Analysing Faecal Samples of Ragworm Predators: Not Just a Matter of Counting Mandibles

Abstract

Ragworms Hediste diversicolor are an important prey item in the diet of many estuary birds. The standard method for estimating the number of Ragworms in faeces or pellets is simply dividing the counted number of Ragworm mandibles by two, which can easily lead to an underestimation of the real number of Ragworms present. Here, I present two alternative methods, which avoids bias in estimates of the number of Ragworms in samples. I compared the three methods by applying them to faecal samples of Grey Plover Pluvialis squatarola and Redshank Tringa totanus from the Tagus estuary, Portugal. The alternative methods yielded significantly higher estimates of the number of Ragworms present in each sample, a difference that can have repercussions on the outcome of dietary studies of Ragworm predators. The pros and cons of each method are discussed with regard to the use of faeces and pellets for ecological research, and the situation in which each method could be applied is discussed.