i6 THE SCOTTISH NATURALIST 



WiGEON. — Two clutches were taken at Glen Luine and Loch 



Cluny, East Ross, in 187 1 (E. Hargitt coll.). 

 Pintail. — A clutch near Knockie, Inverness, 1869 (E. 



Hargitt coll.). 

 Pochard. — Two eggs from Loch Awe in 187 1 (E. Hargitt 



coll.). 

 Scoter. — One egg from Sutherland in 1855 (G. J. Hancock: 



H. B. Tristram coll.). 

 All the above are in the collection of the British Museum. — 

 F. C. R. JouRDAiN, Abingdon, Berkshire. 



An Optical Illusion regarding Herring Gulls. — On several 

 occasions I have noticed and have been deceived by a very 

 peculiar optical illusion with regard to Herring Gulls in certain 

 positions. When standing among stones with the sun shining on 

 a background of water, the legs, to the naked eye, sometimes 

 appear distinctly yellow at a distance of 25 to 50 yards. A prismatic 

 glass at once corrects the illusion and shows the legs to be flesh 

 coloured. Three times, in Orkney, was I so deceived and on one 

 occasion shot the bird. In September last I saw four Herring Gulls 

 in such a position, all of which at 25 to 30 yards' distance, appeared 

 to have yellow legs, although in two others well away from the 

 water the legs showed normal. The same thing has happened 

 to-day, loth January, some of the birds being actually in the 

 shallow water. Prismatic glasses again showed all of them also to 

 be L. argentaius, as they had previously done others in the same 

 place under the same conditions. Lest my eyes might be blamed 

 for the illusion, I might add that others, whose attention I called to 

 such birds, proclaimed their legs also to be yellow to the naked eye. 

 To bring about the illusion the birds must be either in or within a 

 foot of the water and on a level with it, and the water fresh, not 

 salt. To the two previous records of the Yellow-legged Herring 

 Gull in Britain as mentioned by Dr Clarke and Dr Stenhouse in the 

 last number of the Scottish Naturalist, I think, must be added, the 

 one seen by Mr F. W. Holder on the Formby shore, South 

 Lancashire, on loth February 1918, as mentioned in the 1920 Report 

 of the Lancashire and Cheshire Fauna Survey. It was in company 

 with a party of Herring Gulls {Lams argentati/s) and the observer 

 was within 10 yards of the birds. It was in summer plumage, 

 whereas the others were still in winter dress, and he first noticed it 

 as having the mantle and wings darker and duller grey than the 

 others, with the bill and legs exceedingly bright yellow, the red at the 

 angle of the former being specially noticeable. — H. W. Robinson. 



