Monday, May 07, 2007

Blakeman on redtail proliferation and a correction at the end

Female redtail feeding two young at Inwood Hill Park
Photo by Bruce Yolton
- May 5, 2007


The reports of multiple breeding red-tailed hawks in the various boroughs and regions of New York City are particularly gratifying. They substantiate my previous contention that red-tail habitats in the rural areas of The East are saturated with breeding adults. New adults have a hard time finding open habitats or territories, so they've drifted into the cities and begun to adapt their hunting and breeding behaviors to urban environments.

Everyone observing these hawks should understand that these new pairs and nests are essentially unprecedented, beyond any red-tail expert's prognostications. No one ever anticipated this urban incursion. A search of the red-tail literature is unlikely to find many historical accounts of these hawks breeding more than incidentally inside major cities in the past.

First, it was Pale Male in Central Park, and for a decade he and his mates were only a curious anomaly. Those of us who know the species in typical rural areas had to finally accept Pale Male's continuing occupation of Central Park. But honestly, few of us thought that there would ever be more than two or three other incidental urban nests and territories in a place like NYC, either on Manhattan proper, or out in any of the more open boroughs.

All of this is a very recent phenomenon, beyond what anyone would have expected. Until Pale Male began the affair, red-tails just didn't nest on buildings or in dense city centers. And if they did, they certainly wouldn't persist there. Surely, only a few other red-tails would continue the experiment.

But now, red-tail nests are all over NYC. Who knows how many are yet undiscovered. There is no longer any doubt that these big hawks are in the city to stay. Only two big biology questions yet need to be answered, and I post them for everyone's consideration.

(The earlier big, pivotal questions have been answered. What, in the city, are these vole-loving hawks eating, and how are they capturing their prey in the city? NYC hawkwatchers have provided the answers.)

The two remaining questions are these. How closely related are the NYC red-tails? Are they closely-related siblings or cousins, or are they multiple, unrelated individuals from distant rural populations?

The second big question is what will be the final, saturation density of the red-tailed hawk population? In the end, how many red-tails will be able to live

This last question could be answered by simple observational diligence. Someone needs to be putting nest dots each year on a map of NYC, showing annual population increases. In time, the number of dots (as they have out here in rural areas) will stabilize.

But the other question, the relatedness of the population, can be answered in only one way. A number of observers have noted pale-headed NYC red-tails, implying descent from Pale Male himself. Personally, I'm not very convinced by this. I have here in northern Ohio a big female red-tail that I've watched now for two years who has a remarkably pale head. This is coloring is infrequent, but not so much as to accurately impute descent from any particular pale-headed adult.

The only way to determine if any of the CP or greater NYC red-tails are related would be to begin a banding program of the eyasses on nests right now. Eyasses can be successfully banded almost right after hatching. By the end of the first week, banding is always successful as tarsus (leg) width approaches that of the adult. The band just won't slip off then.

And for those who would be concerned that humans reaching into a nest and picking up eyasses would so disrupt things that the adults wouldn't come back or feed the young, let me assure that this is an artificial contrivance intended, perhaps, to keep little boys from messing with schoolyard robin nests. Thousands upon thousands of birds have been banded as chicks in the nest, and parents always come right back and properly attend to their duties. Neither adults nor eyasses will be disrupted by banding. It's not an issue of any concern. I've assisted in the banding of a number of red-tail eyasses on nests, and in every case all was well in the end.

Because the CP and NYC red-tail populations are new and unique, special permission should be gained from US Fish and Wildlife Service authorities to band not only with conventional bands, but also with colored bands to allow easy ID of locally-produced eyasses.

Until the NYC red-tail eyasses are banded, no one should be suggesting that any new birds are descended or related in any way to any other known red-tails. Most of that is mere hope or romance. The biology right now doesn't support such suppositions.

And as I mentioned sometime in the past, genetically, for survival, it would be far better that all of the new NYC parents were unrelated. If the majority of the new NYC red-tails are descended from Pale Male in Central Park, inbreeding will be a problem in consequent populations.

One last point. The question of whether or not maturing young adult red-tails actually return to their natal territories, or even to their general natal regions, is really unanswered. Today, with all the new NYC eyasses on nests, there is a unique opportunity to answer this question.

Once again, a proper study of the NYC red-tails could provide missing information for the species. Raptor biologists don't really know for sure if youngsters return as adults to their natal territories. There are a pile of red-tail eyasses sitting right now on NYC nests awaiting bands to answer this question.
--John Blakeman

PS A correction
Bruce Yolton writes in about yesterday's posting:

In your report on the St. John the Divine nest, you wrote
"Though these hawks hunt in Morningside Park, not Central Park, it is one of the neighboring redtail nests featured here this year, in the sad absence of nestlings of our own."

Although the picture shows a rodent caught on the grounds of the Cathedral, both St. John's adults hawk do hunt in Central Park. Both hawks are seen in the area around the Great Hill frequently and do so both during the nesting period and during the "off-season".