Tidal variation in numbers of waterbirds: how frequently should birds be counted to detect change and do low tide counts provide a realistic average?

Abstract

Capsule Variation in numbers needs to be assessed at both site- and species-specific levels, but low tide counts are representative of average usage in most cases. Aims To determine whether the numbers of waterbirds using six intertidal sites changed across the tidal cycle, how frequently counts were needed to detect change and whether low tide counts were representative of the average numbers using a site. Methods Hourly counts of Shelduck Tadorna tadorna, Wigeon Anas penelope, Oystercatcher Haematopus ostralegus, Dunlin Calidris alpina, Curlew Numenius arquata and Redshank Tringa totanus from six intertidal study sites – in Wales and Cumbria – were analysed using generalized linear models. Results Tidal state was significant in explaining the variation in species' numbers in 20 of 32 cases considered. The relative distribution of birds within one site changed across the tidal cycle for three of four species. In the 12 instances where numbers did not vary tidally, there would be no need to count birds more than once during the exposure period to obtain a representative sample. In the other cases, results suggested 11 instances where birds would need to be counted every hour to detect changes, eight where counts would be needed every two hours and one where counts would be needed every four hours. Conclusions Due to the variation between species and sites, it is difficult to recommend a minimum count frequency needed to detect changing usage across a tidal cycle. However, examination of species' feeding activity did indicate that low tide is the best time for recording the feeding distributions of many waders and the ebb or flood tide more suitable for some wildfowl. Analyses suggested that low tide counts were representative of the average usage of the study sites in 75% of cases.