( 1^37 ) 

 ON INCUBATION. 



BY 

 ERIC B. DUNLOP. 



The majority of British birds do not commence to 

 incubate their eggs until the complement has been 

 deposited. This is obviously to their advantage, in most 

 cases, for if the powers of progression varied to any great 

 extent in nestlings of the same brood, the work of the 

 parents in tending and feeding them would certainly 

 be greatly increased. 



For instance, take the case of the smaller Nidicolce. 

 Many enemies are ready to destroy them as soon as they 

 quit the nest ; the adult birds are taxed to the uttermost 

 to keep the young out of danger, but in spite of their 

 efforts numbers of nestlings do fall a prey to ever-watchful 

 carnivorous birds and beasts. Now, if there were four 

 or five days' difference in the age of the oldest and 

 youngest nestlings, there can be no question that the 

 difficulty of the parent birds' task would be enormously 

 increased, for a few days make a great difference to the 

 powers of flight of a young bird, and to keep all their 

 offspring under control would be beyond the powers of 

 the parents. 



Similarly, in the case of the Nidifugce the dangers 

 would be greatly increased without any advantage 

 accruing. Take, for instance, the Partridge (P. cinerea). 

 Although the male is a careful parent, very few of the 

 young would reach maturity if the hatching-period 

 extended over a fortnight or three weeks ; for upon the 

 brood being threatened by danger their powers of escape 

 would be so varied that several of the young would be 

 inevitably lost. The Anatidce would suffer yet more 

 severely, for in most cases the drake does not assist the 

 duck to rear the young. 



It is thus evident that in most instances it is to the benefit 

 of the species that the full clutch should be deposited 

 before incubation commences. However, even in those 



