Streaked Horned Lark (Eremophila Alpestris Strigata) Nest Predation on Lowland Puget Prairie Remnants, Washington State – The Effects of Internal Edges and Scot’s Broom (Cytisus Scoparius)

Predation is the leading cause of nest failure for the streaked horned lark (Eremophila alpestris strigata). I examined potential factors affecting streaked horned lark nest predation and nest predators at three lowland Puget prairie sites in Washington State. I identify likely primary predators and discuss their potential contribution. Contrary to results reported in the literature for grassland birds, the primary predators of the streaked horned lark are most likely avian, corvids in particular. Many studies document the occurrence of deleterious “edge effects” within 50m of an external edge of both forests and grasslands. Streaked horned larks do not nest within this 50m range, but do nest among light-use roads and airport runways/taxiways present at their breeding sites. Few studies have addressed internal fragmentation of this nature and the effect on breeding birds. At each site, the distance from nest site to nearest internal edge was measured and the corresponding edge type (pavement, gravel, or dirt) was recorded for all nests discovered during 2002-2004 (n = 166). Also measured in 2004 was the distance from nest site to the nearest Scot’s broom (Cytisus scoparius) plant and the estimated percent cover of Scot’s broom within a 25m radius around the nest site was recorded (n = 45). Logistic regression was used to examine the relationship between nest outcome (depredated or successful) and the above listed factors. None of the factors examined affected streaked horned lark nest outcome. Using nearest neighbor analyses in ArcGIS 9.0, significant aggregation of nest sites was detected at two study sites. Nest clustering has been shown to be an effective defense against avian predators. Additional independent data is needed to confirm the aggregation. Future study of nest predation issues such as positive identification of lark predators, the importance of habitat patch size, and the effects of fragmentation and urbanization is needed.