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Alarm calls and chick reaction: comments on Kleindorfer et al. (1996)

Correspondence: K. Halupka, Department of Avian Ecology, University of Wroclaw, Sienkiewicza 21, 50335 Wroclaw, Poland (email: halupka@biol.uni.wroc.pl). In a study of relationships between alarm calling by parents and the reactions of nestlings in the moustached warbler, Acrocephalus melanopogon, Kleindorfer et al. (1996) concluded that the antipredator response of chicks is the proximate cue for adult alarm calls. In this note we review sampling errors in the study and the reasoning behind the chick reaction hypothesis. The most serious problems in Kleindorfer et al.’s paper are pseudoreplication and lack of control for potential confounding factors. In 1991–1994, during 5 months of the breeding season, they made 191 1-h observations of 33 nests. The data (frequency of alarm calling) were treated as independent and pooled without any control for the influence of the nest (parent birds, nest site) advancement of the breeding season and year (all these factors could affect alarm-calling frequency: Montgomerie & Weatherhead 1988). Kleindorfer et al. used 23 nests to perform 250 experiments (observations of the parental and offspring reactions to three species of predators). Thus, on average, there were 10.9 tests per nest and each predator was exposed more than three times at the same nest. However, the data were treated as if they were independent. The following passage (Kleindorfer et al. 1996, page 1201) indicates another problem with sampling. ‘The cases ‘‘without alarm calls’’ include approaches (. . .) to three nests where alarm calls began after 1 min at the nest (these three nests are repeated entries in the ‘‘with call’’ category, because later calls elicited chick jumping).’ Thus, the data set was ‘enlarged’ by using invalid definitions of displays: the same parent birds in the same test could be both active and silent and the same offspring could both jump out and remain in the nest (data prepared in this way were