HOST RELATIONS OF PARASITIC COWBIRDS 15 



Otter Lake, west of Pontiac, Michigan. They followed very closely 

 the breeding activities of several female cowbirds, each of which was 

 readily distinguishable by the consistent size and coloration character- 

 istics of its eggs. One of these laid 18 eggs, all in nests (18 in number) 

 of the yellow warbler. Another cowbu'd was responsible for 6 eggs 

 from May 18 to May 27, also all in yellow warbler nests; a third 

 individual laid 6 eggs between Ma}^ 18 and 28, 5 of which were de- 

 posited in as many nests of yellow warblers and one in a song sparrow 

 nest. On the other hand, another cowbird laid 19 eggs between 

 May 9 and June 19, using as its depositories 11 song sparrow, 6 

 yellow warbler, and 2 Traill's fl^^catcher nests. 



Breeding Success of Host and Parasite 



Wliether the same cowbu'd scatters her eggs in different nests or 

 tends to deposit more than one in the same nest, the resulting fre- 

 quency and degree of parasitism makes one ask what the damage is 

 to the hosts and how they are able to stand up under it. In many 

 accounts, the impression is that the imposition on the host is exces- 

 sive, and yet the situation continues year after j^ear. In only one 

 species, the Kirtland warbler (see pp. 108-111), however, does the 

 situation become alarming for the continued welfare of the host. 



In attemptmg to estimate the effect of cowbird parasitism upon the 

 most frequently used, or at least the most often reported, host species, 

 we may take as a basis for our discussion the data compiled by Nice 

 (1957, pp. 317-318) on altricial birds. The success rate of open nests 

 of altricial birds in the north temperate zone, as recorded in 24 studies 

 on 7,778 nests, ranged from as little as 38 percent to as much as 77 

 percent, with an average of 49 percent. In 29 studies, involving a 

 total of 21,951 eggs, fledging success (that is, success to the point of 

 resulting young developed to the stage wherein the}^ leave the nest) 

 ranged from 22 percent to 70 percent, with an average of 46 percent. 

 Possibly because of the lesser exposure to predators, hole-nesting 

 species averaged a higher degree of success; 33 studies involving 94,400 

 eggs showed fledging success of from 26 to 94 percent, with an average 

 of 66 percent. Lack (1954, p. 87) gave almost the same percentages. 



Although they are included in the above summaries by Nice, it is 

 of interest to review the breakdown by species given in Kendeigh's 

 paper (1942, pp. 19-26). He studied the outcome of 2,725 nesting 

 attempts by 51 species, mostly of forest-edge passerine birds, in the 

 central United States. Considering only species for which he had 7 

 or more nest records, the percentage of nesting successes was 80 to 

 90 percent in the Baltimore oriole, barn swallow, house wren, and 

 cardinal; 70 to 80 percent in the starling, flicker, phoebe, and purple 

 martin; 60 to 70 percent in the catbird, yellow warbler, song sparrow. 



