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They come to the marshes to pass the night with the Red-wings and the Rusty Blackbirds but during the day scatter over the fields on the uplands, where, in small groups, they forage for food." The third phase is marked by the arrival of the resident males, which come on the average between March 23 and April 8. The testes of these resident birds are noticeably larger than those of the migrants, and as all the birds collected at dusk in the marshes averaged smaller testes than those shot from their singing trees, it seems that the two groups do not associate in the marsh, the resident birds sleeping in their territories.
The resident males on arrival at once establish themselves on their posts and remain in their territories for a few hours early in the morning every day, spending the greater part of the day in foraging around for food, often going to stubble fields, and to plowed areas a little later in the season. The migrant females are the first of that sex to arrive, passing to more northern points between March 23 and April 28.
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.
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.For the benefit of the readers who do not own this interesting and comprehensive book, we shall quote from it freely. This indicates that originally it built a nest but no longer knows how. This species is the only one of its group that is not parasitic and doubtless represents the original condition of the Cowbird stock. The weakness in this theory is that such cases of adventitious laying in alien nests must have been very rare at first, and the inherited tendency to repeat the experiment would soon disappear by cross-breeding with individuals of normal breeding habits, unless the habit proved to be beneficial to the species, and no such proof is evident.In his chapter on the origin and evolution of the parasitic habit he writes: The evidence points unmistakably to the view that the Cowbirds originally bred in normal fashion and that parasitism is a secondarily acquired habit. The instincts of nest-building and incubation are so universally present in all groups of birds in all parts of the world that it seems likely that this is the primitive condition of the Cowbirds. All the Cowbird's close relatives are nest-builders; in fact, its family, the Icteridae, is known as a family in which the nest-building instincts reach their pinnacle of development. The North American Cowbird, ) have definite breeding territories and are more or less monogamous. The fact that the Cowbirds are fairly monogamous indicates that they were monogamous originally and probably nested in normal fashion as all monogamous birds do. The most primitive of the existing species of Cowbirds is. We frequently find fresh eggs of robins and other birds laid on the ground, but failure to reach their nests has never developed parasitic habits in these birds.The Greek word Molothrus signifies a vagabond, tramp, or parasite, all of which terms might well be applied to this shiftless vagabond and imposter. The first writer to see that one explanation would not serve for all the different groups of parasitic birds was G. As the pastoral habit of the bird became stronger, it gave rise to the parasitic habit, simply because, in following the roving animals, the bird often strayed from home too far to reach its nest in time for the deposition of the egg, and, being hard pressed, had to look about for another bird's nest wherein to lay the egg. Probably the parasitic habit was developed before the cowbirds invaded North America.It deserves the common name cowbird and its former name, buffalo-bird, for its well-known attachment to these domestic and wild cattle. This writer studied the cyclical instincts of birds and found that not infrequently different parts of the cycle are interrupted by various causes which result in a general lack of harmony between successive parts of the cycle. And we do not know to what extent the primitive cowbirds, in South America, had developed the habit of following the wandering herds. Coues (1874) makes the following suggestion: Ages ago, it might be surmised, a female Cow-bird, in imminent danger of delivery without a nest prepared, was loth to lose her offspring, and deposited her burthen in an alien nest, perhaps of her own species, rather than on the ground.