Estimating the use of tidal flats by waders: inaccuracies due to the response of birds to the tidal cycle

Abstract

Capsule Low tide counts may give inaccurate estimates of habitat use; combining low and mid-tide counts yields better estimates. Aims To evaluate whether low tide bird counts provide a good estimate of habitat use of intertidal areas with different exposure periods, to analyse how differences in the response of the birds to the tidal movement influences these estimates, and to evaluate an alternative to the traditional counting scheme. Methods The total use of intertidal plots by six wader species was estimated through regular bird counts during entire tidal cycles. These estimates were then compared to estimates of use based on both replicate low tide counts, and a combination of low and mid-tide counts. Results For most species, estimates based on low tide counts alone differed substantially from estimates based on counts repeated over the full tidal cycle. This difference tended to be greater in the upper parts of the sediment flats, and in species that followed the tidal waterline while foraging. Using the same number of count replicates, but extending the counting period to the mid-tides, we tended to obtain far more accurate estimates of use of space. Conclusion In studies that compare the use of areas under different tidal conditions, or that evaluate the use of restricted sectors of an estuary, a combination of low and mid-tide counts is preferable to the conventional low tide counts, especially in the case of species that tend to follow the tidal line.