Regional variation in response of Field Sparrows to the threat of Brown-headed Cowbird parasitism

We conducted aggression experiments using model cowbirds on nesting Field S parrows (Spizclla pusilla) in heavily, moderately, and rarely parasitized p opulations. We also documented Field Sparrow morning nest arrival times dur ing the laying period, because Field Sparrows appear to desert nests in res ponse to encounters with laying female cowbirds. Field Sparrows responded m ost aggressively to cowbird models and arrived the earliest in Illinois, wh ere they were most heavily parasitized. Field Sparrows responded the least to models in Pennsylvania, where they are almost never parasitized. Our res ults suggest that those host behaviors result from some aspect of host-cowb ird interactions, but the extent to which such behaviors are genetic or lea rned needs further study.