Information provided by Daguerre (1933), Olrog (1967), and Devillers (1977) already indicated that the Olrog’s Gull feeds mainly on crabs. Recent research shows that this gull species has a rather specialized feeding ecology during the breeding season, depending mainly on three crab species (Neohelice [Chasmagnathus] granulata, Cyrtograpsus altimanus and Cyrtograpsus angulatus) (Delhey et al. 2001a, Herrera et al. 2005). In the Bahía Blanca estuary, the dominant prey during spring and summer is the crab Neohelice granulata (Delhey et al. 2001a, P. Petracci, unpubl. data). At Golfo San Jorge (Islas Vernaci and Isla Felipe) breeding individuals feed predominantly on the crabs Cyrtograpsus altimanus and Cyrtograpsus angulatus, although they also include in low percentages (< 11%) other crab species, mollusks and polychaetes (Herrera et al. 2005). Size of crabs preyed upon at the Bahía Blanca estuary averaged 23.3 ± 7.5 mm (n = 120) (Delhey et al. 2001a), although the value reported includes prey obtained both in the non-breeding and breeding seasons. Detailed information on prey sizes by crab species and sex captured at Golfo San Jorge is available in Herrera et al. (2005). At Bahía San Blas, they feed mostly on Cyrtograpsus altimanus and Neohelice granulata (N. Suárez and P. Yorio, unpubl. data).
Crabs are also an important component of its diet during the non-breeding season, although during this part of the annual cycle it also includes a broader trophic spectrum. Delhey et al. (2001a) report that during fall and winter at the Bahía Blanca estuary Olrog’s Gulls feed mainly on Neohelice (Chasmagnathus) crabs, but they also include a variable proportion of barnacles Balanus glandula depending on the month. At Laguna Mar Chiquita, Buenos Aires, Neohelice (Chasmagnathus) granulata and Cyrtograpsus angulatus are the dominant prey of adults and young individuals during the non-breeding season (Spivak and Sánchez 1992, Martinez et al. 2000, Copello and Favero 2001, Berón et al. 2005a). Other prey items reported by these studies in the Mar Chiquita area, although in low frequencies, are Uca uruguayensis, insects, fish, Adelomedon snaisl (individuals and egg capsules), fish waste and fish-bait. Average size of crab prey at Mar Chiquita lagoon was 28.4 ± 4.0 mm (range 12-44 mm, n = 1,435) (Copello and Favero 2001). Detailed information on crab prey size by species can be found in Spivak and Sánchez (1992) and Copello and Favero (2001). At the Mar del Plata harbor, adults and juveniles include in their diet fishery waste (both from boats and fish factories), mussels (Mytilidae) and sewage (Martinez et al. 2000). Olrog’s Gulls, particularly juvenile birds, are regularly recorded in refuse tips in Bahía Blanca and Punta Alta, Buenos Aires province (J. C. Chebez, pers. comm, D. Rábano, pers. comm.). Olrog’s Gulls have been also reported to feed on grains along the shoulder of the road and in a parking lot besides the port (Petracci et al. 2007) and, occasionally, feeding at sea on discarded offal from fishing vessels (Jehl and Rumboll 1976). Little information is available from wintering areas in Uruguay, but observations indicate that its diet is also mainly based on Neohelice granulata and Cyrtograpsus crabs and occasionally Uca uruguayensis and mussels (Escalante 1970, 1984).
Diet studies on juveniles during November and December at Mar Chiquita and Punta Rasa, Buenos Aires province, showed they feed mostly on Neohelice granulata (Berón 2003).
At Islas Vernaci, Chubut, breeding Olrog’s Gulls outfitted with with VHF transmitters fed mostly within 3 km of the colony (Yorio et al. 2004). The birds were consistent in the use of one particular area, and most of the tracked birds foraged mainly in one or two of the identified feeding areas. Number of trips per day for each tracked individual varied between 2.7 ± 1.7 (n = 30) and 6.1 ± 2.2 (n = 67) (mean 4.1 ± 0.99; range = 1-9; n = 8 birds). Mean trip duration was 109 ± 31.2 min (range = 15-645 minutes, n = 8 birds). Gulls foraged at all states of the tide, although they were more likely to forage during mid and low tides than during high tides. At Isla Arroyo Jabalí Oeste, Bahía San Blas, breeding Olrog’s Gulls instrumented with VHF transmitters foraged along the coast up to 20 km from their nest, mostly to the north of the colony (N. Suárez and P. Yorio, unpubl. data).