Abstract from BTO Research Report No 190:

Dr Andrew J. Musgrove

The relationship between waterfowl counts carried out at high and low tide.

Executive summary

Population estimates of estuarine waterfowl in the UK are mostly derived from WeBS Core Counts (which are predominantly carried out at high tide), with further detail about feeding distribution within estuaries gathered by WeBS Low Tide Counts. This report compares the numbers of birds recorded by the two types of counts, in order to quantify the relationship between the data gathered by the two schemes.

The high tide and low tide count data for selected winters for a set of 39 estuaries were analysed. For all species combined, the low tide counts recorded about 79% of the total noted at high tide. A similar proportion (85%) was noted for all waders combined, but for the combined estuarine wildfowl, the low tide count total was only 48% of the high tide count.

The amount of agreement between high tide and low tide counts varied between individual species. From a total of 39 species analysed, eight showed no significant relationship between high and low tide counts, although these species were uncommon estuarine species. Four species (Grey Plover, Purple Sandpiper, Dunlin and Bar-tailed Godwit) showed regression coefficients which were not significantly different to one, implying that there was no overall difference between high and low tide counts for these species. Two species (Knot and Greenshank) were counted in significantly higher numbers at low tide than at high tide. For the remainder (25 species), counts were significantly higher at high tide than at low tide.

Analysis of the relationship between the difference in low tide and high tide counts, and a range of external estuarine variables (including measures of estuary size, geographical location and climatic variation) revealed that the difference in counts was related most strongly to the intertidal area. The larger the intertidal area of an estuary, the greater the discrepancy between high tide and low tide counts, presumably due to the difficulties of detecting estuarine birds at greater distances. Correlations of the differences in counts with the length of the shoreline and with the number of days with snow on the ground were also found, but ceased to be significant after application of a Bonferroni correction. However, since these two variables were found to be important for a well-defined group of species (principally sea-duck), the effect may perhaps have been a real one.

It was concluded that WeBS Low Tide Counts should not currently take the place of WeBS Core Counts for the purposes of monitoring the size of waterfowl populations on estuaries (obviously, WeBS Core Counts are not capable of replacing WeBS Low Tide Counts because they do not address within-estuary waterfowl distribution). This is because the difference between low tide and high tide counts depends most strongly on the intertidal area of an estuary, which implies that the smaller numbers of birds counted at low tide are mostly due to detectability and not to birds moving in and out of a site. If all of the UK’s larger estuaries had been counted at low tide the discrepancy between low tide and high tide totals would be expected to be far greater than seen in the present study. However, it is quite possible that with a longer run of data, WeBS Low Tide Counts could be used to monitor trends in population sizes of estuarine birds.

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