Seasonal changes in diet and diet selection of great bustards (Otis t. tarda) in north-west Spain

Faecal analysis was used to study the diet of great bustards in north-west Spain on a monthly basis for 1 year. After correction for differential dige stibility, the diet composition by dry weight varied from 48.4% green plant material, 40.9% invertebrates and 10.6% seeds in August, to consisting alm ost completely of green plant material between December and March. At least 65 plant species were represented in the green plant material, but most oc curred rarely and the bulk of this portion of the diet consisted of alfalfa . Eight invertebrate orders were detected in the faeces, of which Coleopter a, Hymenoptera and Orthoptera were most important by numbers. From August t o November when seeds were important by dry weight, wheat and barley seeds were found in more faecal pellets than other species. Estimates of use and abundance were obtained to evaluate diet selection in the green plant mater ial and invertebrate components. Alfalfa was strongly selected throughout t he year, but grasses were used less often than expected. Coleoptera were al ways eaten in greater numbers than expected, Orthoptera were used either in proportion to abundance or less often than expected, and Hymenoptera less often in May, more often in September and November, and as expected at othe r times. The results support the suggestion that increasing alfalfa cultiva tion would be a useful management tool for maintaining endangered great bus tard populations, but further work on the cost-effectiveness of this option is needed.