By flocking together, birds may have trouble adapting.

An article in the latest issue of The Auk (published by the University of California Press for the American Ornithological Union) indicates songbirds may be in even deeper peril than previously thought.

In the piece, Point Blue Conservation Science (formerly Point Reyes Bird Observatory) researchers Renée Cormier, Diana Humple, Thomas Gardali and Nathaniel Seavy addressed the migratory habits of the Swainson’s thrush (Catharus ustulatus), a retiring songbird with a speckled breast renowned for its melodious song (video).

Swainson’s thrushes are a member of a large group of songbirds known as neotropical migrants (or “neotrops” to birders); they typically breed in North America and winter in Mexico, Central America and/or South America.  Many if not most neotrops are dwindling due to urban development in the northern hemisphere, forest destruction in the southern latitudes, and sundry other factors – including predation by domestic cats.   Many scientists think climate change will further stress their populations.

Though ornithologists know the general range for most neotrops, precise details on their migrations are pretty vague. Hoping to remedy the deficit, Point Blue scientists captured Swainson’s thrushes near the organization’s field station in Marin County with mist nets, fitted them with sophisticated telemetry tags known as light-level geolocators, and released them.  When the birds returned over the following two springs, they were re-captured and the information of their movements downloaded from the geolocators to a computer.

Their findings:  Swainson’s thrushes, like many human beings, prefer to travel with friends and family to known locales.  In other words, they don’t disperse hither and thither during their migrations.  They travel in groups, they know where they’re going, and they stay in relatively constricted territories in both their northern and southern ranges. 

This is sobering news, in that such habits increase vulnerability to the species as a whole.  If – as now seems almost certain – Swainson’s thrushes migrate as well-established groups to the same locations year after year, individual populations can blink out when a favorite spot is turned into a shopping mall or banana plantation; the birds may be unable to adapt to new, unfamiliar locales, even those that contain their basic food and habitat requirements.

Point Blue’s Swainson’s thrush work begs the question:  Do other neotropical migrants face similar threats? Henry Streby, a UC visiting scholar, could provide the answer.  He’s conducting his own research with light-level geolocators on golden-winged warblers, another lovely neotrop.  We’ve contacted Streby by email, and hope to have with an interview with him shortly, which we’ll post in due course. 

—Glen Martin

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