Type: report

Article abstract: Here we present information on the influence of vegetation growth, dune habitat, and enclosure fencing on nest site selection and productivity of California Least Terns (Least Terns) at the Venice Beach Colony, Marina Del Rey, California (colony). This study originated from a plan to use heavy equipment to flatten dunes and remove vegetation from the colony, and contradictory observations by project biologists that terns were mostly using those dunes for nesting, and nesting successfully. This study consisted of investigating both nest site selection by the terns and the productivity of those nests under several natural and manipulated treatments. However, heavy egg predation by the American Crow at this colony exerts a strong influence on nest placement and nest success. Our findings indicate that predation by American Crow exerts an "edge effect," with the heaviest predation on individuals away from the center of the colony and closest to the fence. We found that nests were less likely to succeed if they were placed within 20 m of the enclosure fence, in grids with fewer than 5 other nests (<125 nests/ha), more than 5 m from their nearest neighbor and more than 70 m from the center of the colony. Additionally, terns were more likely to be predated in areas with less than 5% vegetation cover, and prefer to nest, and are most successful, in areas with 20-40% vegetation cover. We found that the best vegetation management technique was to reduce vegetation to less than 30% cover, but even this was not as successful as areas that are naturally between 5-30% vegetation cover. The terns also prefer to nest, and are most successful in areas with dunes, although our finding indicate that predation increases with the number of dunes in an area. Based on these findings we recommend: 1. Manage the American Crow and other nest predators with a goal of decreasing the strong edge effect and colony failures. We must continue aggressive management to discourage incursions into the colony and use volunteer observers to inform staff of predation rates. 2. Nesting should be discouraged within 20 meters of the fence. 3. Control vegetation, with a goal of maintaining 20-40% cover in nesting areas within the colony. Vegetation manipulation will likely have the most impact if used to maintain open areas in the vegetated flat (Figure 2) and increase cover in the newly exposed areas (Figure 2). We should accomplish this by first removing non-nativ