Research Article: The Secret Life of Oilbirds: New Insights into the Movement Ecology of a Unique Avian Frugivore

Date Published: December 16, 2009

Publisher: Public Library of Science

Author(s): Richard A. Holland, Martin Wikelski, Franz Kümmeth, Carlos Bosque, Adrian L. R. Thomas.

Abstract: Steatornis caripensis (the oilbird) is a very unusual bird. It supposedly never sees daylight, roosting in huge aggregations in caves during the day and bringing back fruit to the cave at night. As a consequence a large number of the seeds from the fruit they feed upon germinate in the cave and spoil.

Partial Text: One of the most crucial challenges for biologists in the next decade is the understanding of the ecological and evolutionary processes involved in the movement of organisms [1]. This assumes particular significance given the emergence of problems with habitat fragmentation and climate change. The recently defined paradigm of “Movement Ecology” [2] calls for a need for new data not just on sequential positions in space but also on the physiological and/or behavioural state of the organism in order to fully understand why and how they move. Understanding movement ecology assumes particular significance in plants, whose seeds are dispersed by animals in fragmented habitats where avian seed dispersers play a crucial role in the ecosystem [3], [4], [5]. Despite this, high resolution data on the impact of avian seed dispersers are lacking in most cases. Seed dispersal is one of the most important processes in any ecosystem, particularly during times when anthropogenic influences fragment landscapes into small, potentially non-connected habitats [4], [5], [6], [7]. Understanding which animals provide the ecosystem services of dispersing seeds between fragments and over large distances is a major research goal for ecology [4], [5], [6], [7], [8]. Seed dispersal is particularly important in tropical forest ecosystems that suffer considerably under anthropogenic stress [9]. Animals that provide connectivity in fragments of tropical forests should be important targets of conservation [8]. Thus the aim of this study was to better understand the ecological role Steatornis caripensis (the oilbird) in Neotropical forests using high resolution GPS and accelerometry [10], [11].

Experiments on Steatornis caripensis were conducted under permits from the Ministerio del Ambiente (#2255) and Instituto Nacional de Parques (Inparques, #0789). We adhered to the AOU special committee recommendations for the use of wild birds in research. A Venezuelan National Park ranger accompanied our research team during work in the “Monumento Natural Alejandro de Humbolt”.

The data obtained here provide new insights into the behaviour of a unique nocturnal frugivore. Previous evidence suggested that Steatornis caripensis make foraging trips for fruit on a nightly basis, returning to the cave at the end of each night [17], [23]. Observations of high activity at foraging stands suggested that Steatornis caripensis would forage for fruit constantly throughout the night, breaking only to return to the cave [23]. Our data indicate that Steatornis caripensis do not continuously fly throughout the night and that individuals do not return to the Cueva nightly but make extended foraging trips over a number of nights. Whenever birds stay outside the cave for a few days they roost in trees in the forest during daylight hours. Plotting the position of known Steatornis caripensis caves indicates that roosting sites do not coincide with these (Fig. 5). Our data on diurnal behaviours of Steatornis caripensis confirm anecdotal reports of Steatornis caripensis roosting in trees [20], [21] and also supports the data from seed traps at the Cueva del Guácharo which suggest a drop in the number of seeds brought back to the cave at this time of year [25]. Our data also indicate that roosting sites in the forest are not the same place that the birds forage, which also indicates that they are effective seed dispersers. This has major implications for the status of oilbirds within their ecosystem. By staying out near, but not at foraging sites for several nights increases their effectiveness as seed dispersers. The birds foraged up to 75 km from their roost, the Cueva del Guácharo; similar to data from radio tracking which suggested that they may forage 120 km from the Cueva nightly [23]. This distance is also beyond the boundary of the national park put in place to protect these animals. Approximately 40% of roosting and foraging sites were outside the boundaries of the national park. The high efficiency of the flights and the higher speed with which birds flew to roosting sites than to foraging sites suggests that the roosting sites may have been familiar to them, there was no apparent search pattern displayed in the tracks.



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