Interspecific brood parasitism occurs when the eggs of one species are laid in the nest of a different species, but evidence of this phenomenon is scarce. Facultative interspecific brood parasitism has been described in Charadriiformes. However, to our knowledge, our study provides the first documentation of facultative interspecific brood parasitism in the colonially breeding species we studied. The factors affecting its frequency in relation to colony species composition, nest density, and period of the breeding season was analyzed. Between 2006 and 2012, we monitored 2,493 nests of five species: Northern Lapwing (Vanellus vanellus; n = 745), Common Redshank (Tringa totanus; n = 467), Black-tailed Godwit (Limosa limosa; n = 237), Black-headed Gull (Chroicocephalus ridibundus; n = 932) and Common Tern (Sterna hirundo; n = 112). Facultative interspecific brood parasitism was observed in 1.3% of all nests. Typically, a single egg was laid in a host nest (59.4% of parasitized nests), and a maximum of four parasitic eggs were laid (3.1% of parasitized nests). The Common Redshank showed the highest frequency of facultative interspecific brood parasitism (59.4%), followed by Northern Lapwing (28.1%). Facultative interspecific brood parasitism occurred most frequently at the beginning of the breeding season (62.5% between the end of April and 10 May), although the number of active nests peaked later than did the number of parasitized nests. Hatching success of broods containing parasitic eggs was 17.8%, which was lower than that of non-parasitized nests (23.1%). Interspecific brood parasitism may evolve from conspecific brood parasitism in colonies with high nest densities. Low, but constant frequencies of facultative interspecific brood parasitism may suggest either that this is a strategy to increase fecundity or non-adaptive behavior derived from failures to identify the nest due to the high nest density in the colony.