Scarlet Tanager

Piranga olivacea (Gmelin)

The true status of this species as a cowbird host is not well understood. Only about 50 cases were known as of the 1963 review (Friedmann, p. 136). This is not a large number for such a common and widespread species. However, the relative paucity of records may be due largely to the fact that this species has a nest that is difficult to find and to reach, since it is usually 7 or more meters above the ground. Several studies listed in 1963, and based on a series of nests, reported high rates of parasitism. The situation is further clarified by Prescott's (1965) monograph. Eleven of 14 (78.6 percent) nests of known contents found in southern Michigan from 1947 to 1949 contained cowbird eggs. An additional nest contained only an advanced cowbird nestling when found. Six nests (among the 11 parasitized ones) whose contents were known and that were successful fledged 7 cowbirds and 8 tanagers. The detrimental effects of cowbird parasitism were great and seemed to be due largely to egg removal by adult cowbirds and/or to decreased egg production by female tanagers. Prescott occasionally observed female cowbirds visiting tanager nests. Some visits even occurred while nestling cowbirds were in the nest, but no adult cowbirds were seen to feed nestlings. On one occasion Prescott watched a female cowbird lay in a tanager nest. This observation was unusual because the cowbird removed a host egg 6 seconds before laying her own egg, whereas workers who investigated other species of hosts reported that host eggs are not removed at the time the cowbird deposits her own egg. A very high incidence of parasitism is also shown by nests studied near Pellston, Michigan. A survey of unpublished reports filed (as of 1969) at the University of Michigan Biological Station shows that all 5 scarlet tanager nests studied there between 1940 and 1966 were parasitized. One nest, studied by J. K. Boon in 1966, had 2 tanager and 4 cowbird eggs.

Since a number of investigators working in diverse areas (data listed herein from Norris, 1947, and in the Friedmann, 1963, review) have reported very high rates of parasitism, it is possible that the scarlet tanager is one of the most heavily parasitized host choices in certain areas. Bull (1974: 537) mentions 12 instances of parasitism in New York State; 9 of these are additional to our earlier compilation. In the files of the Ontario nest records, at Toronto, are 2 more cases out of 36 nests reported from that province, a much lesser frequency of parasitism.

SUMMER TANAGER