Abstract
Marine predators, such as seabirds, are useful indicators of marine ecosystem functioning. In particular, seabird diet may reflect variability in food-web composition due to natural or human-induced environmental change. Diet monitoring programmes, which sample diet non-invasively, are valuable aids to conservation and management decision-making. We investigated the diet of an increasing population of greater crested terns Thalasseus bergii in the Western Cape, South Africa, during three successive breeding seasons (2013 to 2015), when populations of other seabirds feeding on small pelagic schooling fish in the region were decreasing. Breeding greater crested terns carry prey in their bills, so we used an intensive photo-sampling method to record their diet with little disturbance. We identified 24,607 prey items from at least 47 different families, with 34 new prey species recorded. Fish dominated the diet, constituting 94% of prey by number, followed by cephalopods (3%), crustaceans (2%) and insects (1%). The terns mainly targeted surface-schooling Clupeiformes, with anchovy Engraulis encrasicolus the most abundant prey in all three breeding seasons (65% overall). Prey composition differed significantly between breeding stages and years, with anchovy most abundant at the start of the breeding season, becoming less frequent as the season progressed. The proportion of anchovy in the diet also was influenced by environmental factors; anchovy occurred more frequently with increasing wind speeds and was scarce on foggy days, presumably because terns rely in part on social facilitation to locate anchovy schools. The application of this intensive and non-invasive photo-sampling method revealed an important degree of foraging plasticity for this seabird within a context of locally reduced food availability, suggesting that, unlike species that specialise on a few high-quality prey, opportunistic seabirds may be better able to cope with reductions in the abundance of their preferred prey.
Highlights
Upper trophic-level predators can be used as indicators of marine food supplies as changes in their life history traits, such as their diet, often reflect variations in the environment (e.g., [1])
We report the species’ prey composition over three breeding seasons (2013–2015) to assess how the foraging ecology of greater crested terns in the Benguela region has been influenced by recent habitat conditions associated with regional shifts in pelagic fish availability
The foraging plasticity of the greater crested tern may be postulated as a short-term response to climate change; a key factor that allows this predator to thrive in comparison to other more specialist seabirds. This is the first investigation of diet in a single prey-loading seabird using a method that allows for intensive sampling over protracted periods with little disturbance [31]
Summary
Upper trophic-level predators can be used as indicators of marine food supplies as changes in their life history traits, such as their diet, often reflect variations in the environment (e.g., [1]). Seabirds have evolved several life-history characteristics to help buffer scarce and/ or unpredictable forage resources, they may still be affected negatively by reductions in food availability (e.g., [6]) Such negative impacts are evident during breeding, when prey availability and localized environmental factors affect seabirds because of range restrictions imposed by central-place foraging [7]. Other species are more versatile, enabling them to buffer changes resulting from climate change and/or human exploitation in the availability of their favoured prey species by switching to other, more readily available, but often lower quality food [12,13,14]. Such opportunistic feeding can be sufficient to maintain adult condition and survival, but is not optimal for the growth and survival of chicks [15, 16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
