HOST RELATIONS OF PARASITIC COWBIRDS 113 



In later studies of the ovenbird and the cowbird, Hann (1941, p. 

 212) found a loss of 30 eggs of the host which was attributable to the 

 females of the parasite and, in addition, the loss of 4 cowbird eggs 

 which were removed in error by the parasite itself. Some 40 cow- 

 bird eggs were laid, which made the loss of the host's eggs to be 75 

 percent of the "gain" in parasitic eggs and made the total loss to be 

 85 percent of all the eggs laid. In only one parasitized nest was the 

 fuU number of laid eggs retained. Hann found that the earliest 

 ovenbird nests of the season were parasitized more heavily than the 

 later ones. 



In Ohio, Hicks (1934) found 112 nests, of which 41, or 36 percent, 

 were parasitized; in southern Quebec, only 6 out of 61 nests reported 

 by TerriU (1961, p. 8) had cowbird eggs in them. Many years ago, 

 L}mds Jones (1888, p. 133) wrote that, in his experience in Ohio, 

 "never yet has a nest been found here but two or more cowbu'd's 

 eggs completed or completely made up the set. In one nest I found 

 only three cowbnd's eggs nearly hatched; in another three of the cow- 

 bird with one of [the ovenbird] . . . another contained four of the 

 cowbird with two of the parent bird, and another . . . just five 

 cowbird eggs." In the J. P. Norris collection there were no less than 

 25 parasitized sets of eggs from Pennsylvania. Sage and Bishop 

 (1913, p. 160) observed in Connecticut that 11 nests were para- 

 sitized, out of 30 examined. 



In the local studies referred to above, we find the percentage of 

 parasitized nests to vary from 33 in Connecticut to 36 in Ohio and 52 

 in Alichigan. If we add the six major studies, we find that, out of 209 

 active nests, a total of 89, or about 42.5 percent, were parasitized. 



Ovenbu^ds have been known to successfully rear the young para- 

 sites; they are usually very tolerant of cowbird eggs, provided that 

 the number present is not excessive. As many as 8 cowbu'd eggs 

 have been reported in a single nest of the ovenbu'd by Mikesell 

 (1898) in Ohio. A nest with 7 cowbird eggs was reported by Hess 

 (1910). 



Northern Waterthrush 



Seiurus noveboracensis (Gmelin) 



The northern waterthrush is generally an infrequently used host, 

 but there is some local variation in this regard. Only 15 records 

 have come to my notice, involving two races of the warbler, nove- 

 boracensis and notahilis, and one of the cowbird, ater. The reported 

 instances are from Quebec, Ontario, New York, Michigan, and Iowa. 

 The only place from which a fairly high incidence of cowbird parasitism 

 has been reported is southern Ontario, where Allin and Harrington 

 {in Griscom and Sprunt, 1957, p. 196) found that 8 out of 40 nests 



