Aberrant Yellow-legged Gull (Larus michahellis) in Alicante, July 2016

On 12 July 2916, Willy Dekeiser found an odd, overall white gull, at Altea river, Algar, Alacant. See a photo above.  The gull was identified as a Yellow-legged Gull (Larus michahellis). Indeed it was at the beach with other individuals of the species and shared structure and size, therefore ruling out other vagrant options (Larus hyperboreus, Larus glaucoides) and  an aberrant Lesser Black-backed Gull (Larus fuscus).

The bird was showing a normal pattern in the bare parts (yellow bill with red spot, yellowish legs, apparently 'normal' eye), thus ruling out an albino aberration. The bird did not look entirely white but with a very pale grey mantle (matching in some way an adult Iceland Gull for example). It also seemed to present a dark (not black) tail band and perhaps dark primary tips, despite bare parts coloration remain unaltered. 

The gull was certainly showing a plumage aberration, probably leucism, but the heavy light conditions of the photo did not allow a detailed examination of the feathering to determine if indeed it could have been any other case of aberration involving eumelanins (responsable of black colour), c.f. van Grouw (2006).

Acknowledgements

Thanks to Toni Zaragozí for sharing the observation on behalf of the author

References:
 
Van Grouw.H. 2006. Not every white bird is an albino: sense and nonsense about colour aberrations in birds. Dutch Birding, 28:79-89.