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Breeding Atlas >> Methodology

Methodology


Abundance index maps

For this atlas we estimated the probability of occurrence of a species for all Catalan 1x1 UTM squares (roughly 32,000) by applying niche-based models to the data collected during 1x1 UTM square censuses (3,077 squares in the case of diurnal birds and 1,204 for nocturnal birds). The models developed allow us to estimate each species' response to a series of environmental variables and thereby obtain the predicted probability of occurrence for each species as a particular combination of environmental variables.

In the modelling exercise used in the present atlas, we used a GLM with a logit link due to the binomial character of the presence/absence bird data employed (logistic regression, McCullagh & Nelder 1989). We used a cross-validation procedure to evaluate the accuracy of model predictions (Guisan & Zimmermann 2000). This procedure consisted of dividing the data (1x1 UTM square surveys) into two different sets by randomly assigning 70% of occurrence values for each species to a calibration data-set and the remaining 30% of occurrences to an evaluation data-set. The calibration data-set was used to develop the niche-based model. The evaluation consisted in measuring quantitatively to what degree predictions from the models fitted the independent observations that were not used for the development of the model.

 


Abundance index map (Subalpine warbler). This map shows the probability of detecting the species in each 500x500 m square during the breeding season with two one-hour sampling periods, one in March-April and the other in May-June. This map has been drawn on the basis of data from the censuses in 1x1 UTM squares. It shows the probability of detecting the species during the breeding period.


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ICO-Oficina Tècnica
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08037 Barcelona
Tel: 93 458 78 93
atles@ornitologia.org
 


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Catalan Winter Bird Atlas 2006-2009
 

Institut Català d'Ornitologia
ICO, Museu de Ciències Naturals, Zoologia,
Passeig Picasso s/n, 08003 Barcelona
Tel: 93 458 78 93 Fax: 93 310 49 99 E-mail: ico@ornitologia.org