THE HAZARDS OF NESTING NEAR SHRUBS FOR A GRASSLAND BIRD, THE MCCOWNS LONGSPUR

Half of all nests of the McCown's Longspur (Calcarius mccownii) were d epredated in the shortgrass prairie of northcentral Colorado, and almo st all nesting failures in this species were attributable to predators , primarily the thirteen-lined ground squirrel (Spermophilus trideceml ineatus). Nests placed beside shrubs were 2-3 times more likely to be depredated than nests associated with other types of vegetation. Half of all nests (n = 28) were associated with shrubs in a moderately graz ed pasture, and 80% of these nests were eventually depredated. In cont rast, most nests (57%, n = 21) were placed beside cactus in a heavily grazed pasture, which had little shrub cover. Of the 20% of nests plac ed beside shrubs in this pasture, however, 75% were depredated. Factor analysis of habitat variables measured at several scales around nests generated one factor that significantly explained variation in nest f ate. This factor described shrub cover within 1 m of the nest and midg rass cover and degree of heterogeneity (continuity of the shortgtass m atrix) at 2 m. Fine-scale (1 m) shrub cover was the most important det erminant of predation risk. Nests that were depredated during incubati on had six times more shrub cover within 1 m of the nest than nests de predated during the nestling period; successful nests had no measurabl e (greater than or equal to 5% total cover) shrub cover at this scale. The relationship between shrubs and an increased risk of nest predati on is consistent with incidental predation by ground squirrels, which concentrate their activities beneath the cover afforded by shrubs.