Genetic Structure of Two Populations of White-Crowned Sparrows with Different Song Dialects

son's Thrushes forage along the upper beaches and nest, usually within 500 m of the beach, on stumps and tree trunks in a mixed forest of western hemlock (Tsuga heterophylla) and western red cedar (Thuia plicata). Robins and Varied Thrushes also forage along the upper beaches and nest along the periphery of the island but place their nests higher than those of Hermit and Swainson's Thrushes. Ecological segregation among these thrushes there, particularly between Hermit and Swainson's Thrushes, is achieved primarily by their different breeding phenologies (table 1). The Hermit Thrush returns to Langara Island about 6 weeks earlier than the Swainson's Thrush and the former's young have begun to fledge by about the time the Swainson's Thrush arrives in mid-June. Adult and young Hermit Thrushes forage along the beaches until at least mid-August (when my observations ceased in 1971); adult Swainson's Thrushes and their young, after mid-July, also foraged along these beaches but had dispersed from the island by the end of July. I do not know whether both species took the same prey items. A similar differential timing of breeding also occurs on Forrester Island, Alaska (Willett 1915; Bailey 1927), some 70 km N of Langara Island. These observations were made during ecological studies of seabirds on and near Langara Island from 6 May to 10 July 1970 and from 17 March to 10 August 1971. This work was funded by grants from the National Science Foundation for research in systematic and evolutionary biology at the University of Michigan, the Frank M. Chapman Memorial Fund of the American Museum of Natural History, the Society of the Sigma Xi, and the Canadian National Sportsmen's Show.