Wild Neighbors: Annals of Forensic Ornithology: The Peregrine Report

By Joe Eaton
Tuesday May 24, 2011 - 09:51:00 AM
Adult peregrine falcon at Morro Bay, a long-time nest site.
Kevin Cole (via Wikimedia Commons.)
Adult peregrine falcon at Morro Bay, a long-time nest site.

Sorting through the trash may not be most folks’ idea of a good time, but it can be a boon to science. I’m thinking here about the intrepid researchers who examine the leftovers in or near raptor nests and reconstruct the birds’ diets. 

Two years ago Kristie Nelson performed that task for the peregrine falcon eyries on the PG & E building in San Francisco and the San Jose City Hall. The prey remains were collected in December, long after the young falcons had fledged and the families had dispersed. 

Although her tallies confirmed some assumptions, they also provided a few surprises. 

The San Francisco peregrines—in 2009 that would have been Diamond Lil and Dapper Dan, and their progeny—had a more varied diet than the San Jose birds. Nelson identified the feathers of at least 13 species at the San Francisco eyrie but only 9 at the San Jose site. Prey species represented by more than one individual in San Francisco, in order of abundance, were rock pigeon (the ubiquitous city pigeon), European starling, Brewer’s blackbird, mourning dove, cedar waxwing, and dowitcher, either long-billed and/or short-billed. For San Jose, only the pigeon, starling, blackbird, and dove made the cut. One pigeon was a homer, with leg band number AU 2007 ARPU 79624. 

Among the one-offs in San Francisco were willet, northern mockingbird, acorn woodpecker, Bullock’s oriole, varied thrush, probable hermit thrush, house finch, possible bushtit, and some kind of small duck. The San Jose eyrie had single specimens of American avocet, probable western grebe, Bullock’s oriole, and house finch. Each nest yielded the remains of a budgerigar: blue in 

San Francisco, green in San Jose. 

Those are interesting lists. Where did Lil or Dan find an acorn woodpecker, of all things? San Francisco is not noted for its acorn crop. It, like the orioles, may just have been passing through. Why would a peregrine bother with anything as small as a bushtit? There’s not a lot of meat on those things. No mammals: as a rule, peregrines don’t do rats. 

And notice the bias toward landbirds: only two shorebirds (willet and dowitcher) and two waterbirds (the grebe and the dubious duck.) The San Francisco falcons in particular are handy to the Bay. Peregrines used to be commonly known as duck hawks, and I once saw one at Cesar Chavez Park take down a California gull. This may be a good thing, though. I remember being told by one of the peregrine-watchers a few years back that city pigeons and other urban birds were cleaner, with much less of a chemical load, than shorebirds and seabirds, and that peregrines that hunted the Bay had lower reproductive success rates than their pigeon-eating counterparts. It would be interesting to track down the study or studies that came from. 

According to the peregrine account in the authoritative Birds of North America series, these falcons have been known to kill birds ranging from crane to hummingbird size. Overall, pigeons and doves account for most of the prey biomass, but there’s much regional variation. The diet in Nunavut is heavy on waterfowl and lemmings. Peregrines in the Grand Canyon specialize on white-throated swifts, and secondarily bats. Over three-quarters of the prey of those nesting on the Pacific Coast consisted of auklets, murrelets, storm-petrels, and shearwaters. For North American urban areas, the top six prey species are rock pigeon, mourning dove, northern flicker, European starling, blue jay, and American robin. 

Meanwhile in Berkeley, Allen Fish and his Cooper’s Hawk Intensive Nesting Survey volunteers have been keeping tabs on the diet of another adaptable raptor. As I recall from Fish’s presentation at a recent Friends of Five Creeks meeting, they collected remains from under perches where the local Coops plucked their prey rather than rummaging around in the nests. 

Of the identifiable, 99 percent were birds, 1 percent mammals. “Rat remains don’t get left at pluck perches,” Fish explained. The top three bird species, representing 63 percent of the number of individuals and 79 percent of the biomass, were robins, mourning doves, and rock pigeons. Not to be outdone by the peregrines, one Cooper’s hawk got his or her own budgerigar. The mammalian minority included Norway rats, woodrats, and fox squirrels. I would strongly encourage more fox squirrel consumption. 

Coops, along with the smaller sharp-shinned hawk, were once regarded as “chicken hawks” and generally shot on sight. I am relieved to report an absence of chicken parts in the 2002-03 sample, although you have to wonder if that may have changed with the recent backyard poultry boom. It would be best not to put temptation in the hawks’ path. Keeping your chickens enclosed will also protect them from night-raiding raccoons.