Harri Hakkarainen á Petteri Ilmonen á Vesa Koivunen Erkki KorpimaÈki Blood parasites and nest defense behaviour of Tengmalm's owls Received: 20 October 1997 / Accepted 29 December 1997 Abstract Infectious diseases are expected to negatively in¯uence essential life history traits of an individual, because investment in immunological response occurs at the expense of reduced investment in other functions. Here we present the ®rst observational evidence that the prevalence of blood parasites is negatively associated with avian nest defense. Because the defense of ospring entails a risk of serious physical harm to the parent, it is also assumed to be a good estimate of parental invest- ment. In both 1994 and 1995, the nest defense intensity of male Tengmalm's owls (Aegolius funereus) against a live American mink (Mustela vison) was strongly cur- tailed in parents infected by Trypanosoma avium blood parasites. Our data suggests that investment in repro- duction can be negatively aected by parasitaemia, and that host-parasite interactions may potentially modify hosts' life-history traits, making it important to consider the costs of parasitism in future studies. Key words Blood parasites á Nest defense á Parental investment á Tengmalm's owl á Trypanosomes Introduction The role of parasite-host interactions in animal ecology has recently been under intensive debate, especially in studies of sexual selection (Hamilton and Zuk 1982; Keymer and Read 1991; Read 1988; Toft 1991) and reproductive trade-os (Gustafsson et al. 1994; Forbes 1993; Moreno 1993; Norris et al. 1994; Richner et al. 1995; Sheldon and Verhulst 1996), and the adverse ef- fects of parasites on breeding success have been shown in many recent empirical studies. For example, individuals harbouring blood parasites may delay breeding (e.g., RaÈtti et al. 1993; KorpimaÈki et al. 1995), produce less eggs (Schall 1983; KorpimaÈki et al. 1993) and ¯edglings (Sundberg 1995; Apanius 1991a) compared to those free from parasites. In many birds, nest predation is one of the most important factors aecting ®tness (Clark and Wilson 1981). By defending a nest against predators, parent birds are able to increase the probability of successful breeding (Blancher and Robertson 1982). However, such defense is apparently costly in terms of energy, and in- cludes a risk of injury or even death (Brown 1947). Therefore, the intensity of nest defense has been con- sidered to be an important measure of parental invest- ment (Blancher and Robertson 1982; Montgomerie and Weatherhead 1988), especially because parents must decide immediately how much to invest in ospring (Montgomerie and Weatherhead 1988), when faced with a predator. Parasites compete for energy reserves of their host and antiparasitic adaptations in the host may be energetically costly (Toft 1991; Sheldon and Verhulst 1996). Consequently, parasitaemia along with poor physiological condition may decrease the amount of energy used for other functions, like traits essential for reproduction. Tengmalm's owl (Aegolius funereus) is a small, noc- turnal predatory bird that breeds in nestboxes and nat- ural cavities in Holarctic coniferous forests. In this area, they feed mainly on voles of the genera Clethrionomys and Microtus (e.g., Sulkava and Sulkava 1971; Ko- rpimaÈ ki 1988a). Tengmalm's owls harbour Trypanosoma avium blood parasites (KorpimaÈ ki et al. 1993), which are unicellular hemo¯agellates transmitted by blood sucking culicine mosquitoes, simuliids, dermanyssid mites and hippoboscid ¯ies (Molyneux 1977). Trypanosomes may induce histopathological changes in the host (Molyneux et al. 1983). Although trypanosomes may mostly be non- pathogenic (Baker 1976), their occurence is thought to be an indicator of the poor state of host's immune system (Apanius 1991b). By using the intensity of nest defense as an estimate of parental investment, we investigated whether the Oecologia (1998) 114:574 ± 577 Ó Springer-Verlag 1998 H. Hakkarainen (&) á P. Ilmonen á V. Koivunen á E. KorpimaÈki Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland Fax: 358-2-333 6550; e-mail: HARRI@HAKKARAINEN.utu.®