Sightings of wing-tagged birds that moved 10 km or more from the study areas in north-east England in the same winter and reported by other observers revealed that all moved south, remained on or near the east coast of England, and moved less than 100 km, apart from one bird, which moved more than 350 km. Of the 254 adults marked in winter in north-east England, 19 were seen in Britain during the breeding season. Nine of these birds were recorded within 50 km of the winter study areas, while 51 were recorded in colonies on the Continent, confirming that most of the marked birds studied bred outside Britain.
That some individuals return to the same feeding areas each day is remarkable – in some cases during the field studies, birds commuted up to 25 km twice a day to and from a large overnight roost on land containing at least 10,000 birds. Observations of the Black-headed Gulls leaving their night roost in the early morning revealed that individuals from several parts of the roost left in successive waves spread over the 30 minutes before sunrise. Some of those leaving from different parts of the roost circled, formed flocks and departed inland. The birds arriving at sunrise at an extensive series of sport fields in Durham 25 km away gathered in one or two flocks, and invariably included several wing-tagged birds that were previously marked when feeding there. Yet some of these tagged birds (and probably the others) did not roost close to each other at night. How individuals dispersing from the night roost among thousands of others managed to gather into a flock returning to the same feeding grounds is a fascinating mystery to which I have no answer. How do birds regularly flying to the same feeding area manage to group together into a flock on leaving the large roost? This is an interesting aspect of behaviour that requires more research, which might be assisted through the use of small modern transmitters (here).
TABLE 14. The proportion of wing-tagged Black-headed Gulls in north-east England recorded in subsequent weeks during the same winter in a different feeding area. Data summarised from MacKinnon (1986).
Age structure in different feeding areas
First-year and adult Black-headed Gulls are not distributed in the same proportions in different feeding habitats within the same area. During the studies carried out in north-east England, first-year birds formed only 1–5 per cent of the Black-headed Gulls present near river mouths and on the coast (Table 15), and were more than four times as frequent feeding inland on pastures and grassland, averaging 22 per cent of individuals present. At landfills, first-year birds increased from 11 per cent in autumn to 20 per cent by midwinter. The reason for these differences is not known, but since birds feeding on the coast and those feeding inland used the same night roost, it would appear to be the result of variation in the feeding ability of birds of different ages (Table 16). This suggests that coastal feeding may require greater experience in order to become proficient, as has been shown in other studies of Black-headed Gulls relying on kleptoparasitism as a source of food (here).
TABLE 15. The proportion of first-year Black-headed Gulls in winter counts near the mouths of four rivers in north-east and eastern England, and on the north-east coast. Proportions of first-year birds showed little variation between days or between years.
Interestingly, the cannon-net captures of Black-headed Gulls at landfill sites were composed of up to twice the proportion of first-year birds than those counted at nearby loafing sites before making a capture (Table 15). This probably indicates that the young birds take greater advantage of feeding frenzies and possibly have a greater risk of capture because of their less efficient feeding ability, which causes them to stay and feed for longer at landfill sites.
Feeding rates of Black-headed Gulls differ considerably between different sites (Table 16). Items are taken much more rapidly at sewage outfalls, as these sites offer an abundance of readily available food. Following the plough is less successful as it involves a degree of searching or gaining good positions, while walking over short grass on playing fields or pasture involves considerable searching before each food item is discovered. The figures in Table 16 do not take into account the size of each item consumed, although they were likely to be similar on fields and at coastal sites, and smaller at sewage outfalls.
TABLE 16. Feed rates measured as numbers of swallows per 12 seconds by Black-headed Gulls at different feeding sites. First-year birds were slower feeders than adults at all three sites. Data mainly collected by Gabriella MacKinnon and personal unpublished records.
Weight changes
The studies of Black-headed Gulls in north-east England confirmed that adult males are about 17 per cent heavier on average than females throughout the year, and that both sexes are 15–17 per cent heavier in winter than in summer (Fig. 38), reaching a peak weight in December when daylight is shortest. From February onwards, both sexes gradually lose weight, reaching their lowest weight in summer. Recently fledged first-year birds in July are much lighter than adults, but soon catch up, so that by September they have almost reached the weight of adults of the same sex. They then remain marginally lighter throughout the rest of the autumn and winter. The heavier weights in winter are, presumably, caused by an increased layer of subcutaneous fat beneath the skin, which reduces heat loss and acts as a reserve to tide individuals over difficulties encountered in feeding during severe weather.
The low weights of adults in summer does not seem to be a direct response to the stress and effort of breeding, because first-year birds, which do not breed, follow the same trend from November to March. Lower summer weights are achieved by adults by March, before they migrate and have started to visit their colonies. Between March and August, adults maintain the lower weight, and there is only slight evidence that breeding causes further weight loss. One effect of lower body weight in summer is that the energy cost of flying is reduced. This is at a time when food reserves protecting against cold weather and insulation to reduce heat loss are less necessary, and so there is no need for birds to carry the associated extra weight.
In the studies, only minimal differences were apparent in the weights of Black-headed Gulls caught in July and August just after dawn (before they have eaten their first feed of the day) compared to those caught in the afternoon (Table 17). However, as day length becomes shorter and the period when the gulls cannot feed increases, their overnight weight loss increases progressively. By December and January, the overnight weight loss recorded reached 18.3 g, which is an appreciable 6 per cent decrease of the afternoon body weight, at a time when body weight is at a seasonal maximum (Table 17 and Fig. 38).
FIG 38. The monthly average weights of adult and first-year male and female Black-headed Gulls in north-east England. Based partly