Hanging out on boats, buoys and just about anything which provides a suitable loafing site, since late August a fourth-winter large, white-headed gull has been almost resident at Pitsford Reservoir’s yacht club. Identified by some as a Caspian Gull, it exhibits a suite of features which are not fully consistent with that species and which hint strongly at mixed parentage. It can be safely aged as a fourth-winter as it closely resembles an adult but it has retained extensive black in the primary coverts and alula while the rather liberally ‘messy’ appearance of the bill, with a yellow tip divided from a pale, washed out base by a diffuse, dark subterminal band, is also a pointer to its not being fully mature.
Pro-Caspian features exhibited by this individual include the rather elongated body with a high-chested appearance and attenuated, sloping rear end, long, thinnish legs, largely white head (although there is a dark ‘eyebrow’ with some fine streaking on the ear coverts) with a restricted shawl of dark streaks on the hind neck, ‘Common Gull grey’ mantle and a washed out, green-tinged, yellow bill.
Conversely, anti-Caspian features are the absence of a long white tip to the upperside of the 10th primary (instead P10 has a very small white tip), absence of a long white tongue on the underside of P10 (where there is a single mirror set into an otherwise black distal end to the primary), broad – as opposed to fine – rear neck streaks, a rather deep, powerful-looking bill with a strong gonydeal angle and a sharply-angled and hooked culmen, a pronounced pale straw-coloured iris and a relatively large, ‘full’ head, lacking Caspian’s long, sloping forehead and ‘snouty’ appearance.
While pale eyes do not rule out Caspian Gull (a minority exhibit pale irises) and variation in wing tip pattern can be considerable, bill structure, head shape and jizz normally remain consistent and combine to lend a more ‘gentle’ appearance far removed from what we are seeing here. This individual appears to have a head shape, bill structure and a general ‘mean’ character much closer to Herring Gull or Yellow-legged Gull than to Caspian Gull.
However, the wing tip pattern is very similar to a known 4th year male Caspian Gull from Poland (see here) although the Pitsford individual has larger white primary tips. Interestingly, in threatening behaviour toward other gulls, this bird calls with its wings partly open, the bill pointing down initially before raising it to 45 degrees as it delivers a very convincing Caspian-like laughing call and posture.
It is easy to speculate but difficult to draw any firm conclusions but I would guess this individual comes from the East European hybrid zone where mixed pairs of Caspian Gulls and Herring Gulls of the ‘Scandinavian’ race argentatus are known to occur. Hybrid Caspian Gull x Herring Gull from this zone has occurred in the UK before see, for example, here . However, it can also appear quite square-headed and sometimes takes on the appearance of Yellow-legged Gull, although structure and jizz are not quite right for that species. It is an interesting individual but it is certainly not a pure Caspian Gull. Watch the video below. What do you think? Comments welcomed.
Northants birds 