Sunday, September 05, 2010

Do Very Young Red-tailed Hawks Get the Migration Urge? For That Matter, What About the Adults?

Photo: July 24, 2005 D.B.

Little on fledge day-- one of a pair of young from Charlotte and Pale Male's second clutch of that season.

An Update from Riverside Hawkwatcher Pam Langford--

More good news -- one baby caught a pigeon today! The worrier in me hopes it wasn't an easy catch because of disease. Interesting discussion about migration.


Many thanks for the update.

Indeed, one hopes it wasn't disease though disease isn't as likely as it might seem. Except for frounce which doesn't really affect an infected pigeon, many common pigeon diseases, such as polymiosis virus (PMV), which adversely affects a pigeon's nervous system making them easier to nab, aren't communicable to hawks.

From my contacts with the pigeon folks in NYC, there is some PMV birds out there right now. It's most common in warm weather. On the other hand, the Riverside parents may just have trained their youngsters well and this fledgling is an ace.

Best regards,

And the next day a second email from Pam concerning John Blakeman’s prognosis for the Riverside young--

Mr. Blakeman said that the urge to migrate in juvenile hawks is triggered by shorter days. Do we know how a juvenile hawk's age might affect this response? After all, the Riverside babies are younger than most Red-tails are when September rolls around. I wonder if their age means they won't respond to the same cues that motivate older juveniles because they are at a different developmental stage. Since what happened at Riverside this year is unusual, it's possible that no one knows the answer to my question.

Thanks again for this interesting discussion. By the way, hawk watchers at Riverside yesterday were treated to the sight of a visitor. A swan was making himself at home in the marina! I've never seen that before.


I've done some searching and haven't found a study on late second clutch fledglings and migration so we'll have to go with the limited anecdotal information we have from observation and possible comparisons with other species.

By the way, one or both of the Riverside Fledglings appear to be precocious hunters-first a well hunted mouse and then a pigeon. We've rarely seen this much successful hunting this early in maturation in a young urban Red-tail's life.

John Blakeman has told us about the phenomenon in rural hawks when the young are sometimes chased from their parent's territory. The thought is that the young are pushed out of their natal territory when the prey base has become thin or when it is time for the parents to begin their yearly courtship behavior.

As far as I know, we've not seen this in an urban environment and I've only seen one case in rural Wisconsin. Some years ago in mid-winter I saw a pair of Red-tailed hawks chasing a brown-tail, who looked remarkably like them, (albinistic characteristics) in circles through the trees in a small area. Finally the young hawk was chased and knocked off branches enough times that he at least flew across the road into dense branches, followed closely by the adults, where I could no longer observe their behavior

Though I can't absolutely swear that this young hawk was the progeny of the adults of the territory, I would argue strongly that they were the parents of the young hawk from the observations of their physical characteristics. Therefore we could hypothesize that this young hawk had not migrated in the Fall and it therefore follows that not all first year hawks do migrate or at least don't go very far from their natal territory.

And if that is the case we've no absolute reason to believe that hawks that are so endocrinalogically immature would absolutely respond to the shorter photo period hormonally produced migration urge and whip off to parts unknown.

And another point for thought from NYC Bill from the comments section--
As an ex-NYC tour guide who used to ride around on a double-decker bus pointing out the sights, I was often impressed by the fact that the 927 hawks (PM and Lola) never seemed to go anywhere except across the park to the Beresford (next to the AMNH) for the winter. And that was to the west. I guess I supposed that RTHs didn't migrate very far at all... if ever.

Previous hawkwatchers have reported that some earlier mates of Pale Male had "disappeared" in the winter and returned in time for the yearly courtship and bonding rituals of Red-tailed Hawks. In one mate's case, if the person who purportedly found the deceased mate and her band is believed, she'd only gone across the river to New Jersey, conceivably back to winter in her natal territory.

Pale Male has been closely watched since he arrived and doesn't appear to go anywhere in the winter and never has. He hasn't migrated, even as a Brown-tail. He did have to come from somewhere though as there were no observed nesting Red-tails in Manhattan previous to his nest for a hundred or so years. (Unless of course some pair nested off the birdwatcher radar in Manhattan or he only made a short hop across the river from Jersey, hardly counting as a migration from my definition.)Though as was said, some of his mates seem to have left for the winter. Lola on the other hand, no matter the behavior of her predecessors bides her winters with Pale Male in Central Park.

But what about all those hawks that fly by Hawk Mountain every year?

This is when I had to go back to do even more delving. Sorry it's taking so long. There is a lot out there, but very little of it specifically pertains to the question that we have on the table--specifically urban hawks and migration habits.

It hit me. Wait a minute-- I think I know the definition of migration but let me look that up.

Ah ha! From The Earthlife Web--What is Migration?
"In everyday speech migration is regarded as the mechanism behind the seasonal appearance and disappearance of some species of birds, mammals, fish and insects. To most people migration is birds, and perhaps mammals. Though in fact many insects, some mites and spiders, some reptiles, amphibians and even plants migrate regularly. Here we are concerned with the migration of birds.

A more exact definition of migration is the mass intentional and unidirectional movement of a population during which time normal stimuli are ignored. This allows migration to be distinguished from dispersal.
Dispersal is multidirectional, often only involves part of a population and does not involve the ignoring of normal stimuli."

And there we have it.

Red-tailed Hawks, at least the Red-tailed Hawks I'm familiar with do not migrate. They disperse. And then only sometimes. When it comes to dispersal there is a choice and various stimuli can affect that choice. For instance your parents run you out as food is getting scarce. I've seen it in Wisconsin but never in NYC.

Plenty of food, plenty of unclaimed territory near by? Why go further?

There is also the possibility that one of the reasons that individual hawks become urban hawks and start a line of urban hawks is because they have a low urge to disperse, perhaps we could say, they genetically just have less wanderlust as adults and therefore may have less as immatures. Plus they are genetically capable physiologically of becoming human habituated.

A little like the wolves ages ago who were capable of becoming human habituated genetically and eventually evolved into dogs. And at least with dogs it's believed that taking this path had much to do with their endocrine systems. Hmmm.

And the endocrine system in birds has much to do with migration.

Of course even rural Red-tails sometimes stay around as they are responding to normal stimuli. "Lots of food, not that cold, snows not deep, guess I'll stick around."

Red-tailed Hawks can make a choice. Therefore my hypothosis is that some of those juveniles in Central Park are local.

In order to prove my hypothosis, the young urban hawks would have to be banded and itb is my understanding that at least in Central Park and perhaps any number of other NYC Parks there is no banding allowed.

Donegal Browne