American Robin

Turdus migratorius (Linnaeus)

Information on natural parasitism of the robin has changed little since the 1963 review (Friedmann, pp. 72–73). We include this species mainly to report the experimental parasitizations. The experimental results confirm the widespread belief that the robin typically rejects cowbird eggs and allow a quantification of that belief. Artificial or real cowbird eggs were rejected at 45 of 46 nests. Seven of the rejections were by nest desertion and the remainder by egg ejection (Rothstein, 1975a). These experiments were done in Connecticut, Michigan, Nebraska, Maryland, Manitoba, and New Brunswick.

Recent data of considerable magnitude on the incidence of parasitism on the robin are as follows. The files at Cornell University show 7 instances among 7482 nests reported (a little over 0.1 percent) distributed among Maryland, Massachusetts, Nebraska, New York, and Wisconsin; the files at Toronto show 9 instances among 3586 nests (a little under 0.3 percent) in Ontario. An unusually high incidence of parasitism was reported to us by P. F. Elliott who found cowbird eggs in 2 out of 5 nests in Riley County, Kansas, in 1974 and 1975. The cowbird eggs disappeared, presumably ejected by the robins, on the day they were laid or on the next day. Also, robin eggs disappeared from both parasitized nests, possibly by removal by the cowbirds. It is of interest that cowbirds are able to carry off eggs as large as those of the robin (see also Blincoe, 1935; Rothstein, 1975a).

The robin and the gray catbird both show close to 100 percent rejection, but they differ in the time taken for rejection. Forty-five robin and 52 catbird nests were visited the day after they were parasitized experimentally. The cowbird egg had not yet been rejected at 42.2 percent of the robin nests but only 23.1 percent of the catbirds had not rejected after 1 day. This comparison is statistically significant. Other lines of evidence also indicate that catbirds are more intolerant than robins of foreign eggs (Rothstein, 1975c). Thus, cowbird eggs placed in robin nests are more likely to be found and reported than those placed in catbird nests. Therefore, if numbers of cases of observed parasitism are similar in both species, then the actual parasitism is likely to be higher in the catbird. As it happens, the 1963 review (Friedmann, pp. 69–70, 72–73) noted 26 cases of parasitism for both species. Furthermore, the total number of robin nests that have been examined is greater than that of catbird nests, since the former species is more abundant, is more widespread, and has a more easily found nest. Thus cowbird eggs are observed in a higher percentage of the total of catbird nests examined than of robin nests, even though they probably disappear more rapidly from the former.

These findings strongly imply that cowbirds parasitize catbirds more frequently than they do robins, although they have virtually no chance of success with either species. Whether this probable difference is due to host preference by the cowbird or to some other factor (such as differences in defense of the nest) is uncertain.

WOOD THRUSH