Nests are built on different substrates depending on the location, including rock, gravel, shell, and sand, and sometimes on vegetation debris or low herbaceous vegetation (< 0.3 m), mostly of Salicornia spp. and Franquenia spp. (Yorio et al. 2001, García Borboroglu and Yorio 2007a). Nesting material in southern Buenos Aires consists almost exclusively of vegetative debris, mainly Spartina spp. and Salicornia spp., while at locations in Chubut it includes small bush branches, seaweeds (Ulva spp. or Porphyra columbina and fragments of Gigartina skottsbergii and Macrocystis pyrifera) and lower percentages of mollusk shells, feathers, and bones. The nests’ mean external and internal diameters are 41.69 ± 7.31 cm and 27.26 ± 12.42 cm, respectively, whereas mean nest height is 7.10 ± 3.30 (n = 231 nests) (García Borboroglu and Yorio 2007a). At most locations, Olrog’s Gulls nest in subcolonies. Olrog’s Gulls nest at relatively high densities, with a mean inter-nest distance of 0.66 ± 0.25 m. (range: 0.30–1.93 m; n = 231 nests) (García Borboroglu and Yorio 2007a).
In general, individuals show weak site tenacity, build nests soon after settling and start laying a few days later. In northern Golfo San Jorge, Chubut, gulls start building nests and laying in the last week of October (P. García Borboroglu and P. Yorio, unpubl. data), but laying can start in early September in the Bahía Blanca estuary (P. Petracci, com. pers.). Clutch consists of two to three eggs, which are olive-brown with dark spots and an average size of 72 x 48 mm (Petracci 2002). Incubation period is approximately 30 days. Hatching at Golfo San Jorge starts in early December (P. García Borboroglu and P. Yorio, unpubl. data), while in Bahía Blanca and Bahía San Blas most hatching occurs during early October (L. La Sala and P. Petracci, unpubl. data.) and early November (N. Suárez and P. Yorio, unpubl. data), respectively. At colonies in this area, there can be a marked asynchrony between nesting groups (subcolonies). The plumage color of chicks is dark lead grey, with the lower breast and belly whitish, upperparts uniform and unspotted, and head with numerous and evenly distributed small dark grey spots (Devillers 1977). The bill is black with a pink or reddish pink tip. Chicks fledge between late November and January, depending on the location. After breeding, part of the population migrates north. Some adults and juveniles remain in the breeding grounds during the non breeding season.