Seed size, bill morphology, and handling time influence preferences for native vs. nonnative grass seeds in three declining sparrows | Tamaño de semilla, morfologia del pico y tiempo de manipulación influyen sobre las preferencias de semillas de pastos nativos vs. exóticos en tres gorriones en declive

The invasion of exotic grasses is a potential threat to the winter habitat of migratory grassland birds by reducing native sources of seed food. We compared seed preferences among 3 native (blue grama [Bouteloua gracilis], sideoats grama [Bouteloua curtipendula], and green sprangletop [Leptochloa dubia]), and 3 exotic (Lehmann lovegrass [Eragrostis lehmanniana], buffelgrass [Pennisetum ciliare], and natal grass [Melinis repens]) grass seeds in captive Baird's (Ammodramus bairdii), Grasshopper (A. savannarum), and Savannah (Passerculus sandwichensis) sparrows to investigate factors that determine vulnerability of grassland passerines to exotic grasses in the Chihuahuan Desert. We hypothesized that seed handling time would determine the ability of sparrows to exploit exotic grass seeds, and that larger-billed birds would be able to better exploit a larger variety of seeds, including exotic seeds. We offered seeds in choice and nonchoice trials and determined handling times for the different seed and bird species. The results indicate that handling time in relation to seed size determined seed preferences. Sparrows preferred seeds they could handle more efficiently to maximize energy intake over time. Baird's and Savannah sparrows, with intermediate and small bill sizes, respectively, preferred natal grass seeds of intermediate size and short handling time, and Grasshopper Sparrows, with the largest bill, preferred the larger sideoats grama seeds. Lehmann lovegrass, the smallest seed with intermediate handling time, was avoided by the 3 sparrows. Buffelgrass, the largest seed with the longest handling time, was avoided by Baird's and Savannah sparrows. Blue grama, the second-smallest seed with relative short handling time, was neither preferred nor avoided, and green sprangletop, an intermediate-sized seed with a relatively longer handling time, was avoided in the choice trials but consumed in the nonchoice trials. These results indicate that exotic grass seeds may be a source of food for 3 sparrows wintering in the Chihuahuan Desert, provided they can be handled efficiently. Our results also show, however, that wintering grassland sparrows are probably unable to consume sufficient buffelgrass and Lehmann lovegrass seeds to meet daily energy requirements, indicating that these grasses may be a threat by reducing exploitable sources of seed food.