| Summary: | The seasonal changes in numbers of Common Gulls wintering in the Ythan Valley were studied over a period of three years. The observed changes were related to changes in the availability of food resources. The numbers of Common Gulls present in the study area were highest in autumn when immigrants from Scandinavia and other parts of Scotland arrive. Numbers decline to a midwinter minimum, due to dispersal out of the study area. Mortality is low in winter, and must account for a small proportion of the observed decline. Numbers increase again in early spring, until Scandinavian migrants leave Northeast Scotland in early April. In winter, Common Gulls roost on or near the coast at night and fly inland to feed in agricultural land each day. Three main feeding situations (habitats) are useds ploughing in progress, grassland and stubble. A model of habitat selection is proposed to accotmt for the changes in numbers of Common Gulls. The model is based on Fretwell's (1972) model of habitat selection. It is proposed that feeding habitats can be ranked in order of preferences ploughing is the preferred (optimal) habitat, and grassland is preferred to stubble. The number of Common Gulls using ploughing is positively related to the availability of ploughing, and, on average 92% of all available ploughing sites are attended by gulls (including Blackheaded and Herring Gulls), It is probable that the reason ploughing is preferred is that the energy intake at ploughing sites is much higher than on grassland or stubble. However, evidence suggests that the number of Common Gulls using ploughing is regulated. Flock sizes at individual ploughing sites are positively related to prey density. Feeding rates at ploughing sites are independent of flock, size; since as flock size increases, the rate of prey depletion also increases. It is suggested that flock sizes at ploughing sites are regulated around an equilibrium level. It is suggested that competition for access to ploughing sites means that a proportion of the population feeds in less profitable feeding sites (such as grassland and stubble fields). The use of these habitats is dependent on a number of environmental variables. The number of Common Gulls using grassland declines with decreasing temperature, which is probably due to a decrease in the availability of invertebrate prey at low temperatures, since the proportion of time spont feeding increases at low temperatures and feeding rate (calorific intake) declines. However, the proportion of Common Culls on grassland is negatively related to the availability of ploughing sites, as might be predicted from the model. The number and proportion of Common Gulls using stubble fields decreases with time, as the availability of residual grain decreases and stubble fields are ploughed up. The energy gain from feeding on stubble fields is greater than that from feeding on grassland, though the proportion of Common Gulls on grassland is greater. It is suggested that invertebrate food is preferred to grain, because grain may be a poor source of nutrients (though a good source of energy). An alternative hypothesis is that grain creates a digestive 'bottleneck' which restricts the rate of intake of food. This may explain the increase in use of stubbles in the afternoon, since this 'bottleneck' will be of no importance at the roost where no feeding occurs. In mid-winter when the profitability of feeding on grassland and stubble's is decreased, individuals using these habitats may find it difficult to meet their energy requirements o Because numbers at ploughing sites are limited, these individuals are 'forced' to disperse to more favourable feeding sites. Common Gulls fly further inland than either Herring or Black-headed Gulls. It is probable that Herring Gulls are dominant to Common Gulls, axid hence may be able to exclude Common Gulls from localised feeding sites (such as ploughing in progress). It is possible that Common Gulls fly further inland to avoid interactions with Herring Gulls. The increased cost of this (in terms of time and energy) are easily offset by the high rate of energy return from feeding at ploughing sites. However, as spatial overlap between the two species increases no decrease in the useage diversity of habitat by Common Gulls is seen, nor does overlap in use of different habitats decrease, as might be predicted if the two species are in competition. Thus, the evidence for competition between Common Gulls and the other two gull species is inconclusive.
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