1194 U.S. NATIONAL MUSEUM BULLETIN 237 paet 2 



the last egg. Incubation periods were 11 and 11^'^ days in 1937 

 (Walkinshaw, 1944a). In a letter Walkinshaw defines the incubation 

 period as "that period between the laying and hatching of the last egg 

 laid in a set." On the other hand, G. C. Kuyava, using a similar 

 definition, writes me that near Duluth the period was 14 days in each 

 of 11 nests he observed in 1959. G. A. Fox (1961) gives an incubation 

 period of 11 days in Saskatchewan, and W. R. Salt (1966) cites approx- 

 imately 10 days near Edmonton, Alberta. 



Frederick E. Warburton (1952) states that both parents incubate, 

 but one much more often and for longer periods than the other. The 

 birds change place sUently, the incubating bird leaving the nest as 

 soon as the newcomer approaches. At one nest each bird had its 

 own approach route, about 18 inches in length, consisting of perches 

 used in an almost invariable sequence. By the last day the birds 

 often skipped half the usual stops. When young could receive food, 

 the parents used a new approach, less well defined. G. A. Fox (1961) 

 found that on the first day of incubation at one nest in Saskatchewan 

 between 2:00 and 5:00 p.m. incubation time was divided as follows: 

 female 81 percent; male 13 percent; nest uncovered 6 percent. 



During both incubation and brooding the female sits close, and, if 

 one is cautious, he may part the branches and photograph her before 

 she flies. After an incubating bird flushes, both parents flit from bush 

 to bush, silent but watchful. Birds may return and resume incubating 

 45 seconds after being flushed. The male frequently carries food to 

 an incubating female, who often leaves the nest to meet him. 



G. C. Kuyava writes me that the three eggs in one clutch all hatched 

 between 9:10 and 9:40 a.m. on the same day; the time from the first 

 pipping to complete emergence of the three was respectively 11, llK, 

 and 10 minutes. Clara Hussong (1946) describes a different hatching 

 pattern; she writes that of a four-egg clutch in Green Bay, Wis., 

 two hatched on July 7, one on July 9, and the fourth on July 10. 

 W. R. Salt (1966) points out the unevenness of the hatch, stating that 

 hatching of each of four clutches was completed in 2 successive days. 



A. H. Shortt writes me that in 1932-1933 at Deer Lodge, Manitoba, 

 where cowbirds were active, of 50 nests found, 22 (44.0 percent) pro- 

 duced fledglings. Of 27 first nests only 8 (29.6 percent) were success- 

 ful. Of 21 renesting attempts 12 (57.1 percent) succeeded. Two 

 nests could not be classified as either first or second nests. Of 158 

 eggs found in 50 nests, 80 (50.6 percent) hatched; 73 young fledged, 

 a reproductive efficiency of 46.2 percent. The reproductive efficiency 

 of renestings (62.3 percent) was greater than that for first nestings 

 (34.9 percent). Shortt gives the causes of 52 eggs lost as foUows: 

 removed by cowbirds, 25; lost in deserted nests, 12; lost in destroyed 

 nests, 10; infertile, 5. 



