Such kleptoparasitism of Lapwings and Golden Plovers decreases the waders’ own rate of food consumption, but this is partially mitigated by feeding at night under full moon conditions when the gulls are absent. The fact that Black-headed Gulls resort to kleptoparasitism suggests that they are not as efficient as plovers at searching and finding their own earthworms.
Kleptoparasitism by Black-headed Gulls appears to be a specialised feeding behaviour used by a minority of individuals, and experience is important in successfully obtaining food in this way. Even by their second year of life, young Black-headed Gulls have still not achieved the efficiency of adults when carrying out kleptoparasitic attacks (Hesp & Barnard, 1989).
In urban areas in winter, the tables are often turned on Black-headed Gulls. Those birds finding large food items that they fail to swallow immediately are frequently pursued by Common Gulls or even members of their own species, until they are forced to drop the food to the benefit of the pursuers. Such pursuits are particularly frequent in urban areas during severe winter weather, when large numbers of Common Gulls move from snow-covered high ground and congregate at much increased densities at urban feeding sites.
Feeding areas
In the breeding season, the feeding areas and ranges of Black-headed Gulls are limited by the location of the colony, and well-defined flight lines are often evident to and from large colonies. Many feeding adults forage within 10 km of the colony, although some will travel 40 km or more to favoured sites with a regular supply of food provided through the activities of humans. These may be fields where ploughing is in progress, landfill sites, large car parks (particularly near food stores), coastal picnic sites or riverbanks in towns where people feed bread, chips and other items to birds. Food supplements put out for farm animals are also exploited. Few Black-headed Gulls hunt far from the shore at coastal sites, and only rarely do they plunge-dive and submerge to capture fish.
In the 1960s, along the 18 km tidal reaches of the river Tyne (which was little more than an open sewer at the time), Black-headed and Common gulls were the commonest birds feeding in winter at sewage outfalls and on sewage items floating in the river. Once the sewage was piped separately to treatment plants to clean the river up, the numbers of Common Gulls reduced dramatically, but Black-headed Gulls remained abundant and were still able to find food in the river. This is because the outflow of filtered water passing from sewage plants into the river still contained small items, which were attractive to the Black-headed Gulls but not the Common Gulls.
The habit of large numbers of Black-headed Gulls following tractors ploughing fields is both widespread and spectacular, with many individuals competing for position just behind the plough blades, and diving to catch and consume worms or insects exposed when the soil is turned over. Similar feeding occurs when grass is being cut for hay. The introduction of grass cutting earlier in the year for silage attracts many Black-headed Gulls and has had a marked effect in some areas where the birds feed. Although silage production was introduced many years ago, it became far more extensive from about 1969, and by 1993 it had increased sevenfold as a means of producing winter cattle feed. Grass is now frequently cut twice a year for silage and much earlier in the season than for hay. This has allowed feeding by Black-headed Gulls on insects in these fields to be spread over a much longer period of the species’ breeding season than was once the case, and presumably has been of benefit to the gulls.
Feeding at landfill sites in winter attracts flocks of hundreds and even thousands of Black-headed Gulls. It is a highly social activity and occurs at irregular intervals, often with a resting, inactive flock remaining nearby for long periods. From time to time, one or two individuals leaving the flock will then fly over the landfill working area and alight there or pick up a food item while in flight. This act of feeding by one or two individuals is a major stimulus to those birds in the resting flock, and the great majority will then leave the roost and stream over to the feeding site, land on the refuse and search for food – several hundred individuals may be highly active within a small area. Suddenly, feeding will stop, perhaps as the result of a loud noise, the approach of a person or the arrival of a lorry. On other occasions and for no apparent reason for alarm, the flock will rise as one and return to the roosting site, staying away from the landfill working area for many minutes or even hours.
Food-collecting techniques
Black-headed Gulls use several techniques when collecting food, summarised below:
1. Aerial searching Individuals patrol suitable areas, turning and swooping to pick up items discovered. Used on farmland and in urban areas.
2. Walking over fields of short grass The birds are usually spread out and search for food items on the ground.
3. Feeding while floating on water The birds swim buoyantly, turning rapidly to pick small items such as larval fish from the water’s surface, phalarope-like (Fig. 37). Head-dipping into the water is less frequent and diving does not take place.
4. Feeding frenzies Many individuals converge on an appreciable food source, such as food discarded by the public, a freshly ploughed field or refuse at a landfill site. In all three cases, individuals are attracted from some distance by the erratic flight and calling of those birds already scrambling for food.
5. Feeding on flying insects This is most frequent in autumn, when winged ants take to the air on calm, warm days. The gulls ‘hawk’ after the insects, flying slowly and frequently changing direction.
6. Consumption of items from trees, bushes and low plants This is an infrequent method, with gulls approaching branches or low plants, landing on them momentarily, and plucking abundant insects from the leaves and thin branches. Similar methods are used to obtain acorns from oak trees and berries from Hawthorn bushes.
7. Paddling in shallow pools on intertidal mud This technique is used to disturb, detect and capture small invertebrates, including marine worms. While Herring Gulls also use this method on areas of short grass, this has not been reported in Black-headed Gulls.
8. Kleptoparasitism The birds steal food from Lapwings and Golden Plovers when the waders are feeding on soil invertebrates in pastures.
Use of winter feeding areas
The next sections are based on data gathered through extensive ringing and wing-tagging studies of Black-headed Gulls wintering in north-east England. Observing coloured leg rings on Black-headed Gulls was difficult in resting flocks and when they were feeding on grasslands, so to overcome this problem, plastic wing tags were used (here). These proved more successful in identifying and following the behaviour of individuals throughout the day. The collaborative research involved identifying feeding areas and catching and marking birds prior to and during three years of intensive field studies on the species made, and was carried out by Gabriella MacKinnon at Durham University as part of her doctoral thesis (MacKinnon, 1986). It involved us in many cold early-morning starts in winter to reach the feeding areas and set cannon nets before the gulls arrived at about sunrise from their night roosts.
Most of the wing-tagged individuals studied in north-east England frequently returned to feed at the same sites during the whole winter (Table 14), but some moved away during this period. A marked difference in the behaviour of the gulls was detected between those using coastal sites