Food Profitability and the Foraging Ecology of Crossbills

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Abstract
Observations over a two and a half year period in the Northeast United States and adjacent Canada indicate that White—winged Crossbills (Loxia leucoptera) and Red Crossbills (L. curvirostra) shift their diets among the seeds of various conifer species in a seasonal pattern. Both crossbill species forage on white spruce (Picea glauca) in late summer. White—winged Crossbills continue foraging on white spruce or tamarack (Larix laricina) until late autumn or winter, when they switch to black spruce (P. mariana). Red Crossbills switch to white pine (Pinus strobus) in early autumn and to red pine (P. resinosa) or other pines in winter or spring. This pattern of diet shifts is consistent with the hypothesis that crossbills forage to maximize food intake rate. Profitability (milligrams of kernel ingested per second) was measured for crossbills foraging on each of these conifers by observing rates of seed ingestion in the field, and subsequently measuring dry seed kernel masses. Profitability for a given conifer increases as cones and seeds mature, then declines as seeds are shed from the cones. Because conifers differ in the timing of cone ripening, crossbills experience sequential peaks in profitability. Both crossbill species usually foraged predominately on the most profitable conifer species, with switches in conifer use coinciding with shifts in relative profitabilities. However, crossbills often forage on more than one conifer species at a time, even though intake rates might be maximized by foraging on only the most profitable conifer. Predation does not appear to influence diet selection and large—scale patterns of conifer use. Crossbill movements and patterns of abundance, both on local and continent—wide scales, are correlated with patterns of profitability. Dietary overlap between crossbill species is greatest, often approaching 100%, in late summer when seed is most abundant. Overlap then declines rapidly when white pine cones open, and usually remains negligible most of the remainder of the year. Patterns of dietary overlap are a result of differences in profitability for each crossbill species that are largely inherent in the ripening phenology of conifer cones and seed accessibility, not seed depletion by crossbills. Interspecific competition is most likely between White—winged Crossbills and either the smallest form of the Red Crossbill or redpolls (Carduelis sp.) during the late winter of "invasion" years, which occur every 3—4 yr. Intraspecific competition is also likely to be most intense at these times.