A previously unknown migration hot-spot along the Pacific flyway in California has been detailed in a new study published in Western Birds.
Bear Divide, in the western San Gabriel Mountains in Los Angeles County, received two years of informal surveys and at least one morning of visible migration produced an estimated 13,500 individual birds passing.
The preliminary analyses discussed in the study suggest that the peak of a species' migration at Bear Divide is correlated with the latitude of a species' breeding, being later in the spring as that latitude increases. The data from Bear Divide also provided an independent perspective on migration as quantified by local radar results.
During the springs of 2019 and 2020, some 44,900 birds were recorded passing through Bear Divide. Peak passage occurred in late April, but substantial migration lasted at least from mid-March to late May. In general, migration was concentrated within the first 2-3 hours of the morning. Among the more numerous migrants that were recorded during the two years were Wilson's and Townsend's Warblers, Black-headed Grosbeak and Lazuli Bunting
This magnitude of visible migration at Bear Divide makes it unique along the Pacific coastal flyway, so far as is known. North American localities where morning flights of landbirds can be observed are unevenly distributed and little reported along the Pacific coast.
The study can be accessed here.
Dean C A, Dokter A M, Farnsworth A, Garret J, Hill L, Maxwell D, Terrill R S & Tingley M W. 2021. A novel locality for the observation of thousands of passerine birds during spring migration in Los Angeles County California. Western Birds. DOI: 10.21199/WB52.4.4