Data from: Seabirds vary responses to supplemental food under dynamic natural prey availability and feeding aggregation composition

While foraging, a predator can feed solitarily or in a group. The net
energy gain of joining a group is predicted to vary with prey patch
quality, species-specific prey capture behavior and the size and species
composition of the predator group. In coastal Newfoundland, Canada,
capelin (Mallotus villosus), a key forage fish, migrates inshore to spawn
during the summer, resulting in a dramatic shift in prey availability.
During July-August, 2015-2017, we examined the numerical and behavioral
responses of procellarid (Great Shearwater Ardenna gravis, Sooty
Shearwater A. grisea, Northern Fulmar Fulmarus glacialis) and gull species
(Herring Gull Larus argentatus, Great Black-backed Gull L. marinus) to
fish offal under varying capelin availability as well as flock size and
composition using an at-sea experiment on the northeast Newfoundland
coast. The experiment consisted of providing a offal every 30 s (10-min
experimental period), along with 10-min control periods before and after.
We recorded the species-specific number of birds on the water, the number
of birds simultaneously attempting to capture offal, and the number of
successful attempts (‘foraging success’). The number of birds on the water
was lower during high capelin availability for all species, except for
Northern Fulmar. The number of conspecifics simultaneously attempting
increased with the number of conspecifics on the water, but plateaued at
different numbers (4-17) for each species. The species-specific proportion
of successful attempts (i.e. ‘foraging success’) varied with flock size
and composition (i.e. number of conspecifics, heterospecifics, species).
Foraging success of Herring Gulls and fulmars were moderately affected by
flock size and composition, suggesting that they may be dominant
competitors. Findings suggest that seabirds rely more heavily on
supplemental food sources, such as fisheries discards and offal, when
natural prey availability declines, potentially resulting in a higher risk
of by-catch during fisheries activities as forage fish stocks decline.