How to Study Birds 27
will doubtless be added other causes, as we become more intimate with the facts involved,
Why should some birds raise only one brood, and others two or even three? We should look for an answer to this question primarily in the Number length of time requiredlby'a species to rear its broodi If "f Brawl; the period from the beginning of the nest to the day when i the young are able to care for themselves is so short that the parent birds are still in the physiological condition incident to nesting time, the rearing of a second brood may perhaps be expected: and. under similar circumstances. a third may followr The English Sparrow is re ported to have raised six broods in a season; but. so far as I am aware, no native bird is known to have raised more than three: and authentic instances of this kind are rare. so difficult is it to keep the same in- dividuals under continuous observation, Doubtless. most of the records of late breeding on which the assumption of third broods is based, are due to the failure of earlier attempts at nesting,
But while the reason above given may explain why certain birds do not raise more than one brood, it does not tell us why, among birds in which the period of incubation and growth of the young are about the same,
some species should rear only one brood while others have Tim: af two or three, arrival from The time of a bird's arrival on the nesting grounds tb: Sautb should, of course, be considered here: and we must also take into account the time of a bird's departure for its winter haunts, without in the least being able to say why it should go at a definite time.
Still, with both permanent residents and migrants. which come and go at about the same season, single- and double- or even triplerbrooded birds may be found For instance, of our permanent residents, the Song Sparv row rears two broods, and, on occasions. three, while the Chickadee has but one; and, among migrants, the Robin is two», or, rarely, three- brooded, and the Purple Grackle, which comes to us fully as early as the Robin and remains nearly as long. is oneabroodedi
I confess no satisfactory reason for this difference occurs to me. Doubtless, a tabulation of our birds with regard to the date when they begin to nest. time occupied in rearing a brood, and number of broods reared, would throw some light on the subject; but much of this in- formation, particularly that relating to the time required for incubation and growth of the young, is still to be acquired.
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