Posted by Tony Leukering (205.188.195.21) on March 14, 2000 at 23:54:30:
In Reply to: bluebird primary extension posted by Jim Tietz on March 09, 2000 at 00:40:01:
A veritable host of birds can be separated by wing formula (the relative spacing of primaries), but it is an aspect of bird identification that has seen little work in this side of the Big Pond. I believe that this is mostly due to North America having relatively few groups of birds, the members of which are very difficult to ID by plumage characters. The Europeans, I believe, are well ahead of us in this regard, due (again, I believe) to a plethora of little brown and little green birds.
Bluebirds are readily identified on primary projection and wing formulae (at least, Mountain can be separated from the other two), and I would love to wade in on the bluebird, but my piece of &^$(&^% of a monitor does not permit me the resolution to determine this on the bluebird in question.
The Empidonax flycatchers all have differing wing formulae that can at least help in their ID. And, think about wing formula on the next somewhat-more-challenging-than-most parulid you see.
And, here for those with the fortitude to read, is some of the reasoning behind wing formula differences.
With all else being equal, species with longer migration routes will have longer, more pointed wings than do species with shorter migration routes - thus, they will have greater primary projection (more primary tips beyond the tertials being visible and the entire wingtip protruding beyond the tertials more) and, probably, wider spacing between primary tips.
Also, with all else being equal, species that are canopy inhabitants will have longer, more pointed wings than those inhabiting dense shrub habitats. This is probably an evolutionary adaptation in shrub inhabitants to prevent as much abrasion of primary tips, thus flight performance, as possible (more of the primaries are covered by the tertials on the folded wing, thus are protected from being rubbed against the many branches in the habitat). Canopy inhabitants, which occupy much more open microhabitats, don't have the same constraint on primary projection and/or wing length.
So, lengthy migrations encourage longer wings (more aerodynamically efficient in sustained flight) and shrubby habitats encourage shorter wings (less abrasion). Mixing and matching various ecological parameters (e.g. a shrub-inhabiting, long-distance migrant) and you have the typical wide range of variation so dear to the hearts of biologists everywhere (did you hear the sarcasm?).
(As an aside: Did you ever wonder why Franklin's Gull, a species with one of the longest migrations of all gulls, has such relatively short wings? I bet it's the flycatching.)
Anyway, the main point is that every species has a discretely-different niche from every other one, thus there are no two species with precisely the same primary projection and wing formula.
Now throw in differing wing lengths and shapes between the two sexes and for various age classes and.... Enjoy!