Sunday, May 30, 2010

Cowbirds, Chipping Sparrows, Jay and Kay in Tulsa, and Blakeman on Red-tail Eyass Death

Photo by Donna Browne
Two Cowbird Eggs in a Chipping Sparrow Nest-

In the "row" of three eggs on the right, the top and bottom eggs are those of a Cowbird.

Interestingly the eggs photograph with more of a different hue than they appear in life. I had to scrutinize carefully to decide that two were actually different enough in size to be positive the nest was a victim of brood parasitism which seems obvious in the photograph

Year after year the Chipping Sparrows in my Wisconsin neighborhood are parasitized by Cowbirds. Four years ago, there were three pairs of Chipping Sparrows in my and the neighbor's adjoining yards. All three nests were parasitized. For the past two years there has only been one pair of Chipping Sparrows in the area. I became concerned about the parasitism in my own area and also because the Chipping Sparrows I'd watched in the yards of relatives had disappeared altogether.

Chipping Sparrows were previously considered quite common here but with the increase of the Cowbird's range, human depredation of habitat has expanded the Cowbird's range exponentially to the point where the U.S. Park Service is beginning to be quite concerned about their impact on many species not just the uncommon ones.

In fact, in Yellowstone Park, rangers have begun checking nests for Cowbird eggs. When Cowbird eggs are detected the rangers pick the Cowbird eggs out of the nests, shake them vigorously, hoping to addle them so that they don't hatch, and then place them back in the nest. Evidently, disposing of them may sometimes encourage Cowbirds to lay new eggs in those nests.

As we're always talking about how most birds can't count when it comes to eggs, chicks, and the like, (it's also possible that they ordinarily don't care if an extra eyass or duckling for example shows up), then how do Cowbirds know they should lay more eggs? That their eggs are gone?

Do they recognize their own eggs as a species? Is it a spatial recognition of the nest size and brood mother? Whatever the case it appears they know.

Last year I was surprised (And happy, I admit.) that the Chippies weren't raising a Cowbird instead of the there own chicks. I later was told that a neighbor, unnamed, had found the Chipping Sparrow nest, saw a Cowbird egg in it, and removed it. Evidently in that case no Cowbird noticed and laid a new egg or eggs to replace the missing one.

Well this year, I walked past the Clematis on the north side of the house and a little bird flew out and whipped past me. Then it happened again. Aha! A Chipping Sparrow had zipped out. I investigated, and sure enough there was the little neat nest with five eggs, two of which were slightly different colored and the the big clue, two of the eggs were slightly larger than the other three.

Now I'm in a quandary. It is illegal to disturb bird's eggs, and ordinarily against my ethics not to let nature take it's course, except in cases of sick or injured birds for instance, but now I admit, I'm sorely tempted to addle a couple of eggs.

Next up John Blakeman on Eyass Death--


Here's the problem, with all of the nests and deaths, I think.

Your perspectives on each of these are tainted by your experiences with Pale Male and the other NYC RTs. In fact, eyass deaths are much more common than the urban nests (in former years) have experienced.

Still, each of these needs and has an explanation. For your rural Wisconsin birds, they mirror the ones I see here in northern Ohio. Pigeons are just never a part of rural red-tail diets. They are too hard to capture in barnyards. The rural birds depend on voles, which were common in pastures. But pastures are gone in Ohio, and ever more infrequent in Wisconsin, with modern dairy practices (or conversion to row-crop farming).

Dependence on voles will produce only one or two eyasses. In row-crop areas like southern WI and northern OH, the only vole habitats are grass margins of roadsides. Makes it tough to feed two or three eyasses when they get larger. I'd bet that starvation or starvation-induced disease is the cause of the eyass deaths. Starvation-induced parasitism can be a factor, with capillaria or other gastrointestinal worms overwhelming the bird.

The presumption that 80 or 90% of the hatched eyasses just ought to fledge isn't realistic. That happens only when everything is perfect, as it once was at 927 Fifth Ave and now, for a second year, at the Franklin Institute in Philadelphia.

But the death of eyasses is rather common in nests with more than one eyass. I've never seen a northern Ohio RT nest with three eyasses. There's just not enough food to support these. The average is about 1.7 eyasses. I think a lot of nests have two eggs, but deaths of the weaker eyass bring down the number at fledging.

It's a lot tougher to raise eyasses than shown by PM and the other famous urban nests. Urban areas have both rats and pigeons (non-native species, as it happens) that provide ample prey. Vole-based red-tail populations have it much tougher, as you are seeing.

This contrasts with Western red-tails, where ample ground squirrel populations in the first half of the summer provide large prey bases. Western nests average between two the three eyasses. But eyass survival difficulties are simply postponed until after fledging, as all of the Western ground squirrel species go into estivation, summer hibernation, during the mid- and late-summer drought periods of these areas. Instead of starving on the nests, as in the East and Midwest, Western eyasses starve after they fledge, being unable to find the now-estivating ground squirrels. They have to turn to voles, which can be hard to capture in the taller herbaceous vegetation of summer.
Weather has no effect on frounce frequency. It's always frounce weather in the windpipes of pigeons. Exposure to cold and rain seems to have no real effects. The down of the eyasses just keeps them warm, even when wet.

And the Riverside nest looked to be a winner, I thought. I was rooting for it, as it was the only typical tree nest in the area. The birds were behaving as real red-tails should. But 70-mile per hour wind bursts can blow out any stick nest.

--John Blakeman

Thanks John, you make a very good point. Not all that many calories in a vole. Gotta nab lots of them. Whereas a pigeon or a chubby rat is a much larger chunk of food with far more calories with a single hunt.

I've still never seen a Mourning Dove go into a nest here. They are much quicker than a pigeon so perhaps just harder for Red-tails to predate.

The M pair were able to raise their two eyasses last season but they not only had the verges of the road to hunt they also had the verges of the railroad tracks on the far side of the field they nested in.

I also saw them feed the eyasses a snake at one point, so skilled and adaptive, and though I never saw bunny go to the nest, last year was a very good year for rabbits here. I rarely could tell exactly what they had as they were secretive and of the prey was small so often voles or little rodents of whatever description.

The summer before, I used to watch one RTH in particular, hunting in a mature cornfield by following the rows at tassel top when I'd drive by, though I never saw an actual kill.

I was concerned when I watched the Cathedral nest this season that Norman wasn't bringing in as much food as was delivered in previous years. If that was actually the case, starvation may have had a hand in the smallest eyass death in particular. Isolde spent far more time off the nest than she had in previous years so some of us surmised she was hunting for herself. She may eventually have taken prey to the nest as well but I don't know of a confirmed observation of that so far.


Word from Sally of Kentucky on the Tulsa cell tower nest of Kay and Jay plus the Portland Fire Escape nest--
Kay and Jay of Tulsa have their one eyass busy branching on the cell tower, and so far Portland still has two healthy-appearing eyasses ready to fledge soon, they have been "branching" to the railings and other areas of the fire escape the past few days.

Photo by Donna Browne
This evening I got a call that the moon had risen more reddish gold then usual though at the time it was likely too low on the horizon for me to see from Rainbow Drive. I went out to reconnoiter and discovered a thin place in the trees and so got a photograph of a "vegetative moon".

Donegal Browne

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