Family LARIDAE. 



Larus ridibundus L. - Black-headed Gull. 



(Plates 165—169, fig. a-o). 

 (Siiuiicknt-rt : Avifauna Neerlatidica, p. 112, No. 266). 



Colour of surface of shell: 

 Colour of spots: 



Average dimensions: 

 Average weight of shell: 

 Texture of shell: 

 Shape : 

 Nest: 



Site of nest: 



Number of eggs: 

 Breeding season: 

 Duration of incubation : 

 •Remark: 



olive brown, olive yellow, olive green, and 

 olive grey, in various shades. 



brown and blackish brown, in different tints ; 

 underlying spots show through pale pur- 

 plish. 



53.4 X 37 niillinictres. * 



2.3 grams. 



as a rule slightly finer than in carins. 



very variable; often pointed oval. 



consists of vegetable material, varying con- 

 siderably in circumference. 



on the ground, in marshy places; among 

 the grass in wet grassland; on floating 

 reed-islands, etc (Breeds in colonies). 



generally 3. 



end of April, May, June. 



about 3 weeks. 



a dwarf-set (Schouwen, 1907) in my col- 

 lection consists of three eggs measuring 

 36 : 27, 34 •■; 25, and 33 X 24-' milli- 

 metres ; I mention this as full dwarf-clutches 

 are very rarely found. 



