Case Study: Blue-eyed Shag
Selective forces acting on body size effects animals differently depending on environment. For a marine bird called the blue-eyed shag, variation in body size within populations and between sexes is a result of several selective pressures. Males are sexually dimorphic from females and therefore somewhat larger. Foraging has also been suggested as an other driving selective pressure, as different sized birds can only forage a different depths. This is creating foraging niches both within and between populations.
In other studies mammal and bird body size is positively related to duration of dive and depth. Adaptations for deep diving mammals and birds including oxygen retention and endothermy.
In the reviewed study , findings suggested that blue-eyed shag body sized is related to foraging depth. Foraging at shallower depths related to smaller size, while larger related deeper size. Sexual selection for size may then be furthern enhanced by foraging niche.
[1} Cook, T. R., Lescroël, A., Cherel, Y., Kato, A., & Bost, C. (2013). Can Foraging Ecology Drive the Evolution of Body Size in a Diving Endotherm?. Plos ONE, 8(2), 1-13. doi:10.1371/journal.pone.0056297