The Breeding Biology and Conservation of Audouin's Gull Larus audouinii on the Chafarinas Islands

Bradley, Patricia Mary (1988) The Breeding Biology and Conservation of Audouin's Gull Larus audouinii on the Chafarinas Islands. PhD thesis, University of Glasgow.

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Abstract

The breeding population of Audouin's gull Larus audouinii, a Red Data Book species, is endemic to the Mediterranean. Its population was estimated at 4000 pairs in 1985, but its restricted distribution and suggestions of low fecundity and vulnerability to disturbance and competition prompted this study. The gull's most numerically important breeding colony is on Rey Island, in the Chafarinas archipelago off the Mediterranean coast of Morocco, where in 1983 this three year study began. Its aims were to discover the factors limiting the size of the colony and to formulate a management plan. In comparison with many other Palearctic gull species, the breeding biology of Audouin's gull is little known. Data were collected during the three study seasons at two study sites which were established within the only two habitat types commonly occupied (NORTH SITE an elevated exposed site, little ground cover, high Audouin's gull nesting densities; SOUTH SITE a sheltered gentle slope, ample ground cover, low Audouin's gull nesting densities). Data presented here have shown that although there is considerable variation between years and sites, Audouin's gulls have the capacity to sustain and indeed even increase their population size at this colony. The largest mean clutch size was observed at the North site in 1983 (mean=2.75, s.e. =0.06, n=85) and the smallest at the South site in 1984 (mean=2.38, s.e. =0.09, n=98). A measure of survival to a stage close to fledging was made, that of the number of chicks surviving to twenty days per nest. The highest mean site value was at the South site in 1983 (mean=1.26, s.e. =0.10, n=74) and the lowest was at the South site in 1985 when not one chick survived from 52 nests. In order explain this variation in success, the relationships between breeding success and the following factors: environmental factors; Audouin's gull behavioural factors; and interactions with other species, were examined. Analysis of variance in hatching success in relation to laying period, study site and year, showed that hatching success varied in relation to timing of breeding at both sites in each season. Those breeding earlier were generally those to have the greater success. Hatching success was consistently higher at the North site as compared with the South. 1983 was overall the year when hatching success was greatest and 1985 the least successful. Environmental factors which included: laying date; height of vegetation surrounding the nest; distance to the edge of the subcolony; nesting density; and visibility from the nest, were shown, in analyses of variance, to influence the success of pairs in raising at least one chick to twenty days. Behavioural patterns which influenced breeding success were also recorded. Audouin's gulls are not aggressive, their only defence in the face of attack upon their clutch or brood was to mob the attacker. This proved to be successful only when Audouin's gulls were nesting at high densities. However, it was also found that at high nesting densities the incidence of intraspecific attacks upon chicks was high. Audouin 's gull chicks are generally able to leave their nests when they are only one day old. Many did not return to their nests but remained concealed within the cover of bushes. Chick mobility appeared to be influenced by environmental factors. At sites where nest cover was available, chicks were less likely to leave their natal territories and if they did had a shorter distance to travel whilst exposed to intra- and interspecific aggression. Whilst great variation in survival of chicks was recorded between sites and years, Audouin's gull chick growth took place at a steady and similar rate each season for chicks of various brood sizes, with weight and wing length increasing in a regular sigmoid manner. Together these observations do not suggest that food was in short supply. The most important influence upon Audouin's gull reproductive success on the Chafarinas Islands during this study was the growth of the yellow-legged Mediterranean herring gull colony on Rey. Whilst the number of pairs of Audouin's gulls remained approximately the same over the three seasons (2100+100 pairs), there has been a 79% increase in the number of herring gull pairs. There is no evidence to suggest that the predominantly fish-eating Audouin's gull is in competition with the omnivorous herring gull for food, however the herring gull is certainly larger, more aggressive, and earlier breeding than Audouin's gull. The herring gulls are not only occupying what were previously Audouin's gull nest sites on Rey but have been shown to be considerable predators of Audouin's gull eggs, chicks and adults. Over the course of this study the level of herring gull interference with Audouin's gull breeding attempts intensified. A management plan was therefore developed which incorporates: a) the reservation of nesting space for Audouin's gulls; b) improvement of the nesting environment for Audouin's gull; c) and the control of disturbance and predation of breeding Audouin's gulls by herring gulls. In 1987, the Instituto para la Conservacion de la Naturaleza, acting upon the findings and recommendations of this study, conducted a cull on Rey of 950 adult herring gulls. The breeding population of Audouin's gull has subsequently increased from 1930 nests in 1986 to 2845 nests in 1987.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Ecology, Wildlife management
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77627
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53
URI: http://theses.gla.ac.uk/id/eprint/77627

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