title: Avian blood parasite infection during the non-breeding season: an overlooked issue in declining populations?
abstract: BACKGROUND
  Pathogens and parasites can have major impacts on host population dynamics, both through direct mortality and via indirect effects. Both types of effect may be stronger in species whose populations are already under pressure. We investigated the potential for blood parasites to impact upon their hosts at the immunological, physiological and population level during the non-breeding season using a declining population of yellowhammers Emberiza citrinella as a model.
  
  
  RESULTS
  Yellowhammers infected by Haemoproteus spp. showed both a reduced heterophil to lymphocyte (H:L) ratio, and an elevated standardised white blood cell (WBC) count compared to uninfected birds, indicating an immunological response to infection. Infected birds had shorter wings during the first winter of sampling but not during the second, colder, winter; survival analysis of 321 birds sampled across four winters indicated that increased wing length conferred a survival advantage.
  
  
  CONCLUSIONS
  We suggest that the potential impacts of blood parasite infections on over-wintering birds may have been underestimated. Further research should consider the potential impacts of sub-clinical parasite infections on the dynamics of vulnerable populations, and we suggest using declining populations as model systems within which to investigate these relationships as well as examining interactions between sub-clinical disease and other environmental stressors.
  
  
  BACKGROUND
  Q5.
authors.affiliation: School of Biology, Irene Manton Building, University of Leeds, Leeds LS2 9JT, UK. Jenny.Dunn@rspb.org.uk.
authors.forename: Jenny C
authors.initials: JC
authors.lastname: Dunn
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Goodman
authors.forename: Tim G
authors.initials: TG
authors.lastname: Benton
authors.forename: Keith C
authors.initials: KC
authors.lastname: Hamer
mesh_terms: D000818:Animals; D016787:Haemosporida / Q000302:isolation & purification*; D007958:Leukocyte Count; D016014:Linear Models; D008954:Models, Biological; D008969:Molecular Sequence Data; D046109:Passeriformes / Q000276:immunology / Q000469:parasitology*; D011157:Population Dynamics; D011529:Protozoan Infections, Animal / Q000097:blood* / Q000276:immunology*; D012621:Seasons