Knowledge of the breeding behavior of Evening Grosbeaks (Coccothraustes vespertinus) is based largely on a multiyear study in Colorado during the 1980s. In Colorado, the duration of full incubation bouts by female grosbeaks had a mean of 26.6 min, a value repeated since then in the literature. For one nesting pair of grosbeaks I studied in Montana, full incubation bouts (n = 5) by the undisturbed female during 22 h of direct nest watches on 7 d in June 2018 had a mean of 112.6 min (range = 45–202 min), much longer than the value reported in the Colorado study. Incomplete incubation bouts (female already incubating on my arrival or at my departure) by the undisturbed Montana female (n = 11) during the same observation period had a mean of 56.2 min, 4 of which were a minimum of 94–146 min, further suggesting that longer duration incubation bouts may be the norm for this species. Duration of incubation bouts for Evening Grosbeak in the Colorado study appear to be brief relative to other North American fringillids, but my limited data suggest that Evening Grosbeaks are typical in this regard. Mean duration of 10 nest absences by the undisturbed Montana incubating female was 9.7 ± 7.6 min (range = 3–26 min), similar to the Colorado study (10.1 ± 10.7 min). Male behavior during incubation also appeared similar to results from Colorado (no attending the eggs, frequent exchange of vocalizations with the nesting female, escorting and feeding the female only when off the nest). I conclude that the duration of nest watches (60–80 min) in the Colorado study was inadequate to determine the length of many longer incubation bouts by female Evening Grosbeaks.
The Wilson Journal of Ornithology
Vol. 133 • No. 2
Vol. 133 • No. 2