Anim Cogn (2007) 10:81–88 DOI 10.1007/s10071-006-0055-2 MINI-REVIEW Do avian brood parasites eavesdrop on heterospecific sexual signals revealing host quality? A review of the evidence Deseada Parejo · Jes ´ us M. Avil´ es Received: 20 January 2006 / Revised: 27 October 2006 / Accepted: 31 October 2006 / Published online: 19 December 2006 C  Springer-Verlag 2006 Abstract Eavesdropping can be defined as the extraction of information from the interactions between other individ- uals. It provides a relatively cheap way of gathering relevant information for fitness enhancement. Here, we propose that obligate avian brood parasites, which always lay their eggs in foreign nests of individuals of other species, may eavesdrop on their host sexual signals to locate nests of high quality individuals in which to lay their parasitic eggs. Sexual signal variation can honestly signal parental quality. Thus, by eaves- dropping on sexual signals, parasites may select high quality foster parents for their own offspring. Such a use of sexual signals within host populations by brood parasites differs from signal exploitation theory that proposes that parasite only use signals to locate potential host independently from signaller quality. Here, we review the avian literature con- cerning host choice within a host species by obligate avian brood parasites and find evidence for host selection within individuals of a host species on the basis of cues potentially functioning as sexually selected traits, or at least revealing parental abilities. We have also found support for the ex- istence of benefits linked to host selection by avian brood parasites. Finally, one study reported on the attenuation of a sexual ornament in host populations under strong pressure by brood parasites. Most of these findings have been interpreted as evidence for host selection by avian brood parasites based on the conspicuousness of sexual signals. We suggest, how- ever, that these findings may in fact reveal eavesdropping on host signalling performance by brood parasites which would D. Parejo () · J. M. Avil´ es Department of Functional and Evolutionary Ecology, Estaci´ on Experimental de Zonas ´ Aridas, C.S.I.C. C/General Segura 1, 04001 Almer´ ıa, Spain e-mail: parejo@eeza.csic.es use the information extracted to choose the better individu- als among conspecifics of a given host. This provides a new perspective for the study of host selection in obligate brood parasites, and raises interesting questions for the study of animal cognition that would deserve experimental studies. Keywords Brood parasitism . Heterospecific eavesdropping . Host choice . Public information . Sexual ornaments Introduction Signals used to provide information among organisms usually occurs in communication networks in which mul- tiple signallers and receivers, both conspecifics and het- erospecifics, are present. Hence, signals may inform individuals other than those interacting with the signaller. Eavesdropping is the behaviour involving the extraction of information from signalling interactions between others (McGregor and Dabelsteen 1996) which provides a rela- tively low-cost method of assessment, thereby reducing un- certainty. Signals used during this information process may belong to acoustic/vocal, visual, olfactory, chemical or other communication modalities, whenever their range allows re- ceivers access to the information transmitted (McGregor 1993). Moreover, a single individual may simultaneously provide information by means of several signals and through several sensory channels, which may lead to the use of mul- timodal cues by eavesdroppers. Eavesdropping has been demonstrated in a conspecific context for such phylogenetically distant organisms as fishes (e.g. Oliveira et al. 1998; Doutrelant and McGregor 2000), birds (e.g. Naguib et al. 1999) and mammals (e.g. Barclay 1982; Madsen et al. 2002). For instance, male Siamese Springer