Fledging Dates, Size and Sexing:
Tuesday, July 18 2006 @ 01:57 AM EDT
Contributed by: davidh
July 17/06 -- Obviously by observation Victoria was seen by others to make her first flight July 12 -- Sidney I am told by those constantly watching the site has not fledged yet.
At this latitude, 49th parallel, and with our latitude determined daylength, the average fledging time is just about 12 weeks -- give or take a few days. I am told that in the top of SE Alaska, where the daylength is 2 -4 hours longer in spring and summer, that the extra feedings speed up growth and can shave a week off the time in the nest. It is quite possible, but I know of no accurate record that some of the few bald eagles nesting above the Arctic Circle, where they get 24 hours of daylight for the growing season, that the time in the nest might be even shorter -- and I suspect it would be.
Now add another variable. In literally all the wilderness nests I followed, and by that I mean those nests not around urban / suburban areas .....
see: more comments; ---
..... every beach and every perch tree were proven safe places to sit and eat. Not so in the urban areas. Sure they hunt the beaches, sit on trees, telephone poles or man-made objects, but this is still not quite as secure when someone can run up near to you.. So when newly fledged chicks are first on the wing -- where is the most secure place? I think they take the nest site where they have successfully spent the previous 5 months preparing the site, incubating the eggs and rearing the eggs as the proof of security. The urban eagle, in absence of unlimited alternative sites for perching and encouraging the vulnerable young to land, encourages extended feeding at the nest site.
This does present an interesting variation. First, here at this site I believe the chicks were about a week late in fledging (the male possible due to late development). Second, it is possible this delay is because the adults either had to respond to the late development of the small chick or wanted to further the bond to the site for the reasons above.
In any case the interesting item to me is that with the first chick fledged and the parents still bringing in food to the nest, there is less incentive for the second chick to leave. Why should he leave? Food is still being delivered!
As I have stated earlier elsewhere, it is the deprivation of food being delivered to the nest that encourages the chicks to leave. The urban eagle seems to have changed it response. It will prolong the feeding of chicks at the nest, even after fledging, with the result that the chicks seem to stay in the most time-proven protected place -- the nest, well beyond what wilderness eagles would do. Since the urban eagle has been so successful, at least in this region, this may well be a very positive adaptation. It will be interesting to undertake repeated observations and timing to see if this hold true.
Adult and Chick Sexual Difference in Size:
My measurements show that the bald eagles in our area, southern British Columbia, generally weigh between 5 and 12 pounds (closer average 7 -11 pounds) . When I repeat this to my Alaskan colleagues they point out they have caught several 13 pound birds. And I believe they do.
I actually suspect many are seasonally bigger here as well. An eagle stuffing itself for days on end on salmon gains great weight. Most of the eagles I caught were during the summer when food was less available – and they had to work harder to get it. The general breakdown is 5 - 8 pounds for males and 8 -12 pounds for females.
It is interesting to note that each of the first 5 years of an eagles life its wingspan gets shorter and has less width. The big soaring wings of the juveniles, so necessary to get them to their first free food with minimal energy expansion -- the northern salmon runs -- get shorter and better adapted to fast maneuverable flight to match the eagles flying needs which become greatest when it has to be able to hunt within a home range to support a family. Before that it has more freedom to soar and the larger sails are obviously better adapted to that.
Sexing the chicks: The general and the specific:
Female raptors are all larger than their male siblings. Generally they can be 25 - 50 larger than the males.
Most birds, unlike some reptiles where the percent females is incubation temperature dependent, is generally 50 / 50 for each sex.
Male chicks on the other hand grow and develop more rapidly than females. This might be 4 -6 days faster by my observations spread over the average 83 -- 86 days of growth.
Since incubation starts with the laying of the first egg, the chicks generally hatch 3 days apart -- the time between the laying of the eggs. By flegding date even the second hatching chick, if a male, can be ready for fledging first. Of course if the female hatches first, with its larger potential for growth, it can readily overpower a second hatching male chick. If there is sufficient food for both chicks then the smaller males can catch up and do fine. When there is a shortage of food, it seems the options for a second hatching male chick that is easily bullied by his first hatching sibling, particularly if it is a female, is not good.
So, what do we have here? After spending 50 years looking at full grown eagles and even rearing many eaglets from birth -- or watching them at a distance which is even more difficult, my summary is that it is not always easy to determine one adult from the other or one chick from the other at a quick glance. Certainly there are some tell tale signs.
One of the most indicative signs is that of the calls. The females tend to be higher pitched in my experience than the males. Combine this with close observations from the same perspective and sometimes you are lucky and note that one bird actually has some permanent feather coloration or even a temporary dirt mark on the plumage or skin that lets you, at least for a while , readily determine male from female.
As we saw with these chicks, even some of those persons so intimately following the chicks could mistake, who was who for a moment. And what fun everybody has had with that! But of course it really does not matter for in the end -- the birds know who is who! Of course, and I am not belittling the controversy, we all want to know who is who.
Here it appears that there is quite a large size difference between big Victoria and little Sidney. And I too suspect this is a reflection of their respective sexes. However, as I have been trying to caution, Sidneys late stunted development due in part to him being bullied out of many a meal by Victoria, could account for the apparent size difference. The final sex determination should await wing length measurements -- which we are not likely -- or even hopefully -- to get. In the meantime I support the general consensus that aggressive first hatched Victoria is a female and the second hatched is surely a smaller male.