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The females evidently recognize the limits of the territories of their respective pursuers as they usually fly in wide circles closely followed by one or more males, but do not leave the general vicinity of their territories. The Cowbirds do not make any very spirited attempts to defend their territories and consequently in regions of unusual abundance the territorial factor is much less noticeable.The ovaries of these resident females are considerably larger than those of migrant birds of the same date." The sixth and last phase of the migration marks the arrival of what are apparently the immature males and females. The gonads of these birds are smaller than those of the only other Cowbirds then present (resident birds)." The males of this group do not seem to take much notice of the females, whereas the resident birds do, and, to some extent, so do the last of the migrant males. I have never seen Cowbirds fight and their method of defense is restricted to an intimidation display. Laskey, of Nashville, Tenn., has sent me notes on her 3-years' study of cowbird behavior.
The persistence and determination with which the resident males pursue these females makes one wonder if either ever rest.All Rights Reserved © This electronic book may not be copied, reproduced, or posted elsewhere, by any means, in whole or in part, without the written consent of the designer, editor, compiler, and copyright owner. Otto Widmann (1907) offered the following interesting theory to account for the origin of the parasitic habit: We know that fossil remains of horses, not much unlike ours, are found abundantly in the deposits of the most recent geological age in many parts of America from Alaska to Patagonia. By a combination of favorable circumstances this new way of reproduction proved successful, and the parasitic offspring became more and more numerous.The two most characteristic habits of this bird are indicated in the above names. It was probably at that period that the Cowbird acquired the habit of accompanying the grazing herds, which were wandering continually in search of good pasture, water and shelter, in their seasonal migrations and movements to escape their enemies. In the course of time the art of building nests was lost, the desire to incubate entirely gone, paternal and conjugal affection deadened, and parasitism had become a fixed habit. Friedmann (1929) disposes of this theory as "more interesting than suggestive," and adds: "It is somewhat surprising to find a naturalist of Mr. Probably he meant it more as a suggestion to be taken for whatever it might be worth than as a real attempt at an explanation." The trouble with the theory is that we have no known facts on which to base it, there being no record of a cowbird leaving its nest to follow cattle, horses, or bison.Howard has shown that the territory precedes the nest in the evolution of the instincts of guarding associated with reproduction. *** Spring.--The eastern cowbird has not far to go on its spring migration.If the Cowbirds were parasitic from the very beginning it would be very hard to explain their territorial instincts. It is one of the earlier migrants, leaving its winter range in the southern states during March and reaching the northern parts of its breeding range during the first 2 weeks in April, or sometimes before the end of March. The Cowbird commonly migrates with the Red-winged and Rusty Blackbirds and the Grackles; in fact these three are usually found together.