Colonies have ranged in size between 11 (Isla Vernacci Oeste Noroeste) and 3,114 breeding pairs (Isla del Puerto)( Yorio and Harris 1992, Yorio et al. 1997, Delhey et al. 2001b, Rábano et al. 2002, Petracci et al. 2004). However, except for Isla del Puerto, all colony size estimates have been always less than 500 nests. Total population is currently estimated at 4,000-5,000 pairs (P. Yorio, P. García Borboroglu and P. Petracci, unpubl. data).