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Estimating the rarity of birds and its ecological context in the University of the Philippines Diliman Campus
Benjamin Vallejo Jr.* and Alexander B. Aloy
Institute of Environmental Science and Meteorology, College of Science
Villadolid Hall, University of the Philippines, Diliman, Quezon City

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ABSTRACT
In this paper we use frequentist (classical) and Bayesian inference to estimate the rarity of birds in the University of the Philippines Diliman campus. Rare species have a detection probability of 1%. As sightings of certain species of birds are extremely rare, a frequentist approach to estimation will often result in overestimates of observation precision. Using McArdle’s rarity and Bayesian inference we estimated the probability of detecting rare species as between 4 to 10%. The Bayesian estimates are lower and may be a better method for estimating rarity. The coefficient of variation (CV) of the frequentist (24%) and Bayesian (25%) estimates are similar suggesting imprecision. Rare species are likely to be detected in areas of campus with common and highly abundant species. This may be an artifact of the fast paced Jokimäki method that was used in surveys of bird abundances. The higher probability of detecting rare species from the suggested 1% cutoff and similar CV estimates are likely due to the small data set used in the estimations and the lack of prior information. The ecological context of our observations is related to the increasing fragmentation of habitat in the campus as a consequence of urbanization.


*To whom the correspondence should be addressed. E-mail:bmvallejo@up.edu.ph
Received May 20, 2008; Accepted December 13, 2008.
Editor-in-charge: Porfirio M. Aliño