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.
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.