The Hamilton and Zuk hypothesis on haemoparasite-mediated sexual selection and certain studies of reproductive costs are based on the assumption that avian blood parasite infections are detrimental to their hosts. However, there is no experimental evidence demonstrating harmful e¡ects of blood parasites on ¢tness in wild populations, it even having been suggested that they may be non-pathogenic. Only an experimental manipulation of natural blood parasite loads may reveal their harmful e¡ects. In this ¢eld experiment we reduced through medication the intensity of infection by Haemoproteus majoris and the prevalence of infection by Leucocytozoon majoris in blue tits (Parus caeruleus), and demonstrated detrimental e¡ects of natural levels of infection by these common parasite species on host reproductive success and condition. The fact that some of the costs of infection were paid by o¡spring indicates that blood parasites reduce parental working capacity while feeding nestlings. Medicated females may be able to devote more resources to parental care through being released from the drain imposed upon them by parasites and/or through a reduced allocation to an immune response. Therefore, this work adds support to previous ¢ndings relating hosts' life-history traits and haematozoan infections.

Introduced parasites threaten native host species that lack effective defenses. Such parasites increase the risk of extinction, particularly in small host populations like those on islands. If some host species are tolerant to introduced parasites, this could amplify the risk of the parasite to vulnerable host species. Recently, the introduced parasitic nest fly Philornis downsi has been implicated in the decline of Darwin's finch populations in the Galápagos Islands. In some years, 100% of finch nests fail due to P. downsi; however, other common host species nesting near Darwin's finches, such as the endemic Galápagos mockingbird (Mimus parvulus), appear to be less affected by P. downsi. We compared effects of P. downsi on mockingbirds and medium ground finches (Geospiza fortis) on Santa Cruz Island in the Galápagos. We experimentally manipulated the abundance of P. downsi in nests of mockingbirds and finches to measure the direct effect of the parasite on the reproductive success of each species of host. We also compared immunological and behavioral responses by each species of host to the fly. Although nests of the two host species had similar parasite densities, flies decreased the fitness of finches but not mockingbirds. Neither host species had a significant antibody-mediated immune response to P. downsi. Moreover, finches showed no significant increase in begging, parental provisioning, or plasma glucose levels in response to the flies. In contrast, parasitized mockingbird nestlings begged more than nonparasitized mockingbird nestlings. Greater begging was correlated with increased parental provisioning behavior, which appeared to compensate for parasite damage. The results of our study suggest that finches are negatively affected by P. downsi because they do not have such behavioral mechanisms for energy compensation. In contrast, mockingbirds are capable of compensation, making them tolerant hosts, and a possible indirect threat to Darwin's finches.

Invasive species can displace natives, and thus identifying the traits that make aliens successful is crucial for predicting and preventing biodiversity loss. Pathogens may play an important role in the invasive process, facilitating colonization of their hosts in new continents and islands. According to the Novel Weapon Hypothesis, colonizers may out-compete local native species by bringing with them novel pathogens to which native species are not adapted. In contrast, the Enemy Release Hypothesis suggests that flourishing colonizers are successful because they have left their pathogens behind. To assess the role of avian malaria and related haemosporidian parasites in the global spread of a common invasive bird, we examined the prevalence and genetic diversity of haemosporidian parasites (order Haemosporida, genera Plasmodium and Haemoproteus) infecting house sparrows (Passer domesticus). We sampled house sparrows (N = 1820) from 58 locations on 6 continents. All the samples were tested using PCR-based methods; blood films from the PCR-positive birds were examined microscopically to identify parasite species. The results show that haemosporidian parasites in the house sparrows' native range are replaced by species from local host-generalist parasite fauna in the alien environments of North and South America. Furthermore, sparrows in colonized regions displayed a lower diversity and prevalence of parasite infections. Because the house sparrow lost its native parasites when colonizing the American continents, the release from these natural enemies may have facilitated its invasion in the last two centuries. Our findings therefore reject the Novel Weapon Hypothesis and are concordant with the Enemy Release Hypothesis.

The physiological mechanism underlying the cost of reproduction may consist of immunodepression caused by increased parental e¡ort. Here, we report e¡ects of experimental manipulation of clutch size on T-lymphocyte cell-mediated immune response in female pied £ycatchers, Ficedula hypoleuca. Parents with reduced broods provisioned at lower rates than those caring for control and enlarged broods three days after hatching. Parents caring for enlarged broods provisioned nests at higher rates 13 days after chick hatching than those feeding control and reduced broods. Females with enlarged broods weighed less than females with control or reduced broods. No e¡ect of experimental treatment on nestling mass and size was found. The response to the injection of phytohaemagglutinin in the wing-web of females decreased with increasing brood size and with increasing provisioning rate when the chicks were three days old, when controlling for the negative e¡ect of female mass on response. The T-lymphocyte cellmediated response decreased from the reduced to the control, and from this to the enlarged group, when controlling for female mass. This e¡ect of experimental manipulation of clutch size was signi¢cant and consistent with a trade-o¡ between maternal e¡ort and immunocompetence.