Yellow-legged Gull and Caspian Gull

Yellow-legged Gull and Caspian Gull, Cambridge Research Park, 23rd January 2016 © James Hanlon

'With the legs invisible this species pair on the water can be a bit challenging - particularly at range in fading light (typical conditions when checking a gravel pit or reservoir roost!). I've noticed that Caspian tends to appear longer-bodied on the water and this is obvious here. The head and bill profile are obviously different - Caspian's sloping forehead and long bill making it look like it's been pulled by its bill! It is darker-eyed than the YLG and this holds true for the majority of birds. The eye-ring is another good feature - obvious and red on the YLG but less obvious on the casp - due in part because its head is turned slightly but also because it is orange. Another obvious feature visible here is 'P10' - the underside of the longest primary is mainly white in Caspian (broad black bar separates off the white tip) but mainly black in YLG, with a white mirror near the tip. The mantle shade is almost identical here but averages slightly paler on Caspian (both overlap broadly with Scandanavian herring gull). The narrow, incomplete dark band on the bill of the Caspian is another clue but isn't diagnostic. It is present to a degree on most adults in winter but only on subadult yellow-legged, adults of which also tend to have more brightly coloured (and shorter, stubbier-looking) bills. Beware herring gulls in late winter when they have clean white heads due to losing their streaks - dark-eyed Scandanavian birds are rare but a potential pitfall. The primary pattern is similar to that of Caspian and they are best separated on overall structure.' JH 

Written at my request as this image seemed a suitable opportunity to explain the differences - thanks James. ED