Relative roles of static and dynamic abiotic conditions as drivers of foraging behaviour in breeding Sandwich Terns

In this study, led by Bureau Waardenburg, breeding Sandwich Terns in colonies near the Dutch North Sea coast were fitted with GPS tags. These recorded the date, time, position and speed of each bird at five minute intervals. The study authors broke down each tern’s trip away from the nest into three key categories: ‘resting’, when birds were on the water, ‘transit’ flight, where birds were travelling fairly directly to or from the nest and a particular location at sea, and ‘foraging’, where birds performed a series of turns and concentrated their movements on one area in search of food. 

The results showed that the switch from transit flights to foraging - when birds stopped travelling to look for food - was most strongly correlated with the grain size of sediment in the water column. This is linked to the presence of sandeels, which are the staple diet of Sandwich Terns (and other seabird species) throughout the breeding season. Sandeels associate with relatively coarse, sandy sediments, and tern foraging focused in areas where these conditions were present. Terns also foraged more in areas of higher salinity, which is also linked to higher sandeel abundance, as well as to the presence of Herring, another Sandwich Tern prey species.

The study reveals that Sandwich Terns can adapt their behaviour to changes in their environment. By foraging over broad areas, birds may be able to maximise the prey they encounter despite changeable conditions. However, these conclusions need further investigation in areas this work could not fully consider, such as finer-scale habitat details. Together, these findings could have wider implications for the conservation of Sandwich Terns and related species, particularly given the threats that seabirds are currently facing like climate change, offshore marine developments, disease and alteration to coastal habitats.