Gulls. Professor John C. Coulson

data analysed by MacKinnon (1986) and on unpublished personal data, totalling 1,953 individuals. Note that few first-year birds were weighed in April, May and June.

      There is a widely held view that winter is the period when birds face the highest risk of food shortages, leading to loss of weight and increased mortality. In the case of the Black-headed Gull, however, this is not so. Prolonged periods of hard frost do little to affect the average weight of the birds. Table 18 shows the deviation in weight of Black-headed Gulls from the monthly average in relation to the number of consecutive days with hard ground frosts before capture. Short periods of frosty weather had little impact on their weights, and even with more than four days of continuous hard frosts, their weights had declined by only 3.7 g, or about 1.2 per cent of the body mass expected at that time of year.

TABLE 17. Weight change between dawn and afternoon of adult Black-headed Gulls between July and January. Weights are the average for males and females recorded in north-east England. Significant overnight weight losses are indicated by an asterisk (*).

TABLE 18. The effect of consecutive days with hard ground frosts on the weight of Black-headed Gulls. Results are shown as the deviation from the average weight of adults of each sex in the month of capture in north-east England.

      FIG 39. The percentage of the total number of ringed adult Black-headed Gulls reported dead in each month of the year. The distribution suggests that mortality is relatively low during the winter months and then peaks at about twice the winter rate during the breeding season (May to July). Figures are based on 2,575 recoveries of adults and include individuals recovered abroad.

      With Black-headed Gulls achieving high body weights in winter and sustaining these during cold weather, this begs the question as to when the risk of mortality is highest in the species. This was first investigated by Jim Flegg and Christopher Cox in 1956–74 (Flegg & Cox, 1975), and the amount of related data have increased since then following the recovery of many more British ringed birds found dead from 1975 to 1984 (Fig. 39). In fact, the seasonal pattern of deaths has changed little since the original analysis, with the numbers reported dying each month during the breeding season (April to July) double those in each month between October and March. In the British climate, Black-headed Gulls are at a greater risk of being killed by predatory mammals while breeding (as found by Hans Kruuk at Ravenglass and Clive Craik in the Hebrides), than by food shortages in winter. While the birds are breeding and are attached to a particular colony site, their feeding areas are restricted. Agricultural fields planted with root or grain crops become unsuitable as feeding areas as the plants grow, while those where grass is allowed to grow tall for hay and silage production also become unsuitable until the first cuts are made. Once the grass is cut, the speed at which the fields are then visited by gulls strongly supports the belief that food may be in short supply during the breeding season.

      ANNUAL SURVIVAL RATES

      Considering that the Black-headed Gull is so abundant, there is relatively little information on the survival rates for members of the species. The first estimates were published in 1975 by Flegg and Cox, who suggested that the adult survival rate in the south of England was 76 per cent per year. This value was probably low because some recoveries of old birds were still to be reported, and in addition, doubts now exist about the durability of the rings then in use, which may have reduced the estimated survival rate (see below). More recently, Anne-Caroline Prévot-Julliard, Jean-Dominique Lebreton and Roger Pradel (1998) estimated a 90 per cent annual adult survival rate in France.

Gabriella MacKinnon and I used British BTO ringing recoveries to estimate the annual survival rates of Black-headed Gulls ringed as nestlings in the UK. The rate in the bird’s first year of life proved to be low, followed by a constant rate of 84 per cent for the next 10 years of life (MacKinnon & Coulson, 1987). Thereafter, the annual survival rate declines progressively, with the oldest bird reported at 22 years of age; this is well below a maximum age of 33 years reported by Klaas van Dijk and Rob Voesten in 2014. There are two possible explanations for this pattern. One is that senility sets in after Black-headed Gulls reach about 10 years of age. The second is that the rings on the gulls start to be lost or become illegible after 10 years, resulting in fewer reports of old birds that are of this age in the population. We encountered a similar problem with aluminium and Monel rings used on Kittiwakes and Herring Gulls in the past, and examined both aluminium and Monel rings used by BTO ringers recovered during our studies on Black-headed Gulls in north-east England. We concluded that ring wear and loss was a problem and that a few individuals probably outlived the life of the rings used to mark them. As a result, survival rates were probably underestimated. In addition, based on information available for other seabirds, it is considered unlikely that senility occurs in 10-year-old individuals. We also found that Monel rings, which were introduced to overcome the wear found on aluminium rings, actually lost weight at a similar rate to the aluminium ones and were therefore little improvement.

      Fortunately, methods have been developed that allow survival rates to be calculated based on age-truncated recoveries. MacKinnon and I used these in our studies (with help from Nicholas Aebischer and David Jackson) to determine annual survival rates for each year from 1950 to 1980 using recoveries of birds up to 10 years old (Fig. 40). This method excluded information on survival rates for the oldest 20 per cent of Black-headed Gulls living in the wild, but avoided bias produced by ring wear and whether senility was having any effect. Using this method on the accumulated BTO recoveries, annual survival of adults fluctuated around 83 per cent between 1950 and 1955, and then progressively increased to about 87 per cent from 1974 to 1980. The survival of first-year birds in the 1950s was about 60 per cent and increased progressively to about 70 per cent during the 1970s.

      FIG 40. Estimates of the annual survival rates of adult and first-year Black-headed Gulls in Britain from 1950 to 1980. The adult survival estimates were based on birds recovered within 10 years of ringing to avoid bias. Each year runs from July to June, and the years shown relate to the start of each 12-month period. Both trend lines differ significantly from a constant survival rate over the period considered, and survival has gradually increased from 1950 to 1979. Note the low survival rate from July 1962 to June 1963, when adult survival declined to 71 per cent and that of the young fledged in 1962 decreased to 44 per cent. The winter during these 12 months was the most severe of the 30-year period.

      The results show a number of interesting features. First, the annual survival rate has increased overall in both adults and first-year birds in more recent years. Second, the first-year survival rates show much more between-year variation. Third, in only one winter (1962/63, the severest during the range of years used in the study), do the birds show clear weather-related decreases in survival, with first-year birds managing only 44 per cent survival and adults 71 per cent survival in that year. And finally, the high (84 per cent) survival rate of first-years and above-average survival rate of adults in the 1974/75 winter coincided with the mildest weather at this time of year in England since 1869.

      Regional survival rates

      The survival rates of Black-headed Gulls ringed as pre-fledged chicks or as adults in different regions of Britain (Table 19) show little regional variation, and the small differences are all within the tolerances of the methods used. In general, the mortality rate in the first year of life is about double that of the birds once they become adults. However, the estimates for Ireland indicate that Black-headed Gulls here had both the lowest first-year and adult survival rates, and the lowest adult life expectancy. These