Spatial and temporal predictability drive foraging movements of coastal birds

Abstract

Background: Temporal and spatial predictability of food resources are critical to the foraging efficiency of central place foragers. While site fidelity is often assessed in this context, route fidelity, or the repeated use of the same path while traveling, and temporal aspects of habitat predictability have received less attention. We examined how the use of urban, coastal, and offshore habitats influenced spatiotemporal predictability in the foraging patterns of herring gulls (Larus argentatus) and great black-backed gulls (L. marinus). Since gulls show higher site fidelity when foraging in urban habitats, we predicted that these trips would also show higher route fidelity. Similarly, we predicted that gulls foraging in coastal habitats would adapt the timing of foraging trips relative to tides.

Methods: We analyzed GPS tracks of herring gulls (n = 79) and great black-backed gulls (n = 37)-between 2016-2022 from four nesting colonies whose surrounding areas varied in their degree of urbanization. Fréchet distance, which is defined as the repeated use of the same path while traveling, was used to assess route fidelity, within colonies and between habitat types. We also compared the consistency of foraging trip timing relative to tidal stage and day of week, respectively, across habitat types.

Results: Neither herring nor great black-backed gulls showed higher route fidelity in urban habitats. Herring gulls showed direct travel between urban foraging sites but revisited sites in different orders, suggesting that a mosaic map may be used to navigate between known urban foraging sites. Herring and great black-backed gulls that foraged at coastal sites exhibited patterns in trip timing in relation to the tidal cycle, with foraging primarily occurring at or around low tide. Herring gulls in urban environments foraged more on Fridays and weekends, possibly due to increased or altered human activities on these days.

Conclusions: Our results demonstrate the importance of spatial memory and spatiotemporal predictability of gull foraging habitats and highlight the extent to which gulls adjust their movements based on their foraging habitats.

Keywords: Larus argentatus; Larus marinus; Fréchet distance; GPS tracking; Great black-backed gull; Herring gull; Route fidelity; Seabirds; Tidal patterns.

Fig. 1

Location of four study colonies In New York and Massachusetts on the east coast of the United States. The extent of the detailed map is shown within North America on the left

Fig. 2

Fréchet distances for herring gulls (left) and great black-backed gulls (right) foraging in different habitats from AOVs accounting for colony effects. Values reflect means ± standard error. Urban (coastal/offshore) habitats reflect foraging trips in which birds were exclusively foraging in urban (coastal/offshore) habitats, while multiple habitats reflect foraging trips in which birds used multiple habitats. Note that lower Fréchet distances reflect higher route fidelity

Fig. 3

Bar plots and stacked rose plots showing the average proportion of herring gull foraging points (± SE for bar plots) across individuals in each habitat type and tide time, and frequency of occurrence of foraging points in each two-hour tidal period within urban, coastal and offshore habitats across all individuals at Young’s Island, Jamaica Bay, Muskeget and Tuckernuck, respectively. The tidal cycle is broken up into 2-h periods, low tide, flood tide 1, flood tide 2, high tide, ebb tide 1, ebb tide 2

Fig. 4

Bar plots and stacked rose plots showing the average proportion (± SE) of great black-backed gull foraging points across individuals in each habitat type and tide time, and frequency of occurrence of foraging points in each two-hour tidal period within urban, coastal and offshore habitats across all individuals at Young’s Island and Muskeget, respectively. The tidal cycle is broken up into 2-h periods, low tide, flood tide 1, flood tide 2, high tide, ebb tide 1, ebb tide 2

Fig. 5

Example of consecutive foraging trips (numbered 1–10) of a herring gull tracked at the Young’s Island study colony. The extent of the foraging trip plots is shown in the map of Long Island, New York in the upper left of the figure. The tagged bird made repeat visits to several urban foraging spots, shown with colored circles. Foraging areas that were not visited repeatedly are shown in gray. Colony point colored pink. Arrows along tracks show the direction of travel