Distribuição, Influência de Características de Locais para Reprodução na Ocupação e Estimativas de Abundância de trinta-réis-anãos (Sternula superciliaris), gaivotas (Phaetusa simplex) e corta-águas (Rynchops niger) no Arquipélago de Anavilhanas, Amazonas, Brasil

Abstract

Ecological information on species occurrence and abundance are crucial to assess the influence of environmental factors on the populations, to recognize changes in population sizes and to identify key sites to maintain viable populations. We estimated the distribution, influence of site features on colony site occupancy and abundance estimates accounting for detection probabilities of Yellow-billed Terns(Sternula superciliaris), Large-billed Terns (Phaetusa simplex) and Black Skimmers (Rynchops niger) breeding in the Anavilhanas Archipelago, Amazonas, Brazil. We did repeated surveys on beaches available in 2008 and 2009 breeding seasons and conducted point counts with a double observer approach in 2009. We mapped 26 potential sites in 2008 and 30 in 2009, one site hosted ≥ 28% of total Yellow-billed Terns counted and two sites hosted ≥ 65% of Large-billed Terns counted. Detection probability, p (±SE) in 2008 and 2009, for Yellow-billed Tern was 0.8272 (±0.0534) and 0.8737 (±0.0341), for Large-billed Tern was 0.8668 (±0.0512) and 0.7323 (±0.0577), for Black Skimmer was 0.8321 (±0.0551) and 0.9429 (±0.0392). Site occupancy, psi (SE) in 2008 and 2009, for Yellow-billed Tern was 0.5841 (±0.0982) and 0.6334 (±0.0880), for Large-billed Tern was 0.5025 (±0.0986) and 0.4006 (±0.0896), for Black Skimmer was 0.5432 (±0.0987) and 0.2333 (±0.0772). Terns and Black Skimmers preferred to nest in larger beaches, islands with small percent of or no vegetation cover, beaches more exposed to open water, sites further away from river margin, more isolated from island, and closer to main colonies. A total of 133 Yellow-billed Terns, 72 Large-billed Terns and 103 Black Skimmers were counted in 2008 and in 2009 abundance estimates (N ± SE) were for Yellow-billed Tern = 192.36 (± 1.24), for Large-billed Tern = 79.56 (± 0.77) and for Black Skimmer = 31.22 (± 0.48). Overall detection probability-p (probability of a bird being detected when two observers counting) was high for the three species (p ≥ 0.992), but it was different between species and between observers and was lower for each observer separate. Our results suggest that breeding site use by terns and Black Skimmer may be influenced by a combination among habitat features and intraspecific attraction of these birds. Based in our estimates of detection probabilities we suggest the use of the double observer approach for point counts for more accurate abundance estimates, population changes assessment and conservation planning for waterbirds.