Are we enlightened about the immunocompetence of a severely inbred population of New Zealand robins? Challenges inherent in studies using immunological endpoints.

The relatively new field of immunotoxicology developed as an offshoot of traditional, medically based immunology, to study the effects of environmental toxicants on immune function (Fairbrother et al., 2004). A parallel application of methods used in immunotoxicology has been to investigate the evolutionary significance of a robust immune system to such diverse topics as sexual signalling, reproductive effort and population persistence. In a broad sense, Hale & Briskie’s (2007) work confirms that there is a high level of interest in evaluating the health of wildlife using non-lethal methods, and that tests of immune function may function as sensitive bioindicators of well-being in wild birds. Scientists, as well as concerned citizens, exercise huge efforts to ensure survival of threatened species, although reduced genetic diversity is expected to result in constraints on the adaptability, disease resistance and overall fitness of the individuals in those populations. If we accept that challenges to the immune response include many types of stressors, severe inbreeding in Motuara robins, described by Hale & Briskie, would qualify as such a stressor. The authors cite numerous publications in which inbred populations have suffered disproportionate losses from environmental stressors, including infectious disease, presumably because of suboptimal immune function. They identify a gap in experimental research linking low genetic diversity with compromised immune status, and, with the current study, set out to address this deficiency. The authors study two island populations of New Zealand robins, one of which is the original, while the other has 600 birds that grew from five individuals translocated from the ancestral population 32 years ago. The bottlenecked population has recently shown poor reproductive performance, speculated to be due to inbreeding depression. The authors compare parasitism and several features of the immune systems in these populations. The testing of immunocompetence is a sound and valuable tool for field scientists but we must be cautious about how to proceed. Too often, tests are selected without sufficient attention to their relevance or application. With a system as robust and redundant as the immune system, it is most meaningful to compare birds during periods of stress when it is particularly costly for them to compensate for additional challenges such as infectious or parasitic diseases. Therefore, studies of immune function are most enlightening if they are carried out during socially or physiologically demanding times such as the breeding or winter seasons, rather than the benign preand post-breeding seasons that the authors selected. The following comments are based upon examples taken from Hale & Briskie’s study, but the shortcomings are not unique to these authors. One of the two ectoparasites the authors selected to reflect immunocompetence was a feather mite (Astigmata spp.). There is substantial evidence (Blanco et al., 2001; Pap, Tokolyi & Szep, 2005) that feather mites are commensal organisms with no pathological costs to the host. Thus, mites would be highly unlikely to stimulate any immunological response. Regarding the other costly, blood-sucking hippoboscid flies (Ornithomya spp. andOrnithoica spp.) that they examined, if the birds had been dusted with insecticidal powder, the researchers would have had definitive counts of the flies as well as any other ectoparasites. Instead, we have only estimates of these parasites based on how many were seen on, or flying off the birds. Hippoboscid flies would exert an immunological cost. Proteins present in the saliva of haematophagous ectoparasites are foreign antigens that elicit potent localized immune responses (Wikel & AlarconChaidez, 2001). In the immunologically competent host, the