 The average number of lice per host tends to be higher in large @-@ bodied bird species than in small ones . Lice have an aggregated distribution across bird individuals , i.e. most lice live on a few birds , while most birds are relatively free of lice . This pattern is more pronounced in territorial than in colonial — more social — bird species . Host organisms that dive under water to feed on aquatic prey harbor fewer taxa of lice . Bird taxa that are capable of exerting stronger antiparasitic defense — such as stronger T cell immune response or larger uropygial glands — harbor more taxa of Amblyceran lice than others . Reductions in the size of host populations may cause a long @-@ lasting reduction of louse taxonomic richness , for example , birds introduced into New Zealand host fewer species of lice there than in Europe . Louse sex ratios are more balanced in more social hosts and more female @-@ biased in less social hosts , presumably due to the stronger isolation among louse subpopulations ( living on separate birds ) in the latter case . The extinction of a species results in the extinction of its host @-@ specific lice . Host @-@ switching is a random event that would seem very rarely likely to be successful , but speciation has occurred over evolutionary time @-@ scales so it must be successfully accomplished sometimes . 
