Kleptoparasitism, disturbance and predation of yellow-legged gulls on Audouin's gulls in three colonies of the western Mediterranean

Abstract

SUMMARY: The impact of yellow-legged gulls on Audouin's gulls was studied by means of observations from a blind, at the Ebro Delta, the Chafarinas Islands and the Columbretes Islands colonies, during different stages of Audouin's Gulls breeding cycle. The rates of predation (upon eggs and chicks) and kleptoparasitism (aerial and on courtship and chick-feeding regurgitates) were recorded to evaluate this impact. Kleptoparasitism and predation rates recorded at the three colonies may be considered low when compared with similar studies dealing with other gull species. It seems that interactions did not represent a threat to the population dynamics of Audouin's Gulls in any of the three colonies. The highest rates of aerial kleptoparasitism, courtship feeding and chick mortality were recorded at the Columbretes Islands, and the lowest at the Ebro Delta. The ratio of the number of yellow-legged gulls to the number of Audouin's gulls seems to influence the rate of disturbances. Food availability was also likely to play an important role in the number of interactions, which increased when food was in shorter supply. The study at the Ebro Delta and Chafarinas Islands was restricted to some subcolonies and hence results may not be necessarily representative of the entire colonies. The existence of some yellow-legged gull individuals specialised as predators was recorded. Finally, we discuss the suitability of several conservation measures commonly applied in colonies where both species breed syntopically.Key words: kleptoparasitism, predation, disturbance, gulls, Columbretes, Chafarinas, Ebro Delta, conservation, seabirds. RESUMEN: CLEPTOPARASITISMO, MOLESTIAS Y DEPREDACIÓN POR PARTE DE LA GAVIOTA PATIAMARILLA SOBRE LA GAVIOTA DE AUDOUIN EN TRES COLONIAS DEL MEDITERRÁNEO OCCIDENTAL. -El impacto de la gaviota patiamarilla sobre la gaviota de Audouin se estudió desde un observatorio en las colonias del Delta del Ebro Delta, islas Chafarinas e islas Columbretes, durante diferentes fases del ciclo reproductor de la gaviota de Audouin. Las tasas de depredación (sobre huevos y pollos) y el cleptoparasitismo (aéreo y sobre cebas nupciales y cebas a pollos) fueron registrados para evaluar este impacto. Las tasas de cleptoparasitismo y depredación registradas en las tres colonias pueden considerarse bajas en comparación con estudios similares llevados a cabo con otras especies de gaviotas. Las interacciones no parecen representar una amenaza para la diná-mica poblacional de la gaviota de Audouin en ninguna de las tres colonias. Las tasas más altas de cleptoparasitismo sobre cebas nupciales, cleptoparasitismo aéreo y mortalidad de pollos se registraron en las islas Columbretes y las más bajas en el Delta del Ebro. La relación entre el número de gaviotas patiamarillas y el de gaviotas de Audouin pareció influir las tasas de molestias. La disponibilidad de alimento es probable que juegue también un papel importante en el número de interacciones, que aumentaron cuando el alimento fue más escaso. El estudio en el Delta del Ebro y las...

Managing seabird metapopulations in the Mediterranean: constraints and challenges

“…Counts of nests and some ringing (much less intensive than the programme carried out on Audouin's gull) have also allowed researchers to study the metapopulation dynamics of this species. The yellow-legged gull is considered a pest in the western Mediterranean (Vidal ≠ 1998), although several studies fail to find any population decrease or extinction of smaller and threatened seabird species owing to interference competition (Bosch, 2000;Martínez-Abraín et al, 2003). In 1991, only 710 pairs of yellow-legged gull bred at the Ebro Delta (see Fig.…”

“…1). Data from Oro et al, 1996;Oro, 1998;Martínez-Abraín et al, 2003). Distances are approximate and show an average estimate.…”

“…Oro et al, 1996): the Ebro Delta is a large and good breeding habitat, with a high food supply due to the oceanographic features off the colony and the high availability of foraging habitats (see Oro, 1998 references therein), whereas the Columbretes colony is comparatively a low quality habitat, with very limited suitable breeding habitats and foraging resources and a relatively high number of interspecific competitors (Oro et al, 1996;Martínez-Abraín et al, 2003) (see Table 1). The Ebro Delta was colonised recently and shows a high population increase, whereas the Columbretes were colonised much earlier and show a more fluctuating population dynamics (Oro et al, 1996).…”

Influence of food availability on demography and local population dynamics in a long-lived seabird

“…The site was colonized in 1974 (40 pairs) and breeding numbers reached 625 pairs in 1995. Only yellow-legged gulls (Larus cachinnans) regularly prey upon Audouin's gulls, especially on eggs and chicks (Martínez-Abraín et al 2003).…”

Unforeseen effects of ecosystem restoration on yellow-legged gulls in a small western Mediterranean island

“…Low recruitment of native vegetation, because of herbivory by these alien species, the high density of gull guano and the human disturbance (D. García-Llinares, unpublished data 2003) fostered soil erosion (see Otero & Fernández-Sanjurjo 1999;Vidal et al 2000;Mulder-Christa & Keall-Susan 2001). Eggs were punctured, by means of hypodermic needles, to reduce the number of gull offspring that year and hence contribute to the future reduction in the rate of population growth and decrease the negative effect of yellow-legged gulls on recruitment of Audouin's gulls Larus audouinii (see for example Finney et al 2003;Martínez-Abraín et al 2003) and storm-petrel predation. Vegetation and eggs were only treated at these times.…”