| Summary: | Bird communities at Gibraltar are examined and the ecology and behaviour of two species pairs which dominate numerically, the Blackcap Sylvia atricapilla and the Sardinian Warbler S.melanocephala in Matorral, and the Swift Apus apus and the Pallid Swift A.pallidus in the air, are compared. The communities exhibit seasonal changes in species and biomass probably due to habitat seasonality and geographical location, The latter in part produces the large migrant element in the communities. Largest seasonal fluctuations occurred in the simplest habitats. The number of species which potentially interacted with the two species pairs studied changed with season, The communities at Gibraltar differ from those studied by others elsewhere in the Mediterranean Basin. The Blackcap and the Sardinian Warbler are resident and exhibit morphological deviations from other known forms. The local Blackcap population is intermediate morphologically between the Sardinian warbler and the nominate Blackcap population, which arrives at Gibraltar in winter, The life cycle of the residents is related to climatic conditions, food supply and the presence of migrants. The two species overlap extensively in resource use but the Sardinian Warbler is broader-niched. Interspecific aggression end territoriality spaces individuals all year. Resident warblers accumulated fat during favourable periods, Mortality was high in both species, Northern Blackcaps returned in successive winters. Swifts exploited seasonal resources and left in autumn. Life histories are modified accordingly. Overlaps in resource use are high but there is spacing out of groups. Pallid Swifts are less specialised than swifts and remain longer at Gibraltar, raising two broods instead of one. Pallid Swift mortality is low but higher than for Swift. The two pairs studied may not be in equilibrium and behavioural interactions maintain resources partitioned. The question is raised as to whether territoriality is related to competition or to interference.
|
|---|