WARBLERS 503 



or desert the site entirely. Rarely when their own eggs are 

 incubated they will accept the egg of the intruder. Four or 

 five eggs are usually laid, an egg generally being deposited each 

 day, though rarely sometimes a day is passed without laying. 



In some instances the bird begins to incubate as soon as the 

 first egg is laid, while in other cases incubation has not seemed 

 to be commenced in earnest until a day or two after the last 

 egg was laid. In cases under observation the incubation period 

 has seemed to range from twelve to fifteen days from the time 

 of laying of the first egg to the hatching of the first young in 

 cases where the birds seemed to have actually begun incubating 

 as soon as an egg was laid. Owing to lack of time to observe 

 them closely the exact cause of variation cannot be given but 

 it seems quite possible that difference in persistence in incuba- 

 ting may account for the discrepancy. 



The young birds are naked when hatched, but within two 

 days are covered with mouse gray down. The first pin feathers 

 appear at the end of about six days and in a period varying 

 from eleven to fifteen days they are ready to leave the nest. 



The food of the parents consists of small soft larvae of 

 Lepidoptera such as canker worms, tortricids and similar 

 larvae which they also feed their young in considerable amount, 

 in addition to which small beetles and bugs of almost any sort 

 are eagerly eaten. They catch quite a bit of their prey on the 

 wing, and I have seen them thus take quantities of adult 

 currant saw-flies while they also eat large numbers of the 

 larvae of the currant saw-fly. The male bird sometimes assists 

 in the work of incubation and very rarely indeed even sings 

 while on the nest. He frequently visits and feeds the female 

 while she is incubating, and in fact seems a very model and 

 affectionate husband. 



654. Dendroica ccerulescens (Gmel.). Black-throated Blue 



Warbler. 



Plumage of adult male : wings and tail black, edged with blue ; base of 

 primaries white, thus forming a white spot or bar on wing at end of the 



