Thursday
Mar082018

How far will streaked horned larks disperse to breed in South Puget Sound?

In South Puget Sound, the population size of the federally-threatened Streaked Horned Lark is approximately 250 individuals, all of which live on six sites - four airfields and two prairies. Although recovery of this species will clearly require increasing population size, it will also require increasing the number of occupied sites. Currently, only two occupied sites are found outside of Joint Base Lewis-McChord military base (JBLM). To better understand the likelihood of larks colonizing new sites, CNLM has been investigating how larks move within the region, especially first-year individuals selecting their first breeding location. One of the important factors about lark movements that we’re trying to determine is what proportion of young return to the site where they hatched to breed versus the proportion that disperse to a different site. That would also allow us to measure how far they disperse.

Since 2011, we have been uniquely color-banding nestlings at JBLM, then later visiting all of the occupied sites in South Puget Sound to locate and identify individuals during the breeding season. Over that time, we have re-observed 139 first-year adults originally banded as nestlings. Most of these first-year adults (74%; 103 of 139) bred at the site where they hatched, but 26% dispersed to different sites (See figure). Of those individuals dispersing to a different site, most traveled around 10-15 kilometers, but some individuals traveled over 35 kilometers (e.g., JBLM to the Olympia Airport). No birds from JBLM dispersed to Sanderson Airfield in Shelton, WA, a distance of about 45 kilometers. The longest dispersal event was a female banded at McChord Airfield, who was subsequently detected on an island in the Columbia River nearly 135 kilometers away, although we found no evidence of breeding.

Map showing dispersal of Streaked Horned Larks from original hatching sites.

These results indicate that first-year larks in South Puget Sound appear quite capable of traveling the necessary distance to colonize other sites within the region. Most of the prairie conservation lands in the South Puget Sound that could serve as potential sites for larks (e.g., Glacial Heritage, Scatter Creek) are within 10-30 kilometers of currently occupied sites, and thus seem available for lark colonization. Curiously, the distribution of larks in South Puget Sound has remained consistent for at least two decades, whereas in other regions of its range, such as the Willamette Valley and Columbia River, larks often colonize new sites as habitat is created.

We think there are a number of hypotheses for why larks haven’t colonized new sites in the South Sound. First, there might be too few birds to expect colonization of new sites (Allee effect) or population growth is too low to create enough surplus individuals. Second, larks may have difficulty finding new prairie sites because they are disjunct from occupied sites and surrounded by forested habitat. Finally, habitat at unoccupied sites may be unsuitable, even though it may look suitable to a human’s eye. Investigating these hypotheses will be important for advancing recovery of the Streaked Horned Lark.