I've been observing hummingbirds for quite a few years, and in hummingbirds that breed in the United States, almost all have a white, post-ocular spot just behind the eye. I've asked this question in various forums, but no one seems to know if it has a function. I haven't been able to find any research papers that address this topic either. I was wondering if anyone might have some insight as to the function of this common plumage characteristic in hummingbirds?
the white post-ocular spot behind the eyes is present in both sexes for most species of hummingbirds, sometimes the post-ocular spot is modified in white post ocular stripes behind the eyes. These are white small feathers characteristic in hummingbirds. I suppose is only ornamental.
These papers may be of interest on this topic
http://rstb.royalsocietypublishing.org/content/287/1018/63
http://www.jstor.org/stable/221684?seq=1#page_scan_tab_contents
Thanks, everyone, for the responses. I was just curious that it might have an intended function because it is so widespread across hummingbird species throughout the Americas. Kind of like the thought that white tail spots are used in aggressive displays when tails are fanned. It just made sense that it might serve a function, perhaps with their vision. But, it looks like maybe just work for future study. Thank you.
Just a follow up comment - 'intended function' - empirically no such assertion can every be made about functions as it is teleological. The universe may be teleological, but it is outside the scope of empirical observation to demonstrate purpose for the origin of adaptation or anything else - in my opinion. If we can observe some sort of interactive effect between a biological structure and the environment (abiotic or other organisms) we call it a function, but here function is just a label to recognize a particular theory of interaction. Similarly, its tricky to refer to a feature 'serving' a function. It is possible that white tail spots have an interactive effect as part of an aggressive display, but they are not 'used' for that purpose unless the bird is conscience of this effect and displays the spots with that intent.
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