Observations of Herring Gull (Larus argentatus) arrival in the large gull roost at Hilfield Park Reservoir in Hertfordshire during 2004 suggested that a substantial proportion of the birds were arriving from a source well to the south-east. It was hypothesised that this was central London, near Westminster, and by means of observations from locations including Deacons Hill (Borehamwood), Featherstone Hill (Mill Hill), Woodfield Park (Brent Reservoir), Parliament Hill (Hampstead Heath), Hampstead Parish Church (Hampstead) and Primrose Hill (Camden) this roost flight-path, over Paddington, out through Maida Vale, over Brent Reservoir and up through Colindale, was confirmed. Due to the required longer travel distance (13 miles) and greater ascent (over 100m) these birds tended to arrive relatively late in the roost compared to most other gulls of presumed more local origin. Small numbers of other gull species, in particular Lesser Blackbacks (Larus fuscus) and Black-headed Gulls (Larus ridibundus), were also found to follow the same flight-path out of north-west London, though their ultimate departure points remain uncertain.
Of interest is that fact that at the observation points most distant from the Hilfield roost, particularly Primrose Hill and Westminster but also Hampstead, most gulls, including Herring Gulls, were moving in a transverse direction to/from the major roosts in the Lea Valley at William Girling and King George V Reservoirs. These roosts are more accessible from central London, both in terms of distance and altitude, as are those on the Thames Estuary and the roost at Queen Elizabeth II Reservoir, Walton-on-Thames. It might therefore be deduced that the Herring Gulls coming to Hilfield could be deriving some extra benefit from attending this roost, perhaps in terms of increased security or information exchange with birds from other feeding areas, to justify the extra effort in travelling there. The former reason would also appear to be consistent with the observed steady increase in numbers roosting at Hilfield in recent years and seems most likely. Factors such as these could well apply more generally in motivating Herring Gull attendance at different, and less accessible, roosts, but further much more detailed studies would be required to systematically evaluate the reasons. Considering also the continuing status change since the in-depth research of Hertfordshire’s gulls by Sage in the 1950/60’s and Gladwin in the 1980’s the general need for more extensive new studies of large gull movements in the area is now apparent.
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