Forster's tern chick survival in response to a managed relocation of predatory California gulls

Abstract

Gull populations can severely limit the productivity of waterbirds. Relocating gull colonies may reduce their effects on nearby breeding waterbirds, but there are few examples of this management strategy. We examined gull predation and survival of Forster's tern (Sterna forsteri) chicks before (2010) and after (2011) the managed relocation of the largest California gull (Larus californicus) colony (24,000 adults) in San Francisco Bay, California. Overall, survival of radio‐marked Forster's tern chicks from hatching to fledging was 0.22 ± 0.03 (mean ± SE), and daily survival rates increased with age. Gulls were the predominant predator of tern chicks, potentially causing 54% of chick deaths. Prior to the gull colony relocation, 56% of radio‐marked and 20% of banded tern chicks from the nearest tern colony were recovered dead in the gull colony, compared to only 15% of radio‐marked and 4% of banded chicks recovered dead from all other tern colonies. The managed relocation of the gull colony substantially increased tern chick survival (by 900%) in the nearby (<1 km) colony from 0.04 ± 0.02 in 2010 to 0.40 ± 0.12 in 2011 but not at the more distant (>3.8 km) reference tern colony (0.29 ± 0.10 in 2010 and 0.25 ± 0.09 in 2011). Among 19 tern nesting islands, fledging success was higher when gull abundance was lower at nearby colonies and when gull colonies were farther from the tern colony. Our results indicate that the managed relocation of gull colonies away from preferred nesting areas of sensitive waterbirds can improve local reproductive success, but this conservation strategy may shift gull predation pressure to other areas or species. © 2014. This article is a U.S. Government work and is in the public domain in the USA.

Evaluation of Caspian tern (<em>Hydroprogne caspia</em>) and snowy plover (<em>Charadrius alexandrinus nivosus</em>) nesting on modified islands at the Don Edwards San Francisco Bay National Wildlife Refuge, California—2016 Annual Report

“…Of these predators, California gulls were by far the most numerous, and are well-documented predators of waterbird eggs and chicks in south San Francisco Bay (Herring and others, 2011;Ackerman, Herzog, Hartman, and Herring, 2014;Ackerman, Herzog, Hartman, and Takekawa, 2014). California gulls were particularly abundant at Pond A16, making up 56 percent of the surveyed birds observed.…”

“…The California gull (Larus californicus) population in San Francisco Bay has increased from fewer than 200 breeding gulls in 1982 to more than 47,000 breeding gulls in 2015 (Strong and others, 2004;Washburn and Butler, 2016). They occur in large numbers around the project area, and have been identified as dominant predators of waterbird eggs and chicks (Herring and others, 2011;Ackerman, Herzog, Hartman, and Herring, 2014;Ackerman, Herzog, Hartman, and Takekawa, 2014). Thus, gull dissuasion was considered a potentially important component for successful nesting of Caspian terns and snowy plovers.…”

Social attraction used to establish Caspian tern (Hydroprogne caspia) nesting colonies on modified islands at the Don Edwards San Francisco Bay National Wildlife Refuge, California—Final report

“…The California gull (Larus californicus) population in San Francisco Bay has increased from fewer than 200 breeding gulls in 1982 to more than 38,000 breeding gulls in 2016 (Strong and others, 2004;Tarjan and Butler 2016). Gulls occur in large numbers around the project area, and have been identified as dominant predators of waterbird eggs and chicks (Herring and others, 2011;Ackerman, Herzog, Hartman, and Herring, 2014;Ackerman, Herzog, Hartman, and Takekawa, 2014). Thus, gull dissuasion was considered a potentially important component for successful nesting of Caspian terns and snowy plovers.…”

“…12C). California gulls, a major predator of waterbird eggs and chicks (Herring and others, 2011;Ackerman, Herzog, Hartman, and Herring, 2014;Ackerman, Herzog, Hartman, and Takekawa, 2014), were abundant at Pond A16 during all years. However, at Pond A16, the average count of California gulls in May and June decreased 57 percent from an average of 270 birds in 2015 to an average of 115 birds in 2017 ( fig.…”

“…Of these, California gulls were by far the most numerous, and are well-documented predators of waterbird eggs and chicks in South San Francisco Bay (Herring and others, 2011;Ackerman, Herzog, Hartman, and Herring, 2014;Ackerman, Herzog, Hartman, and Takekawa, 2014). California gulls were particularly abundant at Pond A16.…”