Pollen analysis from nest provisions of two populations of Osmia lignaria propinqua Cresson from the same valley over a 2-yr period showed a seasonal pattern beginning with willow, Salix spp. (Salicaceae), and progressing to bitter brush, Purshia tridentata (Rosaceae). Population I used twice as many pollen species but had similar niche breadths and constancy measures to Population II. There was a yearly difference in the percentage of the dominant pollens collected. The percentage of cells with one pollen showed between year, population, and season differences. There were no differences in the populations' nesting successor offspring production. The foraging differences apparently resulted from the proximity of the dominant pollens, Salix and Purshia, to the nest sites. Salix was a minimum of 600 m (one way) from Population I, but only 300 m from Population II. Purshia was within 200m of both populations. The increased distance traveled by Population I bees allowed foraging on other pollen sources: Camassia (Liliaceae), Ceanothus (Rhamnaceae), Delphinium (Ranunculaceae), Phacelia (Hydrophyllaceae), and Potentilla (Rosaceae). These pollen species were available within 200 m but were rarely used by Population II. Bees were using as dominant pollen sources cornucopian species in the form of shrubs or small trees having showy floral displays of flowers with exposed, erect anthers extending beyond the petals. © 1990 Entomological Society of America.
