Strictly diurnal, foraging during the day and returning to the colony at night (Murphy 1936, Zavalaga and Paredes 1999). Nest colonially, with densities up to 3.0 ± 0.5 nests per square meter (Duffy 1983a). Known to fly in a “V” formation (Johnson 1965). Will form a “compass raft” of birds, with individuals departing from colony aligning in the water with those returning overhead (Weimerskirch et al. 2010).
Competes for nesting space with Peruvian Boobies (Sula variegata) and Peruvian Pelicans (Pelecanus thagus) and will jab at them with their bills (Duffy 1983a).
Social and interspecific behavior
The Guanay Cormorant is often associated with the Peruvian Booby (Sula variegata) and the Peruvian Pelican (Pelecanus thagus) as the three principle surface-nesting bird species of the Peruvian Coastal Current. Stemming from this, all three were historically important for the guano industry. The three compete to some degree for nest sites, though they each have different preferences (Duffy 1983a).
Kleptoparasitism or piracy of food is a known occurrence of the pelicans on the cormorants (Duffy 1980).
Known nest predators include Andean Condors (Vultur gryphus), Turkey Vultures (Cathartes aura), Kelp Gulls (Larus dominicanus), and Band-tailed Gulls (Larus belcheri) (Murphy 1936, Duffy 1983d).