Supplementary Material 3

Supplementary material 3

Accumulation function

The duration of scan sampling observations was determined using accumulation functions, which allow the estimation of the required effort (in terms of time) to register an arbitrary proportion of the function asymptote (Shiu and Lee 2003; Soberón and Llorente 1993). In order to assess how individuals are accumulated over time in each focal tree, we used the linear dependence model, defined as:

where S(t) is the number of bird individuals recorded until time t, and a and b are parameters that were estimated by means of a nonlinear model fitted to scan sampling data (see text). The a/b ratio represents the predicted asymptote (i.e., the expected number of individuals at the focal tree). The count duration required to reach 97.5% of the model asymptote (T) can be estimated from:

For this analysis parameter a was estimated as 0.0595 (SE=0.0008, p <0.001) and parameter b as 0.0591 (SE=0.0012, p <0.001).

References

Shiu HJ, Lee PF (2003) Assessing avian point-count duration and sample size using species accumulation functions. Zoological Studies 42: 357-367.

Soberón J, Llorente J (1993) The use of species accumulation functions for the prediction of species richness. Conservation Biology 7: 480-488.