[Tweeters] Anna Notes and Surmise
flora.fauna at live.com
Mon Mar 30 11:49:35 PDT 2009
Now that Rufous are here and proceeding as they should, I want to start winding down this unofficial column. A few weeks ago I asked if anyone might know or surmise,why male Annas Hummingbirds, out of all the hummers one might normally encounter in North America and parts of Mexico,and with the exception of a couple of populations of Costs'a, are heavier than the females. No responses were received perhaps because nobody is reading this column!! However I want to question a little further. One might guess that because the males do elaborate dive displays and vigorous territorial defense that females select males who are bigger and stronger. But let's look at some other non-passerine groups first, where females can sometimes be bigger than males. In Arctic breeding shorebirds, which have long migrations females of some species have the brighter plumage but do not incubate. But out of many shorebird species, females weight (body mass) is bigger than males in a small minority of species e.g. Baird's and White-rumped Sandpiper and Red-necked Phalarope, but even then their is overlap between sexes.
Raptors and owls are also groups where the females are usually heavier than males. A limey ornithologist ( Ian Newton) suggested that in Accipiters, size dimorphism helps a breeding pair extend the weight of their prey base, but I am not sure that this idea is still accepted.Recent research suggests that male raptors may be smaller because they indulge in significant aerial displays in the mating period.
As you can see, there are several different explanations in non-passerines, for size dimorphism. In shorebirds that have long migrations, females who bear the burden of egg-laying are necessarily bigger. In birds of prey there are different explanations, prey-base, display, non-incubation, why males might be smaller. My surmise and that is all it is, is that virtually all N.American hummingbirds are migratory, especially Rufous and Ruby-throated. With the burden of migration, egg-laying and feeding the young, hummers select for larger size. But the same is not true for Annas. In the literature, Annas is described as migratory in southern California, with movements up-slope in summer and significant movements eastwards to Arizona and beyond. But what if these are not real "migrations" but merely "dispersals"?. Annas may well be sedentary throughout large parts of its range of occurrence. There is no evidence for north-south migration or up-slope movements in most parts of Annas range. Are other species becoming sedentary: I don't know? But I believe that Annas certainly is and this is the only difference I can discern between it and the other hummer species. This may be the reason males are selected bigger or females smaller. If you have found research that might help me answer these questions,or I have overlooked significant factors, please let me know DH
David Hutchinson, Owner
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