Adaptive geographic variation in Western Scrub-Jays

Bill size and shape of Western Scrub-Jays (Aphelocoma californica) vary,geo graphically in relation to habitat. Scrub-jays occupying pinyon-juniper woo dlands ("pine" scrub-jays) have relatively long, shallow bills, while scrub -jays occupying oak woodlands ("oak" scrub-jays) have relatively, stout, de curved bills. We captured five pine and five oak scrub-jays, and submitted them to feeding trials on pinyon pine (Pinus monophylla) cones and acorns ( Quercus agrifolia) to determine whether geographic variation in bill struct ure was adaptive for foraging on these critical food resources. Three lines of evidence indicated that scrub-jays occuring in habitat dominated by eit her pinyon pine or oak had adaptive bill structure. First, scrub-jays exhib ited a steep trade-off in feeding performance on pine cones and acorns. Pin e scrub-jays extracted and consumed pinyon seeds more than 30% faster than did oak scrub-jays. Oak scrub-jays penetrated and consumed acorns twice as fast as pine scrub-jays did. Second, measures of feeding performance were m oderately to strongly correlated with different measures of bill structure. Third, bill shape as measured by the ratio of bill length to depth was the metric that (1) best accounted for variation in feeding rates and (2) was related to the relative amount of pine and oak in habitats of 12 different populations of scrub-jays. These results, as well as others, were consisten t with observed geographic variation in scrub-jays.