Gray Catbird

Dumetella carolinensis (Linnaeus)

Before the experimental studies discussed earlier in this paper, the gray catbird and the American robin were the only species known to eject cowbird eggs frequently. But the actual incidence of ejection was not known. Experiments on 53 catbird nests resulted in 50 (94.3 percent) ejections. The cowbird eggs in the 3 remaining nests were accepted. The experiments were conducted in Connecticut, Manitoba, Nebraska, Michigan, and Maryland (Rothstein, 1975a).

Experiments on 17 additional nests show that after egg laying has ceased, catbirds correctly distinguish between their own and foreign eggs in a number of different contexts. Catbirds always ejected cowbird or other types of foreign eggs even when these egg types outnumbered the catbird's eggs or were the only types present (Rothstein, 1975c). These experiments were done to test Rensch's (1925) question of whether birds that reject parasitic eggs actually recognize either their own or the parasitic egg, or whether they simply reject the egg type that is in the minority. In nearly all cases the parasitic egg is in the minority, so rejection could function efficiently as a host defense even if birds did not recognize their own or a parasitic egg. Contrary to the generally held belief, the experiments showed that catbirds do indeed “know” the appearance of their own eggs. Other experiments (Rothstein, 1974) suggested that catbirds learn the appearance of their egg type from the first egg(s) they lay and that it is possible to “teach” a catbird that a cowbird egg is its own egg type if eggs are experimentally switched shortly after laying begins.

The Ontario nest records files at Toronto include 9 instances of cowbird parasitism on the gray catbird; these constitute 1.3 percent of 707 nests reported; the files at Cornell University record 4 instances (out of 367 nests in all), 3 from Michigan and 1 from Kentucky.

BROWN THRASHER