina and Santa Cruz islands, was particularly impacted.
On Catalina Island breeding commenced 2 months late and only 5 (out of 45) females built nests. Of these, only 1 laid eggs and those young eventually starved.
Santa Cruz Island had a little more winter rainfall than Catalina so more females nested and laid eggs. They did not, however, raise many young as many nests were predated, primarily by Island scrub-jays.
The winter rains are important to dusky orange-crowned warblers as they entice oaks to put forth new leaves. These new leaves provide tender forage for caterpillars which in turn are the primary food source for warblers, especially for nestlings.
In a year with average rainfall the warblers produce an abundance of young. But with climate change models predicting sparser precipation for southern California in the future, the fate of the dusky orange-crowned warbler is precarious.
This article summarizes the information in this publication:
Langin, K. M., Sillett, Terence Scott, Yoon, J., Sofaer, H. R., Morrison, S. A. and Ghalambor, C. K. 2009. Reproductive consequences of an extreme drought for orange-crowned warblers on Santa Catalina and Santa Cruz Islands. In: Garcelon, D. K., Proceedings of the Seventh California Islands Symposium. Institute for Wildlife Studies, pp.293-300.
Bird populations on the California Channel Islands are adapted to a Mediterranean-type climate with extended dry periods and considerable annual variation in rainfall. The winter of 2006-2007, however, was outside the normal range in southern California and ranked as one of the driest on record. Here we report how these drought conditions affected the reproductive output of a small songbird, the "dusky" orange-crowned warbler (Vermivora celata sordida), on Santa Catalina and Santa Cruz islands in the spring of 2007. Warblers on Catalina Island began establishing territories in February, yet they did not commence breeding activities until mid-April. Even then, only 5 of 45 females built nests and only 1 female laid eggs. The young in that nest ultimately starved; thus, the reproductive output of our study population on Catalina Island was zero. This contrasts starkly with the population on Santa Cruz Island, where all individuals studied attempted to reproduce and began nesting in mid-March. Nevertheless, only 11% of Santa Cruz Island warbler pairs fledged young due to high nest predation. Taken together, our observations from the two islands reveal marked differences in the consequences of severe drought and in the factors that limit the reproductive output of terrestrial birds on the Channel Islands.
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