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 



length of time required by a species to rear its brood. If 



the period from the beginning of the nest to the day when 

 of Broods , , , r 1 1 • 11 



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 follow. 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 difScult 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 



Time of two or three. 



arrival from The time of a bird's arrival on the nesting grounds 



the South 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 triple -brooded birds 

 may be found. For instance, of our permanent residents, the Song Spar- 

 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 one -brooded. 



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. 



(TO BE CONTINUED) 



