Robins resemble other species--like bluebirds (see Patrick Coy's post
of yesterday), or tree swallows--who form large, sometimes huge, flocks
of fruit-eating individuals during the colder months, after leading
relatively solitary lives eating insects, worms, etc. during the summer.
Others more well-traveled are welcome to correct me if I'm wrong, but
many such species seem to turn to a fruit diet and flocking behavior in
winter even in southern climes where there are more insects available.
At our latitude, it's increasing as an adaptation to warmer winters,
along with extensive plantings of fruit-bearing ornamental trees in
urban areas in Ohio, such as in the mulch islands at the local mall.
        Mockingbirds (see other recent posts) have inched north in Ohio in just
such a way over the past century, taking advantage of new fruits
available in winter, but recently they must deal with new hordes of
robins, a losing contest I witness in my back yard every November.
Still, having lots of fruit around does not seem to attract northern
frugivorous birds--pine grosbeaks, for example--anymore: the last
confirmed record of this once much more common bird was 20 years ago.
Observations by birders are the way we can keep track of changes like
this. Changes we humans impose on the environment--winters growing
warmer so quickly, or row upon row of winter-fruiting ornamentals--seem
to favor the most adaptable, commonest species, and the outermost
branches of the evolutionary tree shrivel as a result.
Bill Whan
Columbus

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