Climate alters the size, color, shape and many other 

 characteristics of birds, but does it change the incubation 

 length? This remains to be demonstrated. If it does, it is 

 probably exceedingly slow in etfect, and only through 

 minute increments slowlj^ accumulated, and probably always 

 producing alterations on the side of a shorter true length of 

 the incubation period. 



Weather 



It is said that varying weather conditions change the 

 incubation period, and while the writer has found in his 

 study of the house finch (77) that, short of actual freezing 

 or prolonged chilling, the weather has no influence whatso- 

 ever on this species' incubation period, and Macdonald's 

 (146) report on the incubation of the horned lark seems to 

 point the same way, yet Job (95) states definitely that the 

 incubation period of the quail is one day longer if the 

 weather be very cool or wet, and Knight (105) states that 

 he has recognized a prolongation of two to four days in the 

 incubation periods of certain species of Lanius and of Geo- 

 thlypis because of varying weather conditions. It is possible 

 that prolonged droughts might also prolong, or, in fact, 

 render unsuccessful natural incubation because of a tendency 

 to egg dessication or to over-heating if the parent were 

 com])elled to leave a nest uncovered too long. I believe that 

 if there be any difference in the length of incubation seem- 

 ingly referable to weather, that future careful study of such 

 effects will show that the true length is not altered, but 

 that the change is merely one of prolongation due to cooling. 



Locality 

 Geographical locality has been mentioned a^ a cause of 

 differing lengths of incubation in similar, and in unrelated, 

 species. I know of no indubitable support for this theory. 

 There is no evidence that a robin's period is longer or shorter 

 in New York than in Georgia, or Colorado, or Canada; on 

 the contrary, all the facts seem to support the opposite idea 

 that there is no change in the period, whatever the locality. 



Site of Nest 



Casey Wood (104) says that the English sparrow's in- 

 cubation period "varies slightly between twelve and thirteen 

 days, depending on the weather, the locality of the nest*, 

 and the amount of time the bird is on the nest," a variation, 

 so far as the nest site is concerned, patently due to the vary- 

 ing temperatures produced by a well-sheltered or an exposed 

 nest, the sheltered nest promoting optimum conditions and 

 a resulting tnie length of incubation. 



However, one writer (162) states definitely that site of 



♦Italics by W. H. B. 



42 



