HOST RELATIONS OF PARASITIC COWBIRDS 115 



Several times nests were deserted where the cowbu'd had deposited 

 her egg." 



More than most of the regular hosts, this species is afflicted with 

 multiple eggs of the cowbird. In my earlier summary (1929, p. 248), 

 I noted that, of 55 parasitized nests, 25 held 1 cowbird egg each, 

 20 held 2, 7 held 3, and 3 held 4 eggs of the parasite. The ability 

 of this waterthrush to rear a large brood of mixed offspring is revealed 

 in a case listed by Wood (1951, p. 412): a nest found in Michigan by 

 Walldnshaw contained three young warblers and two young cowbu'ds, 

 all about ready to fledge. 



Kentucky Warbler 



Oporornis formosus (Wilson) 



The Kentucky warbler is a locally common victim of the brown- 

 headed cowbird (race ater). About 150 records have been noted, 

 ranging from Delaware, Pennsylvania, Ohio, Indiana, Illinois, and 

 Michigan to Iowa, Kansas, Oklahoma, and Kentucky. The greatest 

 number of records are from Pennsylvania, a fact which may reflect a 

 local difference in the incidence of parasitism but which also may be 

 due to the presence in that state of two unusually successful and 

 devoted egg collectors: J. Warren Jacobs (1893, 1938) and J. Parker 

 Norris (1892a). Jacobs (1893) found cowbird eggs in no less than 47 

 nests of this warbler in Greene County, Pennsylvania. Many years 

 ago Norris wrote me that he had in his collection 54 parasitized sets 

 from Pennsylvania and Delaware. In Greene County, Jacobs (1893) 

 estimated that about one-fifth of all the bkds' nests which were 

 found by him with cowbird eggs consisted of this species. He noted 

 that of the 47 parasitized Kentucky warbler nests, 39 held 1 cowbhd 

 egg each, 7 held 2, and 1 held 3. These figures present a considerably 

 different picture from that which we described for the Louisiana 

 waterthi'ush. 



In a later paper (1938), discussing his long span of observations 

 near Waynesbm'g, Pennsylvania, J. W. Jacobs stated that he had 

 examined several hundred nests of the Kentucky warbler over more 

 than half a century. He kept records of 133 of these and found that 

 60 nests, or 45 percent, were parasitized by the brown-headed cow- 

 bird. Of 42 nests containing 5 warbler eggs each, 8, or 19 percent, 

 held cowbird eggs; of 56 nests containing 4 warbler eggs each, 26, or 

 46 percent, held cowbird eggs; of 35 nests containmg 3 warbler eggs 

 each, 26, or 74 percent, contained cowbird eggs as well. In the 73 

 unparasitized nests, the preponderance of full clutches of 5 eggs 

 brought the average up to 4.3 eggs per nest. The 60 parasitized 

 nests showed an average of 3.8 warbler eggs per nest. In comparison 

 to this, it should be noted that there were 0.25 cowbird eggs per nest; 



