Many birds in this order used to be placed in the Pelecaniformes. Nowadays, the order Suliformes comprises four families: Fregatidae, Sulidae, Phalacrocoracidae and Anhingidae.
Populations of Great Cormorants (Phalacrocorax carbo) in the UK consist of two subspecies, namely ‘North Atlantic’ (P.c. carbo) and ‘Eurasian’ (P.c. sinensis) birds, that probably hybridize (Goostrey et al., 1998; Marion & Le Gentil, 2006).
Hybridization between Blue-footed Booby (Sula nebouxii) and Peruvian Booby (S. variegata) was reported in Peru (Ayala, 2006; Figueroa & Stucchi, 2008), and subsequently confirmed by morphological and genetic analyses (Taylor, Zavalaga & Friesen, 2010). Further studies on these species indicated a bimodal hybrid zone with limited introgression (Taylor et al., 2012), that is asymmetrical (from Peruvian into Blue-footed Boobies) and restricted to autosomal and sex-linked markers (Taylor, Anderson & Friesen, 2013).
Ayala, L. (2006). Apparent hybridization between Blue-footed Sula nebouxii and Peruvian S. variegata boobies on Lobos de Tierra Island, Peru. Marine Ornithology 34, 81-82.
Figueroa, J. & Stucchi, M. (2008). Possible hybridization between the Peruvian Booby Sula variegata and the Blue-footed Booby S. nebouxii in Lobos de Afuera Islands, Peru. Marine Ornithology 36, 75-76.
Goostrey, A., Carss, D. N., Noble, L. R. & Piertney, S. B. (1998). Population introgression and differentiation in the great cormorant Phalacrocorax carbo in Europe. Molecular Ecology 7, 329-338.
Marion, L. & Le Gentil, J. (2006). Ecological segregation and population structuring of the cormorant Phalacrocorax carbo in Europe, in relation to the recent introgression of continental and marine subspecies. Evolutionary Ecology 20, 193-216.
Taylor, S. A., Anderson, D. J. & Friesen, V. L. (2013). Evidence for Asymmetrical Divergence-Gene Flow of Nuclear Loci, but Not Mitochondrial Loci, between Seabird Sister Species: Blue-Footed (Sula nebouxii) and Peruvian (S. variegata) Boobies. Plos One 8.
Taylor, S. A., Anderson, D. J., Zavalaga, C. B. & Friesen, V. L. (2012). Evidence for strong assortative mating, limited gene flow, and strong differentiation across the blue-footed/Peruvian booby hybrid zone in northern Peru. Journal of Avian Biology 43, 311-324.
Taylor, S. A., Zavalaga, C. B. & Friesen, V. L. (2010). Hybridization Between Blue-footed (Sula nebouxii) and Peruvian (Sula variegata) Boobies in Northern Peru. Waterbirds 33, 251-257.