Monday, April 18, 2005

Blakeman answers some previous questions

4/17/05

Marie,
Got some time this afternoon. I was to do a nice 50-acre prairie burn, but rain doused that idea, quite literally.
Mai Steward submitted some good questions. Here are my thoughts.
1. I've been wondering about all the digging / footwork that goes on in the nest, as noted by Donna Brown -- are the hawks just making the nest more comfortable for themselves (esp. Lola, since she spends so much time just sitting there), or is there another reason for this foot activity, perhaps connected with the incubation of the eggs?
Mai, Donna and some others have noted this. I don't know what it is. I don't recall my captive breeders doing any over-the-eggs-dance, and I don't recall seeing this in wild nests. But the birds do a lot of wiggling, rocking, and maneuvering when settling down. Perhaps this appears to be digging, from distant, lateral viewpoints. But it may also be a real digging motion. Perhaps this relates to the pigeon spikes below the nest. Perhaps the birds are repositioning lining materials as they feel the ends of the spikes below the nest. Nesting red-tails often reposition lining and even some feathers around the eggs in the nest. But I'll have to plead ignorance on this.
2. I've noticed how protective PM is of the nest site, it's really cute and touching -- probably this is deeply ingrained instinct -- and also how Lola seems to know, many minutes in advance, of PM's arrival, even when he's out of sight -- as reported by Donna Browne, Lola's head is up, she's alert, looking intently in the very direction from which PM soon swoops in .
Remember, red-tailed hawks are animals of vision, not smells, or even much of sounds. What they see is essential to their lives. And they have remarkable eyesight, as everyone knows. So yes, they are in constant visual contact, or at least they have a good perception of where the other mate is when out of site. Lola, when sitting on the nest, has a very good idea of what Pale Male is up to when he comes into sight. If he’s got a food morsel, she can see it from across the Park or way down the street. If PM comes back with nothing, she understands that he’s gliding in to give her a breather, a chance to stand up, defecate (“slice”), and even take off on wing for a time. (We human dads aren't nearly so instinctively understanding and accommodative of the similar needs of our wives have while tending to our children. We have some lessons to learn here.)

3. Regarding the ledge-nesting of not just PM, but also the younger pair on CPS, I thought this was the result (esp. in PM's case) of harassment by crows and blue jays -- that PM + mate had attempted several times to create nests in trees, but they were forced to give up because these birds made it impossible for them -- or am I mistaken? In fact, all the birds in CP live quite close together, compared to out in the wild -- Was this a factor in PM's retreat to 927?
This is an interesting, even plausible reason for the abandonment of initial tree nests inside the Park per se. But, I don't think so. Doubtless, red-tails can be harassed unmercifully by crows and jays, and they often just retreat to some distant perch when so pestered in non-breeding areas. But don't forget that should a red-tail elect to do so, it can grab a mobbing jay or crow right out of the sky.
We know from one or two very well done studies that most birds are able to detect the aggressive intentions of red-tailed hawks, especially as they relate to the hawk’s hunger. I've personally seen this with perched captive red-tails in my backyard, and occasionally with wild eyasses out on their first personal hunts in July. If a red-tail is hungry and on the hunt, many species of smaller birds can detect this from the hawk’s body stance. The hawk leans over and has an “I'm hunting” look. Humans can discern this after a bit of time. (This is a wonderful experience for me. As I travel down country roads and see red-tails perched, I can almost always read the bird’s mind set, as expressed by it’s posture. I can tell when it’s hunting, loafing, or watching nearby threats. It’s all body posture. Birds have learned this instinctively.) If the hawk is hungry, songbirds will mob the hawk, trying to drive it off. If it’s not hunting, the other birds pay no attention to it. as it presents no threat.
But here’s a remarkable story, one that my compatriot doing the two-year study of Ohio red-tails observed. It was noted that almost all red-tail nests in flat, Lake Plain northern and northwest Ohio had the remains of male red-winged blackbirds. Red-wings commonly nest in harems in hayfields and along ditches. A proud, testosterone-warped male (with the bright red epaulet wing feathers) stands guard over his several wives raising young in nests within the male’s territory. Now a red-tail can't possibly capture a free-flying, diligently observant red-winged blackbird. When being chased by a hawk, they fly too adroitly to be captured. We wondered for some time where and how so many red-tails were capturing so many red-wings, and why they were always just males, never a female.
The answer may relate to jay and crow harassment in Central Park tree nests. (Or, maybe it doesn't – but it’s good story, anyway.)
Here is what was observed. Red-tails are famous for “doing the rounds,” for punctually moving from one hunting perch to another while circulating around a hunting territory. At 10:30 AM a resident male could be seen sitting in a particular tree, peering out over the landscape for prey. Every day at about 10:40, the bird would then take off and fly a half mile to another, well-used hunting perch. After a time there, the bird would move on again. In watching this every day, we were able to put dots on maps signifying perches, along with arrival and departure times. The hawks are noted to be very methodical and punctual in covering the entire hunting territory, often several square miles. None of this was random wandering.
It was noted, however, that when moving from hunting perch to hunting perch, the hawks often passed over hay fields or stretches of roadside ditches with resident red-winged blackbird populations. And as mentioned above, the patriarch red-wing males could see that the hawk passing above was on the hunt. The red-wing flew up to the passing hawk on the first day, mobbing as it flew over. It was trying to protect its females and young in the nests below. Just as soon as the hawk passed out of the red-wing’s territory, the blackbird dropped back down to attend to its females.
But because the hawk flew over the red-wing area each day on its hunting rounds, the red-wing male (males being males), became ever more perturbed with the passing hawk. Each day, the mobbing red-wing would fly every closer to the passing red-tail. At first, the blackbird stayed 4 or 5 feet away in its pesterings. The next day, it got closer. Finally, on a final day, after being impossibly emboldened, the red-wing actually dropped down onto the red-tail’s back – a fatal mistake. At this point the hawk turned over and snatched the red-wing out of the air with instant ease.
This was seen several times, and we are absolutely certain that by flying repeatedly over the red-wing areas day after day, the red-tails were setting up the red-wings for an easy, effortless kill. They deliberately suckered the red-wing males in by slightly slowing their flight and dropping down to just 20 or 30 feet above the nests of the blackbird nests below each day. The red-wing males perceived, quite erroneously, that they were successfully driving off the marauding red-tail each time. But in fact, the red-tails were setting things up for an easy meal for their eyasses back at the nest. The hawks had to fly over the red-wing territories each day anyway while making their hunting circuits. It was an easy and smart thing to lure in the hapless, testosterone-polluted blackbird males. Smart hunters, our red-tails.
What might that have to do with blue jay and crow harassment around a tree nest in Central Park? I have no doubt that should a red-tail wish, it could easily reach up and grab a mobbing jay or crow, after luring it in. Red-tails can maneuver their legs and feet as quick as cat, and they have a very long reach. If a Central Park red-tail wanted to put an end to smaller-bird harassment, I think it could very quickly.
Well why, then, didn't that happen? Not sure. But jays and crows, as we know, are very intelligent. If one is lost to a scheming hawk, the others will have the good sense to retreat and stay way. Obviously, the Central Park red-tails didn't grab many crows (or any at all). As I asked before, were the hawks overly occupied and concerned with the large numbers of ubiquitous Central Park animals that seemed to pose a continuing threat to nests and eggs? Those, of course, may have been both humans and dogs, species the hawk knows that it can't fend off by a quick punch of an extended leg.
Your observation that bird populations in Central Park are compressed and compacted is astute. That may be the real answer. Collectively, there may be too many birds, mammals, and other environmental disruptions too close to the nest to allow calm and peaceful nest building and incubation. The confluence of other species of birds, wild mammals, multitudes of people, dogs, bicycles, and whatever else frequents Central Park, is too great to allow normal red-tail nesting in trees just above this persistent clamor. I continue to marvel at the disruptions that Central Park red-tails abide; ones that my wild rural hawks wouldn't at all.
My wild, rural red-tails might ask a Central Park hawk, “Are you nuts? How can any self-respecting red-tail live here with all of this?”
4. If the younger hawks in CP are PM offspring, are their mates, as well? (Which would probably not be good for their reproduction, as JB has mentioned) Or is it possible that other female hawks were somehow attracted to these males, the way PM's mates have just shown up?
Mai, you got this question quite right. If all or the majority of CP red-tails are Pale Male progeny, we are watching a chapter near the end of the book, which will be a tragedy, expressed in the final chapters with accounts of failed nests, genetically deficient eyasses, and the eventual loss of the formerly productive, aged-out parents. The genetic difficulties of inbreeding are surely a problem for red-tails as much as any other species. If all the red-tails are related, even with different mothers, the inbreeding loads on the population will eventually overwhelm it. Unless some unrelated outsiders elect to come in to broaden the genetic base (or have in the past), we would be watching only a curious, even aberrant natural history diversion, not a continuing red-tail occupation of Central Park.
Therefore, I'd like to think that at least the mates of the 927 progeny are unrelated. Once again, everyone can see the unparalleled value of banding data. As before, we are guessing. Until we can get some reliable ID info, we'll just have to hope and guess.
There may be so many prey species in Central Park, so many easily captured pigeons, squirrels, and rats, that parents are neither motivated (by hunger or shortage of food specimens) to drive out the new eyasses each summer, nor to drive out nearby new nesting residents. There may be too much food around, allowing offspring to hang around home and never go off to college or work (to a new, distant territory). If all the hawks are related, this is going to genetically complicate matters rather lethally in a generation or two.
Lots more to ponder. Super observations and questions. As always, wish I had more definitive answers. But those of us who initially study wild species in wild habitats (even in Central Park) are always frustrated by the inherent murkiness of our understandings. None of this is physics, where hard numbers quantify the truth. It’s all a bit “soft,” but wondrously so. Everyone, keep watching and thinking,.
Sincerely,

John A. Blakeman