Striking plumage anomalies in two Tyrannidae (Passeriformes): Vermilion Flycatcher Pyrocephalus rubinus and Tropical Pewee Contopus cinereus from

Plumage anomalies may be caused by differing amounts and distributions of pigments usually present in feathers, chemical changes to pigments resulting in abnormal colors or changes in feather structure (Harrison 1985) or genetic, environmental or dietary factors (Dorst 1971, Goncalves Junior et al. 2008). The most extreme variations of abnormal pigmentation occur in individuals that show marked reductions or increases in the normal pigments present (Harrison 1985). Though probably not uncommon in nature (Hosner & lebbin 2006, van Grouw 2013), individuals exhibiting plumage aberrations frequently find themselves at a selective disadvantage and thus may be short-lived, meaning that they are rarely observed. Some aberrant individuals are more conspicuous to predators, and reduced pigmentation may weaken feather structure causing accelerated wear and affecting mobility (Harrison 1985). Additionally such individuals may suffer harassment from conspecifics ( nero 1954, Harris 1983, witHGott & McMaHon 1993).