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THE EFFECT OF FOREST DISTURBANCE ON BIRD POPULATIONS

AIM:

To determine the effect of disturbance on the population of each species.

OBJECTIVES:

Does the number of species differ between each grid?
Does species abundance differ between the grids?
Does the habitat differ between the grids?
What are the disturbances?
Are the species more nucleated in the disturbed grid?
Have these hotspots changed since last year?

INTRODUCTION:

The biogeography of the Wallacea region is responsible for a high number of endemic bird species on the island of Buton. Deep trenches in the sea surrounding the islands within the area have ensured that populations on Buton are cut off from those on the surrounding islands and, therefore, have evolved independently of others. This leads to endemic species and sub-species which are not found in any other part of the world and are very well adapted to their natural habitats on the island. The area consists of a mixture of both Australasian and Asian avifauna making this a truly unique area. Unfortunately, some of the forest has become highly disturbed due to selective logging and rattan collection and it is hypothesised that this may cause the depletion of some of the bird species as natural habitat is lost. This study has been designed to determine if species are indeed under threat and if so, protected parks can be set up where habitats and, therefore, the associated bird species can be preserved without danger. This is important because once species have been lost it is not possible for them to be regenerated and due to the high endemicity of species in the area, many cannot be found anywhere else in the world. It will be possible to determine how much the habitat has changed in one year by taking data collected by botanists last year and seeing how quickly habitat depletion is happening. This will give some idea of the urgency of the problem.

Three grids have been chosen in different areas of disturbance. The very small Kakenauwe reserve is broken by a road and selective logging has been high in the recent past with a low rattan diversity and abundance; this has been taken as our disturbed grid. The Lambusanga reserve is much larger and contains our two other grids. Work done by botanists has suggested that the area called Anoa is much less disturbed than our third area, Lapago, and we have taken this advice when positioning the grids.

Birds call the most during dawn and dusk, these times are not dark but are not too hot and it is at these times that birds are most likely to been seen, heard and recognised. Taking this into account, all our data is collected during these times

METHOD:

The study was conducted during the months of July 2002 and August 2002 in two reserves on Buton Island, Sulawesi. Three 1 km by 1 km study grids were used; one highly disturbed in Kakenauwe, one of medium disturbance in Lapago and one undisturbed in Anoa. The latter two located in the Lambusanga reserve. Thirty six equidistant points were used on each transect and each species heard or seen was recorded as well as their approximate distance from the point. Each point was surveyed twice with aim to complete one late and one early count at each. Counts lasted ten minutes each following a two minute ‘settling period’ to minimise any influence our arrival may have had on the data. The data collected was then entered into the DISTANCE® programme which can calculate population densities within the grids.

The dawn chorus was also monitored at nine points on each grid. This was completed between the hours of 5.30 am and 8.00 am. No repetitions were made. The time was divided into 15 minute intervals but data was only recorded during five minutes out of each 15 minutes. Again, birds heard were noted along with their approximate distance from the observer. This data was then entered into an Excel spreadsheet and graphs of the species present during the progression of the dawn chorus were plotted for each grid. Another graph was produced to show the number of individuals, regardless of species, calling through the morning in all grids with error bars added to show significance. Finally it was possible to show the number of species calling through the morning in all grids and again error bars were added.

Inter-observer consistency was ensured by each observer working for two weeks alongside a more experienced observer before recording alone. Unidentifiable calls were recorded using a microphone and mini-disc player to later compare with a comprehensive library of identified bird calls already held.

Group size data was also collected during afternoon walks, between 3.30 pm and 5.30 pm where birds were looked for and information regarding flock size was recorded. This information was then useful when determining the number of individuals in social species that were calling at the same time during the dawn chorus.

INITIAL RESULTS:

Initial data suggests that time of morning is important to the composition of the dawn chorus, with some species beginning earlier than others and some finishing early whilst others continue later in the day. The number of bird groups calling at different times in the morning is greatest in all three grids at around 6.00 - 6.15 am and the number of species is greatest at around 6.30 am.

There is a significant difference between the number of species calling during the morning in Anoa and the other two grids, with Anoa having fewer species. The effect of altitude, humidity, soils and other physical factors as well as edge effects have to be considered before further conclusions can be drawn.

REPORT:

A dissertation entitled The effect of forest disturbance on bird populations will be produced by Jennifer Smith, University of Aberystwyth, Antoinette Gregory, University of East Anglia and Sally Griffiths, University of Leeds by April 2003.