Ciclo reprodutivo e densidade populacional da Araponga (Aves: Cotingidae) : uma abordagem metodológica

Abstract

Resumo: Frequentemente a abundancia de especies raras nao pode ser estimada por metodos convencionais, como consequencia, o tamanho populacional de muitas especies neotropicais permanece desconhecido. Dentre estas encontramos as arapongas (genero Procnias), as especies deste genero sao de interesses conservacionistas, pois vivem em florestas pouco alteradas. Porem sao dificeis de amostrar, pois habitam florestas densas com a visibilidade prejudicada, a femea possui coloracao criptica e comportamento reservado, os machos vocalizam somente durante o periodo reprodutivo e a captura destas especies e dificil. A araponga (P. nudicollis) e uma especie ameacada e endemica da Mata Atlantica, mesmo assim nao existe dados nem sobre sua dinamica populacional nem sobre seu ciclo reprodutivo. Neste trabalho foi acompanhado todo o ciclo reprodutivo da especie, onde, por meio de contagens, foram obtidos dados sobre a fenologia da atividade dos machos para determinar as dinamicas de canto e abundancia ao longo do ciclo e descobrir o melhor periodo pare gerar a estimativa de densidade. Foi testada a a eficiencia da amostragem por distancia baseada em um transecto em linha em estimar a densidade populacional de Procnias nudicollis, usando triangulacao acustica baseada em duas coordenadas obtidas por GPS, onde foram determinados o angulo de direcao ao macho com uma bussola. Usando as coordenadas e os angulos foram comparados tres metodos para medir a distancia de cada individuo e esta informacao foi usada no programa Distance para estimar a densidade da araponga. Assim, o metodo baseado em triangulacao, onde as distancias e angulos foram estimadas por um script escrito no programa R, e nao necessitava trabalho adicional, reduziu consideravelmente o tempo para a analise, alem de minimizar o erro potencial nos calculos necessarios nos outros dois metodos. Existem aproximadamente 15 machos adultos km-2 (95% CI 11.9-18.7, CV = 11). Assumindo as premissas de razao sexual igual e razao etaria de 2:1, a ultima obtida pelas contagens, a populacao total da area estudada (87 km-2) pode ser estimada em 3915 individuos. O ciclo reprodutivo da araponga durou 163 dias, do inicio de setembro ate o meio de fevereiro. O numero de machos vocalizando aumentou gradualmente durante o ciclo, com o pico dos machos adultos no final de novembro. Machos adultos e jovens apresentaram diferentes periodos de atividade, e possivel que a araponga seja um migrante diferencial, pois o inicio tardio da vocalizacao dos jovens pode indicar padroes diferentes de migracao entre classes etarias. Como o estudo foi realizado em uma area bem preservada e com uma populacao saudavel, este resultado pode ser comparado com estudos populacionais em areas ja degredadas ou em que a especie sofra maior pressao de captura, para ter ideia de como a populacao reage. O metodo de triangulacao acustica pode eficientemente estimar a abundancia da araponga e tem potencial para ser largamente aplicado nao somente para aves, mas para especies florestais de dificil visualizacao que apresentam comportamento de lek ou vocalizam de um ponto por tempo suficiente para triangulacao.

Abstract: Abundance of rare or hard-to-see species is often difficult to estimate and, as a consequence, population sizes of many species of tropical birds remain unknown. This is true of the bellbirds (genus Procnias) that may be good indicator species because they are associated with healthy forests. Due to their lekking behavior, where males sing in tree tops often within the canopy, they are not easy to find. Also, females are silent and cryptically colored and males are silent when not doing their display. The Bare-throated Bellbird (P. nudicollis) is a potentially threatened endemic to the Atlantic Forest of Brazil. While it is known that this bellbird has declined in several states, due to the difficulty of study, population dynamics are unknown. Here, we begin to fill this lacuna by observing males for the reproductive season and based on those observation, developing a technique whereby the abundance of adult males can be estimated. We used a triangulation method based on two coordinates obtained by GPS at which compass bearings to the singing males were noted. Using these coordinates and angles, we compared three methods of estimating the positions of the singing males and using that information in the program Distance to estimate bellbird density. All three estimates and their confidence intervals were similar. Yet, a method based on triangulation in which distances and angle to the singing bird were all estimated by a script written in the program R and which required no additional work, reduced the time required considerably and also reduced or eliminated the potential error associated with hand calculations used in the other two methods. There are approximately 15 adult males km- 2. (95% CI = 11.9 - 18.7, CV = 11%). Also, during counts, we noted how many adults and young males vocalized. Using those number, the ratio of adult to adolescent males was approximately 2:1. If we assume that females and males are equally abundant, and if we assume that adult females are approximately equally abundant, there are approximately 3915 bellbirds in the 87 km2 study area. The breeding season lasted 163 days, from early September to mid February, as measured by the first and last sightings of vocalizing males. Rhe number of singing males gradually increased over time, and reached its peak in late November. The number of young (adolescent) males that vocalized, although not with the same finesse as the adult males, also increased over time, beginning a month or so later, but with a later peak and ending at the same time as the adults. Migration of the bellbird has been suggested, and these data suggest that young males arrive on the breeding grounds later than the adults, but that all birds migrate out of the breeding area at the same time. This first study with an estimate of abundance of the bellbird and which was carried out in well-preserved forest, now provides a baseline with which other studies may be compared. Also, we have shown that bellbird abundance can effectively be estimated by triangulation on vocalizing males. Bellbirds in other areas may now have their densities estimated by this method, as well as other vocal but difficult-to-see birds. Indeed, triangulation may be a better and faster method for estimating distances to animals than other methods used in Distance. We suggest that the Bare-throated Bellbird has a healthy population here in the Serra do Mar of Parana (at nearly 4000 individuals in the state park alone) and recommend that other regions estimate abundance so that better understanding of population dynamics, and their threatened status, may be better understood.