Thursday, June 21, 2007

WHY CAN SOME FLY AND OTHERS CAN'T? Branching and Begging

The stance of a newly fledged Red-tailed eyass while begging.

Why is it there is a wide difference between the flight ability of newly fledged eyasses from different nests and even more disparate between other species of birds and Red-tail young? Wouldn't there be an evolutionary advantage to youngsters with immediate flight?

Little from the 2005 Trump Parc nest of Charlotte and Jr. According to report, Little came off the Trump Parc nest beautifully, no wobbles for this guy. He then spent a number of days on the roofs surrounding and below the nest making very short flights between railings and roofs, one roof to another of equal height, and very short trips of a foot or two from a rooftop to a slightly higher place such as a chimney, before flying into Central Park. Pale Male Jr. conducted hunting lessons on nabbing pigeons in flight for the eyasses during this time.

Charlotte on the Trump Parc nest, 2005.
Though the nest is reasonably roomy compared to some there was some question as to whether Little had flapped the needed amount, as his sister Big had appeared to have done more. Both eyasses exercising at the same time on this nest was possible but it was a tight fit, therefore near fledge time when they were larger, they tended more toward separate bouts.

The Cathedral Church of St. John the Divine Nest, 2006.
Eldest flapping on St. Andrew's hand but a grip is necessary not to catapult prematurely into fledging. No running and hopping possible while flapping in that spot. Youngest behind the Saints head on the nest, has some room to run but not much room for full flapping. Because of the layout of the nest, and even more so in the 2007 season since the seasons added nest material created higher nest walls, space was even more limited. And of there were three eyasses on the nest in 2007.

Isolde and Youngest, 2006. Even then there wasn't much room to run, hop, and flap.

Though fledging into the trees beyond the high chain link fence, in the next day or two both eyasses were discovered back on the roofs of the Cathedral and inside the enclosure of the fence.
They used the cathedral and the scaffolding that was then erected beside the cathedral for branching. Though the rake on the roofs and the small landings on the scaffolding were not conducive to hopping, flapping, and running at the same time.
Not long many days after fledgling, youngest found himself grounded outside the fence attempting to get back to the nest area. He was only able to hop perhaps a foot and he didn't fully spread his wings while doing it. His mode of transportation was to take to his heels. the only way he could gain elevation was by hop flap climbing into bushes and trees.
What about this inability to fly right away and running instead? This made me think of Robins. They are completely grounded as well immediately after fledging. When necessary to avoid harm they run and hide. Why can't they fly? There's got to be an evolutionary advantage in there somewhere? What is it?

I started thinking about this particular Robin chick. He came off the nest and ran. Then ran some more. Usually they run towards Dad who is going to feed them. Having made me think in the past that the urge was Dad not the running that was at issue. But this guy had a very big urge to run though the part about "towards Dad" didn't kick in right away. Dad had to chase him so he wouldn't run off and get lost. Interesting.
Do RT eyasses have an urge to run. I'd say that was a big yes, it's just that sometimes they've no room to do much of it in.

The best nest to run in as far as I know in the city, is Fordham's. (Pale Male's used to be good too, though we've not had a chance to see how it works with the cradle and its slightly more exaggerated curve that's now built in.)
Look at the Fordham nest. Not only is it great for running, the platform affords plenty of width for flapping and elevated space for hopping as well. It's a virtual airport runway.
Not only that but look at how close the tree branches are to the ledge. Also that tree and those near it have branches that rise up to a higher roof on a near-by building. And that building's roof gives access to a whole panoply of other roofs and branching opportunities.

This eyass was first seen off the nest on this day's morning. He saw his mother with food on a roof to the left. Yet he walked off the pipe onto a roof to the right though further from the food to make the flight to the food. Why?
Try this for size.
BECAUSE he needed to take off from a roof in order to have a "runway".
The pipe didn't afford what he needed in the way of a run/step hop surface to help his wings get him off the "ground". Plus the roof on the left was lower in one spot and peaked in the other, neither as advantageous to getting to his final destination as was the roof on the right, which was of a more equal height.

Here he is, far left, having flown to where a parent was eating. This flight was very well done and he landed perfectly on the exact spot for which he was aiming.(Not with polite hawk manners as he almost nailed his mother, but very good aim for his age anyway. Manners will come later. ) It looked perfect. Which made him look perfectly flighted though he wasn't. It was a flight of some length but he didn't have to gain altitude and he had an excellent runway to start from.
It wasn't that he just had the skills to choose wisely, he also had the advantage of more muscle. His increased muscle strength was acquired because of the many advantages built into the nest he hatched on, a large sumptuous adjacent branching area, and conceivably a few days to practice before we noticed he'd left the nest.
A caveat here. Keep in mind, Fordham eyasses are notorious for getting on and off the nest. Though on a building they have the advantage of being able to branch on and off with relative ease. A rarity for most urban nests. And because one can't see all the way to the back of the Fordham nest from the ground, one must use a roof for the full view, it's almost never clear who left when or who left and came back before anyone noticed. Chis Lyons one of the major watchers of this nest felt this eyass may have fledged a day or two before the sighting.
A commodious nest, good branching opportunities, and a couple extra days off the nest before we may have noticed? No wonder he looked perfectly flighted, though he wasn't. Back to the question. What is the evolutionary advantage that is so important that it's worth unflighted young coming off the nest?

Another tack, let's look at chicks, like this little House Finch, that come off the nest flighted. They aren't going to be flying to South America tomorrow but they still get around very well immediately. Once off the nest they fly to the food following along after a parent. Food doesn't come by delivery in the way baby hawk food does. Do these babies do something that hawks don't that helps build muscle before leaving the nest?
Categorically they do. They beg with their entire bodies including their wings from the time they can remain upright long enough.
Scroll back up to the top of the post and look at the begging eyass. Her wings are hanging down as if they are too heavy for her to hold up. And they probably are as she is putting every ounce of her being into begging as loud and long as possible. There will be some wing use once the parent gets close but until then it's about putting everything the youngster has into sound.

Note the posture of the House Finch as she watches her father shuck seed into his mouth. She's stiff. She's contracting all her muscles, though she's hunkered down. I'm theorizing that as she comes from a bowl nest with no running room, her legs aren't all that strong and well muscled at first so between bouts she has to rest them. Though her little body has had lots of practice vibrating and flapping her wings.

When the chick has decided that Dad has about filled his mouth with shucked seeds, she jumps onto her feet, vocalizes and beats her wings often running part of the way to Dad. When Dad has decided he has a big enough mouth, in this instance he went right a few steps and plucked a few more before deciding he had a full load. Note the position of the maple seed up of his tail. He will turn towards her and they both run to meet.

Dad then turns takes a few quick steps and by then she's there and he feeds her. Note position of Maple seed again. Also note that their wing and breast muscles are reasonably similar but Dad's muscles for running are quite a bit more developed than hers are.

Dad pauses for a moment, check out his lower legs and feet.

While Dad pauses the chick also pauses in what previously seemed like constant tension in her body, a spring winding tighter and tighter towards the feeding moment. (The combination of intermittent contraction and release/relaxation builds muscle strength.) She's still keeping a major eye on him though.

Dad turns and begins looking for some choice seeds and...

She goes back into begging mode.

She holds it, holds it, holds it. Dad is taking forever. And wait a gosh darn minute Dad just gave that mouthful to a nest mate.

She releases the tension and decides to try doing it herself.

She was able to swallow one. Though she doesn't look all that happy about it. It's just not the same having to feed oneself. But as more of the broods chicks arrive for Dad's care day to day, she does more of it.
Okay so where does all of that get us in the way of finding some possible answers to the questions?
You know, I could use a recap of the questions myself.
What evolutionary advantage or advantages are so worthwhile that sacrificing immediately flighted hawk fledglings or even flighted Robin chicks right off the nest makes that advantage worthwhile?
The Part B, being why are some species fledglings downright unflighted or nearly so and some species can come off the nest with reliable wing power?
(You realize of course, we're not going to come up with a real answer. We're going to come up with possible answers, decide which are likely. Decide which we like the best... a hypothesis which could be tested. And if observation and testing worked out, it might actually be a real answer....but there would have to be more testing and observation of other possibilities. In the end there would probably be multiple contributing reasons as to why. Not really as satisfying as the epiphany of THE answer in a moment so let's not think about that part right now.)

So what did we come up with?
The more advantageous the Red-tail nest site, the somewhat more flighted off the nest may be the eyasses. Though as we know, even less advantageous nest sites still produce young that become fully flighted, hunt, do perfectly well, and go off on their own.
Though one could surmise that less of a percentage do. Compare the number of problems of the young before being able to fly off with their parents safely to learn hunting skills amongst the 888, the Cathedral, and the Fordham nests.

Next, Red-tail and Robin young have a noticible urge to run compared with House Finch young.
(Couldn't be because over milennia it was developed because they can't fly? No actually, probably the urge to run was there because running appeared before flying.)
Okay so why don't RTs beg with their wings to build them up? Because they flap to build them up. And while flapping they run, both needed for such large birds. Ahhhh. that's one of the advantages. Robins are large for songbirds, most songbird chicks can fly once off the nest but they're small with skinny light legs. Robins have sacrificed immediate flight for size. So most likely have RTs with their strong legs, heavy feet--their hunting survival gear.
What are the other differences? Robins are omnivores, Red-tails carnivores, Finches are regularly herbivores. Robins and Raptors have to hunt other creatures most days of their lives. They must learn hunting skills and strategies from the moment they leave the nest in order to get those skills honed enough before limited prey or migration turns off the parental food faucet ?. In fact then, is coming off the nest early unflighted more important to species survival than waiting for flight and having less time to be taught? Could be.
(As for the House Finches, as seeds don't run when you see them, perhaps learning to find them and eat them, needs a shorter learning window. Wait, they just have to learn to eat them. They can continue to follow a parent to "where" for a good long time as seeds aren't singular, they are usually found in large groups. I can think of several reasons why finches might need wing action immediately because of peculiarities of plants but enough already.)
Time is limited in Robins and Red-tails for learning hunting skills. For Robins, there can be numerous broods a summer. If the first batch doesn't learn to feed itself before the second comes hard on the first's heels, they are out of luck. And we know that in Red-tails eventually the parents will stop feeding the young hawk as prey becomes less available when the year wears on so the young better be able to outsmart the prey by then.
The questions just multiply. So why don't RTs beg with their wings early as well as later doing the flapping, hopping, and running? Perhaps the gain wouldn't be worth the calories? Or do they really need their entire being for volume when begging?
See what I mean?
There are those who are fascinated by how many of a species and where they all are. The how many and the where. And I'm glad that they do or how else would I know where to find them. My fascination is the what they are doing and why? Come to think of it I'd probably have to add who, as well. Because at least in the amazing adaptable Red-tail, the who has a great deal to do with what you're going to get for a what and why?
Because as long time urban hawk watcher Ben Cacace wrote in an email to me the other day referring to Charlotte and Jr. picking up the eyass's care as if she'd never been gone...."One should never underestimate a Red-tailed Hawk." And that's a fact.
Donegal Browne
Donegal Browne
Donegal Browne

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