Plumage Coloration and Conspicuousness in Birds: Experiments with the Pied Flycatcher

Abstract

-Hypotheses concerning predation, sexual selection, and communication in birds often assume that individuals differ in conspicuousness. However, few studies have tested this by measuring the response of individuals that receive the signals. We investigated the effect of plumage coloration on conspicuousness by presenting caged Pied Flycatchers (Ficedula hypoleuca) to unmated territorial males and measuring the time until a response (either courtship or aggression) from the territorial male was observed. In the first experiment, we presented caged males and females at various distances and degrees of habitat openness. Caged birds elicited responses more rapidly in open than in closed habitats, and response time increased with distance. Black-and-white-colored males seemed to trigger a response more rapidly than brown-colored females, at least in open habitats. In the second experiment, we presented bright-colored males, dull males, and bright-painted dull males. All trials were made at the same distance. Caged birds again elicited more rapid responses in open than in closed habitats. Bright-colored males seemed to trigger a response more quickly than dullcolored males, but only in closed habitats. Painted birds elicited response times intermediate to those of bright and dull males. The difference in interaction between color and habitat in the two experiments is difficult to explain but may have been caused by differences in background related to seasonal development of vegetation. One possible problem with experiments on conspicuousness is that the response may be delayed after the receiver has detected the signal. Field observations of the behavior of territorial birds suggested that this probably did not confound the results of our study. Thus, the results support the intuitive but previously untested hypothesis that a bright and contrasting coloration makes birds more conspicuous to conspecifics. Received 30 December 1995, accepted 10 April 1996. GREAT VARIATION exists in animal coloration both among and within species. This may have evolved in response to predation (Cott 1940; Baker and Parker 1979; Harvey and Paxton 1981; Guilford 1990; Endler 1991; Dumbacher et al. 1992; Gotmark 1992, 1993, 1995), sexual selection (Hamilton and Zuk 1982, Parker 1983, Hill 1991, Sxtre et al. 1994; review in Andersson 1994), intraspecific communication (Rohwer 1975,1982; Whitfield 1986; Slagsvold and Lifjeld 1988; Butcher and Rohwer 1989), or for species recognition (Rowland 1979, Endler 1983, Andersson 1994). Several of these hypotheses assume that an individual with bright or contrasting coloration is conspicuous (i.e. more easily discovered) than one with a dull and cryptic coloration (Cott 1940, Parker 1983, Slagsvold and Lifjeld 1988, Butcher and Rohwer 1989, Endler 1991). 1 Present address: Agricultural University of Norway, Department of Biology and Nature Conservation, P. 0. Box 5014, N-1432 As, Norway. E-mail: svein.dale@ibnf.nlh.no Testing whether various plumage colors and patterns differ in conspicuousness is a difficult task. Birds of different color may differ in detectability to humans (Gotmark and Unger 1994, Gotmark and Hohlfalt 1995), but bird vision is different from and perhaps better than that of humans (Goldsmith 1990). Ideally, response by receivers of the signal (e.g. color), such as a conspecific or a predator, should be measured (Bennett et al. 1994). Experiments also must be designed to ensure that a response can be observed once the subject has received the signal. Pied Flycatchers (Ficedula hypoleuca) vary in plumage color. Males range from black-andwhite (i.e. bright) to brown (i.e. dull), whereas all females are brown (Drost 1936). Males benefit from a bright plumage because they are preferred by females as mates (STtre et al. 1994), but bright males seem to pay a cost in terms of a higher predation risk (Slagsvold et al. 1995; but see Gotmark 1992, 1993, 1995). The benefit of bright plumage may increase if bright individuals are more easily discovered by intruding males and prospecting females (Slagsvold and Lifjeld 1988). However, the cost