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the cuckoo-y tastes of peregrines

Seeing a Peregrine Falcon is not very unusual in southeast Michigan.  Although they were not historically found in these parts, they were introduced into the city of Detroit beginning in 1987 as part of the Midwest Peregrine Restoration Program (MPRS). For the last 10 years, there have been five to seven Peregrine territories in southeast Michigan, including some now-traditional nest sites on rather famous
buildings in downtown Detroit, including the Whittier Apartments, the FisherBuilding, and the Book Building.  So, we have resident birds in addition to migrants (the Southeast Michigan Raptor Research hawkwatch tallies about 50 each fall). I’ve even have one on my yard list.

Therefore it wasn’t too surprising that a pair (the male was banded with a MPRS band) took up housekeeping this spring on the central campus of the University of Michigan in Ann Arbor, on the Burton Memorial bell tower.  Although a nest couldn’t be seen, the behavior of the birds indicated the female was on eggs for a time, but no young were produced (although the music was halted so as to not disturb them).

The most interesting part of this story, to me, was that my friends over in the bird division at the Museum of Zoology routinely collected body parts at the base of the tower that had been dropped or not eaten by the falcons.  They kept a running tally of prey items from the beginning of April through the end of October, with a two-week gap when the collection manager took a vacation.

Urban falcons are frequently said to favor Rock Pigeons as food.  This pair had much more unorthodox tastes.  I was quite astonished to see that the remains of at least 71 cuckoos were found at the tower, with slightly more Yellow-billed than Black-billed. Granted, Ann Arbor has quite a bit of green space, and there is a large cemetery, the arboretum, and parks along the Huron River in the vicinity, but I would not have imagined there being that many cuckoos around, especially careless ones.

These Peregrines must have specialized in secretive prey, as the leftovers also included nearly 20 rails, mostly Soras but including, incredibly, a Yellow Rail, a state-threatened species considered one of North America’s most elusive and mysterious birds, a species last recorded in Washtenaw County in 1925! Least Bittern was also on the menu, along with regular servings of Pied-billed Grebe, Green Heron, and Northern Flicker.

Not eaten regularly, despite their ubiquity, abundance, and reported favor, were Rock Pigeons, of which it appeared only about a half-dozen were taken.  House Sparrows and European Starlings were represented on only two occasions each.

I found few readily available lists of Peregrine prey items.  One from Regina, Saskatchewan was also heavy on grebes and Soras, but Rock Pigeons also came in at 18% of the prey. Prey analysis of Peregrines nesting at the University of Alberta in Edmonton show few Rock Pigeons but lots of gulls, and the web site notes the importance of marsh birds even in urban Peregrine diets. In the U.K., an observer of Peregrines in Bristol has come to the conclusion that the idea of pigeons as a staple food for city falcons is something of an urban myth.

This may be just as well.  Analysis of a dead Peregrine from Baltimore in the 1980s [1] determined it had died of a gram-negative infection, associated with sublethal lead ingestion in other raptors.  Pigeons made up 93% of this bird’s diet, and samples indicated very elevated levels of lead in the blood and organs of the pigeons — a common occurrence, at least at the time, for pigeons that lived in cities with high traffic density.

This might not be as great a threat now that leaded gasoline has been phased out.  But studies in British Columbia have shown that although DDT has not been used since the 1970s in a local agricultural area, even short-lived birds are still contaminated with the break-down products.  It was estimated [2] that feeding on as little as 10% of species such as starlings, robins, and gulls would have damaging effects on Peregrine eggs.  Detroit Peregrines eat Ring-billed and Herring Gulls, which nest in the Detroit River. Levels of PCBs, DDE, and dioxins have remained steady or increased in local Herring Gull eggs the last decade.  Contaminated prey in urban areas is just one more thing to consider when evaluating the wisdom of introducing wildlife into cities.

Hopefully, those cuckoos were clean-living, and the Ann Arbor Peregrines didn’t get a dose of unwanted chemicals with their exotic meals.

[1] DeMent, S.H., J.J. Chisholm, Jr., J.C. Barber, and J.D. Strandberg. 1986. Lead exposure in an urban Peregrine Falcon and its avian prey.  Jrl. Wildlife Diseases 22:238-244.

[2] Elliott, J.E., Miller, M.J., and Wilson, L.K. 2005. Assessing breeding potential of peregrine falcons based on chlorinated hydrocarbon concentrations in prey. Environ Pollut. 134:353-61.

Photos from Wikipedia, falcon by Joe Kosack / PGC Photo.

Filed in Birds, Natural history

Comments on this entry are closed.

  • Gavan Watson November 28, 2006, 10:03 am

    Fascinating and very cool. Thanks for sharing. So now I wonder (and I think you're hinting at a hypothesis) how Peregrine diet is affected by the urban-ness (for lack of another word) of their home range.

  • Nuthatch November 28, 2006, 10:17 am

    Er, well, yes. I have a bit of a jaundiced view of some of the "reintroduction" projects I have seen in urban areas. They are sometimes championed and even undertaken by well-meaning citizens without strong scientific backgrounds, and then — incredibly, in my mind — backed by wildlife agencies because of all the high profile good will, I guess. I don't necessarily include the Peregrine project among these, and I think it has been a nice success story. I do wonder about the amount of money spent on urban falcon introductions when history and common sense would seem to dictate focusing restoration on areas where they once occurred.

  • Gavan Watson November 28, 2006, 2:23 pm

    Well, let's not forget that when humans are involved in the natural world, it's never so cut-and-dry that "scientific" decisions get made for "scientific" reasons.

    I wonder, now that Peregrines are enjoying some kind of success with re-introduction (albeit perhaps not based on the most sound reasoning) if we will get to a point where they will make a transition from success story to pest, especially given the information that this pair of Peregrines hunted a state-threatened species (not in great numbers, it seems).

  • Nuthatch November 28, 2006, 2:30 pm

    They are already unpopular with racing pigeon owners! Take a look at this item, one of many references I found to this "problem." Apparently pigeon fanciers have illegally killed Peregrines and other birds of prey believed to have taken their birds. Heavy sigh.

  • Clare November 28, 2006, 2:55 pm

    Cool, once again you've shown me another way about thinking about a species I thought I knew.

    I'm amazed at the inclusion of rails in the diet. Again, my perception was that they would have mostly taken birds on the wing and rails are a species I don't associate with a lot of flying.

    I'm also thinking I should visit underneath our peregrine's nests and see what they are eating. I'm guessing that it includes a lot of gulls.

  • Roger B. November 28, 2006, 4:52 pm

    Here in the UK, the peregrines which nested on Derby Cathedral brought home a wide variety of prey.

    Some of the prey species were rather unexpected. The list included water rail, common tern, golden plover, lapwing, turnstone, redshank, woodcock, snipe, jack snipe, dunlin and gadwall, all of which are scarce in urban environments. This suggests that either the peregrines were ranging far and wide from their city base, or they were preying on birds which migrate unseen through (or over) urban districts. It's believed that this particular pair of peregrines specialised in hunting nocturnal and crepuscular species.

    There's more info on the Derbyshire Natural History discussion group at http://pets.groups.yahoo.com/group/derbyshirenaturalhistory/

  • Clare November 28, 2006, 5:42 pm

    I was also going to mention that when I was at Depot (The RCMP Training Academy) there was a pair of Merlin nesting in a tree just beside my dorm and the parade square. In a pretty unscientific survey I'd have a look under the nest when I could when we were forming up for parade. To my casual eye it seemed that much of the prey was House Sparrow.

  • Nuthatch November 28, 2006, 5:54 pm

    Perhaps Clare has the answer to the Ivory Gull decline

  • Mikko November 28, 2006, 6:11 pm

    The Yellow Rail brings into my mind few events of the same kind from Finland:

    About a year ago, a dead Siberian Rubythroat (Luscinia calliope), third record for Finland, was found from a nest-box among other dead birds. Apparently a Pygmy Owl (Glaucidium passerinum) had stored it there for the winter.

    And in 1990's a single leg of an Eurasian Stone Curlew (Burhinus oedicnemus) was found from a Peregrine's nest in North Finland. This was the 9th (or so) Stone Curlew for Finland.

    Peregrines mainly eat waders and ducks in Finland and northern Sweden, but also many thrushes and crows. (They nest almost entirely on fens here.)

  • Nuthatch November 28, 2006, 6:20 pm

    Do they nest on the ground in fens?

  • Mikko November 29, 2006, 3:03 am

    Yes, they nest on the ground in fens, usually on open tussocks in the impassable (wet) parts of the fen.

    Before the decline in 1940's/50's peregrines also commonly nested on cliffs in Finland, but they have returned only to the fens. (An estimate from 2002 tells that there are 150-170 pairs on fens and 12 on cliffs.)

  • Nathan B. December 1, 2006, 10:55 am

    There's a pair living in the clock tower of the University of Detroit Mercy McNichols campus here in Detroit, MI.

  • farlane December 4, 2006, 10:00 am

    Finally was able to get a link to this great post up on Absolute Michigan.

    Back in May we featured the Macomb County Falcon cam. Apparently the birds abandoned their nest soon after. Is that a common behavior for peregrines or is it likely a result of the more chaotic urban environment?

  • Nuthatch December 4, 2006, 12:16 pm

    Both of the Macomb nestings failed when the eggs broke or disappeared, probably removed by the parents when they were found not to be viable. So disturbance was not the reason, but egg shell thinning or infertility, which in turn might be caused by… that's right, a contaminated diet. I know at least one unviable egg was taken for tests, but I don't know the outcome.

  • Robert DeCandido, PhD December 23, 2006, 1:51 pm


    You might want to mention that much of the hunting of cuckoos, rails and other passerines by Peregrines occurs at night. There has been some nice research on this lately…The use of skyscrapers as a research site (to study night migrating birds) is just catching on.

    One other question: did the array of cuckoos caught come from certain time frames or…you get the drift of my question I am sure.