eWestern BirdsThe Quarterly Journal of Western Field Ornithologists
Vol. 35, No. 2
Western Field Ornithologists
Clutch Sizes and Nesting Habits of Birds at Tioga Pass
Martin L. Morton, and Maria E. Pereyra
ABSTRACT: We recorded clutch sizes for several birds, mostly passerines, during a lengthy study of vertebrate populations in the Sierra Nevada of California near Tioga Pass. Here we report frequencies of the various clutch sizes observed, descriptions of nest sites, and other natural-history notes. In at least three species, the Hermit Thrush (Catharus guttatus), American Robin (Turdus migratorius), and White-crowned Sparrow (Zonotrichia leucophrys), clutches in this montane setting tended to be larger than those reported for lowland regions. In the two species studied in greatest detail, the Dusky Flycatcher (Empidonax oberholseri) and White-crowned Sparrow, clutch size decreased through the breeding season with date of clutch initiation. We suggest that this commonly observed seasonal trend is cued by decreasing day length and that it is an expression of a gradual, as opposed to abrupt, onset of photorefractoriness.