Wednesday, July 23, 2008

Decline of the Rusty Blackbird

The current issue of ZooGoer, the member magazine of the Smithsonian's National Zoo, features an article on the current population of the rusty blackbird (Euphagus carolinus). It has been trending downward, to put it mildly.

More than 40 years ago, visionary ornithologist Chandler Robbins and his colleagues at the Patuxent Wildlife Research Center (now part of the U.S. Geological Survey or USGS) established the continent-wide Breeding Bird Survey (BBS), which organizes volunteers to survey birds along some 3,000 miles of secondary roads each summer. Because there are few roads and even fewer birders in much of the rusty blackbird’s summer range, the BBS data on this species are spotty and meager. Population trend estimates are based on fewer than 100 routes scattered from Maine to Alaska. Still, USGS and Canadian Wildlife Service (CWS) statisticians show a robust decline in rusty numbers that surpasses almost all other North American birds in the steepness of its descent. The 40-year trend for the species is phenomenally depressing: 12 percent per year, or a whopping total decline of more than 97 percent over four decades.

The wintering ground, located largely in the southeastern United States, is mostly accessible, making annual winter counts there much more complete, thanks in part to one of the greatest events on the birdwatching calendar for the past 50 years—the Christmas Bird Count (CBC). Birders participating in a CBC record all the feathered creatures they encounter in one day within a 15-mile-diameter circle. The rusty blackbird’s winter range is dotted with more than 1,600 “count circles” within which the birds’ relative abundance has been reported over the past 40 years. National Audubon Society ornithologist Daniel Niven, working with John Sauer at the Patuxent Wildlife Research Center, has kept tabs on the CBC numbers for the rusty blackbird. The story is clear: The estimated annual decline is 4.5 percent per year, or an 85 percent decline over the past four decades.
The trends present in BBS and CBC data have been confirmed by more detailed breeding surveys in northern Canada. Surveys there have also shown that rusty blackbirds are disappearing from portions of their former breeding range.

There is a lot more at the linked article, including descriptions of rusty blackbirds' former abundance, the work being done to stop their slide, and possible reasons for the decline, so I recommend reading it.