Foraging specificity and prey utilization: evaluating social and memory-based strategies in seabirds

in Behaviour
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This study explores the capacity for seabirds to exhibit behavioral plasticity in response to the predictability of resources. Using direct species-comparisons, I tested the hypothesis that roseate terns (Sterna dougallii), dietary specialists, rely more heavily on foraging site-fidelity to pursue persistent prey, whereas common terns (S. hirundo), prey generalists, depend more on local enhancement by exploiting mixed-species assemblages. I analysed chick-provisioning observations and the bearings of commuting trajectories between the shared breeding colony, foraging areas, and feeding flocks. Foraging route patterns in roseate terns were consistent with a strategy based more heavily on spatial memory than social cues, in contrast to common terns, which associated more readily with nearby feeding aggregations, in line with social facilitation. Only during years of high prey abundance did roseate terns outperform common terns in nest productivity and the quality of prey delivered to chicks, suggesting that opportunistic tactics support resilience to sparse prey availability.

Foraging specificity and prey utilization: evaluating social and memory-based strategies in seabirds

in Behaviour



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    Wind directions at the study site. (a) One of the largest shared breeding grounds of both common and roseate terns in North America: Buzzards Bay, MA, USA, which includes Bird, Ram and Penikese Islands. (b) Buzzards Bay magnified: rose diagram (circular frequency histogram) of mean wind direction for all colony-level scans, at Bird Island. The black arrow represents the mean, and the black arc represents the 95% confidence interval. Data were downloaded online for the mouth of Buzzards Bay via NOAA weather station BUZM3 (2009), and for Marion inner harbour via KMAMARIO3 (2011). These maps were produced using ArcGIS Desktop 10.0 (ESRI 2012), with land feature and bathymetry data layers from the online Office of Geographic Information (MassGIS). This figure is published in colour in the online edition of this journal, which can be accessed via

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    Study design to test three foraging strategy hypotheses from correlations among flight directions and bearings of mixed-species feeding flocks. Positively-correlated angles of individual and colony-level departures and returns, and bearings of mixed-species feeding aggregations, are consistent with three foraging strategy hypotheses (Davoren et al., 2003; Weimerskirch et al., 2010), across three seasons at Bird Island. Axes denote cardinal directions on a compass (large black circles), with colony location at the centre. Black arrows: returns; dashed arrows: departures; single arrows: mean directions of marked individuals; multiple arrows: mean directions of the sampled colony; bird in flight: mixed-species feeding assemblages. Rows define the tested hypotheses, and columns represent sampling periods: either 1-h stints observing marked individuals, or 40-min scans of the colony. Correlation tests were conducted on commuting trajectories within sampling periods (left column) and across consecutive sampling periods (‘∼’ indicates between scans, within days). The relationships of significant correlations (positive or negative, p<0.05) indicate species-specific predictions and results, comparing roseate (RT, Sterna dougallii) to common terns (CT, S. hirundo). Significant positive correlations between: (a) individual departures and colony-level departures or returns show reliance on individual-level foraging fidelity (a combination of social and memory-based tactics); successive, positively correlated departures of marked individuals are in line with heavier dependence on spatial memory than information exchange, as predicted in roseate terns. Positive correlations among (b) colony-level departures and returns within scans, or between scans within a day, provide evidence for colony-level fidelity, as expected for both roseate and common terns. Positively correlated (c) bearings of feeding flocks and mean directions of colony-level departures or returns, within a scan, illustrate social facilitation (local enhancement) as predicted in common terns; negative correlations point towards competitive interactions (as with roseate terns). See Figure A4 for more detail. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Size of the three dominant fish delivered to chicks, by tern species, year, and prey category. Roseate terns (RT) delivered longer sandlance (‘Sl’), especially in 2009–2010. For the three years combined, (a) roseate terns delivered sandlance (μ=1.7) that were significantly longer than those delivered by common terns (CT, μ=1.6, p<0.05), and significantly longer than herring (‘Hg’) or anchovy (‘An’) delivered by either tern species (p<0.001). More specifically, there was a significant interaction of year with (b) tern species (F2,995=31.9, p<0.001) and with (c) prey category (F4,995=11.2, p<0.001). In the first two years, roseate terns delivered significantly longer sandlance than did common terns (p<0.05); compared to 2009, significantly shorter sandlance were delivered in 2011 by both roseate and common terns (p<0.001). Points and bars indicate mean ± standard error. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Sampled spring abundance of the three dominant prey categories by year. Sandlance (‘Sl’) was significantly more abundant in 2009 than in 2010–2011 (p<0.05). There was a significant effect on abundance (average number at length per 15 × 15 km cell) by prey category (F2,231=4.1, p<0.05) and year (F2,231=4.5, p<0.05). Anchovy (‘An’) abundance was negligible, as they are more common in the fall. Points and bars indicate mean ± standard error, herring = ‘Hg’. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Directions and relative frequencies of population-level routes to and from Bird Island during the 2009–2011 breeding seasons, observed from the lighthouse. The black arrow represents the mean direction, with a narrow 95% confidence interval arc, calculated by simple bootstrap using the concentration parameter kappa for a von Mises distribution, for: (a) common tern (CT) departures (N=12284), (b) common tern returns (N=10148), (c) roseate tern (RT) departures (3606), and (d) roseate tern returns (3467).

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    Relative frequencies of individual-level departures from Bird Island during the 2010–2011 breeding seasons, observed from blinds. The black arrow represents the mean direction, with a 95% confidence interval arc, for: (a) common terns (CT, N=204 departures of 61 individuals) and (b) roseate terns (RT, N=92 departures of 43 individuals).

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    Predicting foraging strategy and tactics from prey utilization, in the context of prey specificity and availability. If roseate terns pursue specific prey items that highly dominate their diet and are highly predictable in their availability, then it would be adaptive to use a highly memory-based strategy to revisit feeding hotspots, with spatial memory as the tactic of choice. Because roseate terns (grey shading) have a diet that is low in diversity, and low in transience (high in persistence), then they are less likely to use a social or facilitative strategy. Common terns rely on prey that are low in dominance and low in predictability, therefore, they are less likely to use spatial memory. Instead, they select a highly diverse set of patchy, highly transient prey, and thereby benefit from the use of a highly social strategy for opportunistic foraging on facilitative cues (e.g., local enhancement). In other words, if terns cannot remember where the food is, then they may turn to others for clues.

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    Sampled spring distribution of the three dominant prey categories by year. Counts of (a) sandlance, herring, and (b) anchovy (up to 15 cm long), averaged into 30 square grid cells, each 15 × 15 km (225 km2). Interspersed black dots represent prey sampling stations, white cells indicate missing data (areas of no survey coverage). The black boxes (a) outline cells of persistent prey distribution across the three inshore bottom trawl surveys (mid-May, 2009–2011), conducted by the Massachusetts Division of Marine Fisheries (MADMF; King et al., 2010). This figure is published in colour in the online edition of this journal, which can be accessed via

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    Depiction of Table 2 to visualize correlations in flight patterns, for tests of foraging strategy hypotheses. Relationships (positive or negative) of significant circular correlation coefficients (∗∗∗p<0.001, ∗∗p<0.01, p<0.05) among theoretical angles of departure or return (single arrows, marked individuals; double arrows, colony), and feeding flocks (tern in flight) indicate which test results are consistent with common (CT, blue) and roseate tern (RT, red) foraging fidelity, social interactions (facilitation or competition), or memory use. Observations recorded within 40-min colony-level scans are separated by minutes; within-day correlations between observations, up to hours apart, suggest the persistence of patterns, either between scans or between 1-h individual stints. Axes denote cardinal directions on a compass, with Bird Island at the centre. See Figure 2 for more details. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Per capita chick provisioning rates by tern species. Across all years combined, provisioning rates of common terns (number of prey delivered per hour) were significantly higher than those of roseate terns, both by nest (t147.2=3.8, p<0.001), and by chick (t147.8=2.8, p<0.01, above, mean ± standard error bars).

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    Size of prey deliveries related to travel directions. Terns delivered significantly longer prey to their chicks after departures to the southeast, according to a generalized additive mixed model (GAMM), using individual as a random effect: F2,53=12.7, R2=0.26, p<0.001.

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    Images of a common and roseate tern carrying sandlance. A common tern with sandlance (a) and roseate tern with two sandlance (b), a rarely photo-documented event since terns are generally single-prey loaders. This figure is published in colour in the online edition of this journal, which can be accessed via


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