Linkages among foraging movements, prey provisioning, and reproduction in a central place foraging seabird.

Abstract

Background: A seabird's ability to efficiently find prey directly affects reproduction, and seabird population declines are frequently linked to forage fish depletion. Reduced forage fish availability often limits seabird egg production and chick provisioning, but impacts to foraging movements remain largely unresolved, as does the effect of movement flexibility on provisioning and reproduction in conditions of reduced forage fish availability.

Methods: This study assesses drivers of foraging movement and the role of foraging movement in mediating provisioning and reproduction in a central place foraging seabird, the common tern (Sterna hirundo), by integrating movement (GPS telemetry), chick provisioning, and reproductive productivity data. Data were collected across four breeding seasons (2021-2024) on the largest common tern colony in the Gulf of Maine, where ocean warming is precipitating shifts in forage fish populations.

Results: In poor prey and productivity years, when clutch size (eggs per nest), fledging success, and provisioning of preferred prey (herrings [Clupea harengus, Alosa spp.], hakes/rockling [Merluccius bilinearis, Urophycis spp., Enchelyopus cimbrius], sand lances [Ammodytes spp.]) were reduced, common terns increased foraging trip durations by 23%-87% and expanded foraging ranges by 17%-51%. Range expansions occurred regardless of sex or breeding status and were driven by increased use of distant waters (up to ~150 km from the colony) rich in forage fishes preferred by common terns. In years of increased reproductive success and high preferred prey provisioning, foraging movements were shorter, and foraging locations focused on waters immediately surrounding the colony (within ~10 km), as well as two nearby river mouths (Piscataqua River and Merrimack River).

Conclusions: Our results suggest that common tern foraging movements are flexible in response to forage fish availability, but that increased movement effort in years of reduced prey availability may be insufficient to maintain chick diet and survival. In regions like the Gulf of Maine, where ocean-warming continues to induce shifts in forage fish populations, additional protection may be critical to central place foraging seabirds. Without proactive management, breeding seasons marked by increased foraging movement effort with little resulting reproductive success may become increasingly frequent.