M. Bertellotti et P. Yorio, Age-related feeding behaviour and foraging efficiency in Kelp Gulls Larus dominicanus attending coastal trawlers in Argentina, ARDEA-T NED, 88(2), 2000, pp. 207-214
We studied age-related differences in feeding behaviour and foraging succes
s of Kelp Gulls Larus dominicanus feeding on fishery discards at trawl vess
els in northern Patagonia, Argentina. Kelp Gulls consumed fish by direct ca
pture and intraspecific kleptoparasitism. Direct capture rate increased sig
nificantly with age. Juveniles dropped a higher proportion of handled fish
than older birds. Intraspecific kleptoparasitism involved gulls of all age-
classes and was recorded in all hauls. The percentage of successful attempt
s was similar among different age-classes. However, juveniles and immatures
attempted to steal prey more frequently and therefore obtained more prey t
han expected. The proportion in which each feeding method (direct capture a
nd kleptoparasitism) contributed to overall fish consumption varied signifi
cantly among age-classes. Juveniles obtained most of the food through klept
oparasitism (72%), while immature, subadult and adults consumed prey mostly
through direct capture (77, 91 and 92%, respectively). Using both feeding
methods, Kelp Gulls swallowed 87.6% of the discarded fish that they handled
. However the percentage of fish swallowed over total handled fish were dif
ferent among age-classes (juveniles 45.5%, immatures 70.8%, subadults 83.3%
, adults 94.8%). Kleptoparasitism may be used as an alternative strategy to
compensate the lower efficiency of young birds.