Seed selection by dark-eyed juncos (Junco hyemalis): optimal foraging with nutrient constraints?

Observations of the foraging behavior of six captive dark-eyed juncos (Junco hyemalis) are used to test the assumptions and predictions of optimal diet choice models (Pyke et al. 1977) that include nutrients (Pulliam 1975). The birds sequentially encountered single seeds of niger thistle (Guizotia abyssinica) and of canary grass (Phalaris canariensis) on an artificial substrate in the laboratory. Niger thistle seeds were preferred by all birds although their profitability in terms of energy intake (J/s) was less than the profitability of canary grass seeds. Of four nutritional components used to calculate profitabilities (mg/s) lipid content was the only characteristic that could explain the junco's seed preference. As predicted by optimal diet theory the probability of consuming niger thistle seeds was independent of seed abundance. However, the consumption of 71-84% rather than 100% of the seeds encountered is not consistent with the prediction of all-or-nothing selection. Canary grass seeds were consumed at a constant rate (no./s) independent of the number of seeds encountered. This consumption pattern invalidates a model that assumes strict maximization. However, it is consistent with the assumption that canary grass seeds contain a nutrient which is required in minimum amounts to meet physiological demands (Pulliam 1975). These experiments emphasize the importance of incorporating nutrients into optimal foraging models and of combining seed preference studies with studies of the metabolic requirements of consumers.