Catching bias in cannon and mist netted samples of Redshanks Tringa totanus on the Inner Moray Firth

Abstract

A series of five cannon and nine mist net catches in a small area of the Inner Moray Firth during the autumn of 1994 provided the opportunity to examine the differences in the proportions of juvenile and moulting and non-moulting adult Redshanks caught by the two methods. This comparison showed significant differences with mist nets apparently catching a higher proportion of juveniles. This difference was highly significant for early autumn (13 August to 15 October) catches, when adults were undergoing active wing moult, but was no longer significant for late autumn (17 October to 24 December) catches when moult had been completed. The proportion of juveniles caught in cannon nets remained similar at 42.9 and 45.6% during the early and late periods. During the early period 67.6% of the birds caught by mist netting were juveniles, but this reduced to 43.9% in the late period, similar to the proportion found by cannon netting. It is suggested that in the situation on the Moray Firth the cannon netting techniques used were giving a representative sample of the population present. It also appeared that the number of juveniles caught by mist netting was proportional to the number of juveniles present in the population, but this appeared to be biased in the catch make-up because mist netting failed to catch a representative proportion of the moulting adults present. The significance of the difference between the proportions of moulting adults caught by the different catching techniques remained very high for the early autumn period but reduced for the late autumn from mid October onwards. It is suggested that the bias against catching moulting adults in mist nets was related to the progress of moult, and that once this had been completed there was little or no evidence for a bias against catching adidts. This suggests that the apparent bias in mist netting towards juveniles noted by previous workers is not related to any lack of experience or flying ability on the part of young birds, but is caused by some difference in the behaviour of moulting adults. © 1997 British Trust for Ornithology.