Common Yellowthroat

Geothlypis trichas (Linnaeus)

Over 300 cases of cowbird parasitism on this warbler have been reported. These should yield some significant indices, but the evidence is mixed. Thus, in the collections of the Western Foundation, 124 sets of eggs of eastern races include 13 that are parsitized (10.5 percent); 159 sets of California races include 12 parasitized sets (7.5 percent). However, data from a few eastern areas where sizeable samples are available (Friedmann, 1963:117–120), yield the following: southern Quebec, 113 nests, 8 of which (7.1 percent) are parasitized; Ohio, 41 nests, 19 (46.3 percent) parasitized; Michigan, 90 nests, 35 (38.8 percent) parasitized.

Two records of parasitized nests of the host race G. trichas campicola from Alberta, in the Prairie Nest Cards Scheme, are the third and fourth instances for that subspecies of the yellowthroat. Another record, from Canston, British Columbia, listed previously (Friedmann, 1963:120) as G. trichas occidentalis is probably better considered as G. trichas campicola. A new, and valid, record for G. trichas occidentalis, from Barr Lake, Colorado, is in the collections of the Western Foundation.

As an indication of the ability of the yellowthroat to rear a sizeable brood, mention may be made of a nest observed in the Goleta Slough, Santa Barbara County, California, by Kelly Steele, that fledged 1 young cowbird and 3 young yellowthroats.

HOODED WARBLER