Oystercatcher specialisation : fitness implications and population consequences

The aim of this thesis was to investigate the fitness implications and the population consequences of individual feeding specialisations. In particular, it was concerned with how individual specialisations may affect the population consequences of habitat loss or change, a subject of particular relevance to conservationists. A review is made of individual feeding specialisation in shorebirds, the mechanisms involved in such specialisations and the different benefits and risks that may be associated with particular habitats or diets. Individual feeding specialisations were found to be widespread among many shorebird groups and most shorebird feeding specialisations were found to be constrained by differences in morphology, social status or individual skill It is concluded that individuals or groups that are unable to change foraging area, diet or feeding method Ar social or morphological reasons would be the most vulnerable to any deterioration in their food supply. Research into Oystercatchers Haematonus ostralecus overwintering on the Exe estuary, south-west England is presented and individual feeding specialisations described. Young birds and females specialised in different diets and feeding methods fi"om adult males. Sex ratios changed with age such that 50% of immatures and 67% of adults were male. Individual fitness was measured in terms of body condition and mortality rates of ringed birds. Mussel- hammerers, the m^ority of which were male, had higher body condition indices and lower rates of mortality than mussel-stabbers and worm/clam feeders. Worm/clam feeders, the majority of which were females and young birds, had lower body condition indices and higher rates of mortality than mussel feeders. It is suggested that young birds and females on the Exe had higher rates of winter mortality because of the lower payoffs and/or higher risks associated with their 6eding specialisations. A modelling approach is used to predict the population consequences of dif & rential mortality between age and sex groups. Increasing the mortality of young birds or 6males substantially reduced population size. Increasing female mortality resulted in a male biased population. Any increase in mortality which affected one sex more than the other resulted in a greater reduction in population size than if the increase affected both sexes the same. It is concluded that studies designed to predict the effect of habitat loss or change on shorebirds should be particularly aware of age and sex-related feeding specialisations that may lead to age and sex differences in mortality and age and sex diGkrences in resfwnse to change.