Wednesday, November 14, 2007

Blakeman on Cooper's and Kestrels Plus a Krideri Update

American Kestrel, Falco sparverius
Raptor-man, John Blakeman takes on some of my questions concerning Cooper's and Kestrels.

The Cooper's Problem is a severe pressure on kestrel populations in the East, South, and Midwest, where both species tend to coexist. Until recent years, Cooper's were wary and tended to stay in uninhabited forests and woodlots. In the 20th century, there are virtually no records (except in the last decades) of Cooper's ever breeding in cities and villages.

The bird has changed. Natural selection has disfavored the naturally wary birds. After DDT disappeared in the environment, Coop's resumed producing 4-5 eyasses each year out in the deep wilds. As with the red-tail, they saturated remote breeding territories, and a few callous ones drifting into cities, where, lo and behold, they discovered a Cooper's hawk feast in every backyard.

It's birdfeeders that have changed everything. They concentrate dickeybirds, allowing Coop's to simply pluck off the day's calories with remarkable ease and consistency. Twenty years ago, a few bold hawks started to disregard humans, cars, cats, dogs, and everything else and errantly began to nest in cities. Everything else is now history. The bird has self selected for urban habitation and human tolerance, making the Cooper's hawk population a literal order of magnitude greater than ever before.

And they find kestrels just out of the box, little so-called "branchers," the easiest of all protein sources. In Ohio, we are finding virtually no immature survival in areas with resident Coop's. And we also think they take an occasional adult. Our kestrels survive only out in wide open row-crop areas where Cooper's don't hunt.

Frankly, it's all caused by backyard bird feeders.

The exact same thing happened in the 19h century, with red-tails in the East and Midwest. In presettlement times, RTs were found only in savannas and forest edges that overlooked large open uplands. The conversion of eastern North America to farmlands made them into open savanna-like expanses, into which the red-tail invaded and has lived well ever since.

Your Coop's are likely to stay for the winter. Will those with bird feeders stop feeding the dickeybirds in winter? As long as the birds are their, the Coop's will hang around, unless you are way up in the boreal parts of WI. It's the kestrel that more likely to head south, if and when winter snows hide the voles.

And here's something to watch for. From Ann Arbor and Detroit south, into my northern Ohio, a moderate number of kestrels stay for the winter. At higher latitudes, they all leave.

But in these northern fringes of the winter range, there is a marked high proportion of kestrel males, just like in your photos. When I was doing my undergraduate kestrel studies, I noticed about a 3 to 1 male to female ratio in winter. In Kentucky, it's equal, or slightly in favor of the females. Why?

I think it's this. Females are just a bit larger than males, about 120 to 130 grams or so. Males are 95 to 110 grams or so. I prognosticate that the females will need to consistently catch two voles a day to survive, whereas a male can persist on an average of one vole, or so. Now it may be 3 and 2 voles, I'm not sure. But at the southern WI and northern OH latitudes, with our shortened winter day length and deeper and longer-lasting winter snows (which hide the voles), the big females just can't always capture enough voles over time to stay healthy. They get hungry and head south. The smaller males, in a week's average time, need just a fewer voles to survive, and they are able to capture them. Hence, the curious winter sex ratios at our latitudes.

And yes, kestrels cache their food, especially voles caught in the afternoon or late in the day. When they wake up, they will eat an entire vole. But the second or third one can be too much, so they save it for breakfast.

Sincerely,John A. Blakeman
My thought about bird feeders is that they've become the replacement for hedgerows, unmown road verges, pastures, plowed prairies, rows of crops without herbicides and the areas where seed heads used to mature and where winter birds, the voles, all the rodents, used to feed themselves. For without winter residents who subsist on seed, and take cover in scrub, there too go those creatures that are supported by them.
Of course this is no more than an untested hypothesis, at least as far as I know, so more data needs to be collected.
Already the Loggerhead Shrike, who's habitat consists of open country with scrub is declining in many areas and is endangered in numerous states. It is more wary of feeders and therefore is less likely to find prey in the manner of the feeder haunting Northern Shrike who's populations are not declining as rapidly.
Plus, there was another sighting of the Krider's Red-tail. Rushing off to the lawyer's office, I heard the train whistle and by the time I made it to the long line of cars waiting at the track, there the Krider's was. Flying from the rear of the train over the top as the train went into the distance, she flapped towards the trees. She headed straight for a large tree adjacent to the Dollar General and inside of going under, over, or veering to the side, at the last minute I taught she might have been going for a perch at top speed, she turned sideways and zoomed through the branches without touching a one and then continued to flap to the refuge of the woods. It was an amazing piece of flying
Donegal Browne

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