干旱区生态环境知识资源中心

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 handlingtime, was avoidedby the3 sparrows. Buffelgrass, the largest seedwith the longest handling time, was avoided byBaird’s and Savannah sparrows. Bluegrama, the second-smallest seedwithrelative short handlingtime, wasneither preferrednor avoided, andgreensprangletop, anintermediate-sizedseedwith a relativelylongerhandlingtime, was avoidedin thechoice trialsbut consumedin thenonchoicetrials. These results indicate that exoticgrass seedsmaybea sourceof foodfor3 sparrowswinteringin the ChihuahuanDesert, provided they can be handled efficiently. Our results also show, however, thatwintering grassland sparrows are probably unable to consume sufficient buffelgrass and Lehmann lovegrass seeds tomeet daily energy requirements, indicating that these grasses may be a threat by reducing exploitable sources of seed food.