2^4 Kellogg, Some Parasites of Birds. Llulv 



distribution is also presented to us in a less striking, perhaps, 

 but no less real phase among the hosts of our own continent. 

 A Mallophagan species is by no means certainly limited to one 

 American host species. Indeed a majority of North American 

 Mallophagan species have been recorded from two or more 

 American host species. How does this condition come to exist ? 

 We have to do with a wingless parasite but a parasite with 

 good legs. Our parasites can migrate from host to host when 

 this migration can be performed on foot. But as a matter of fact 

 this migration does not take place unless the host bodies are 

 close together or in actual contact. Such actual contact takes 

 place between male and female and between old and young. 

 Thus is explained the perpetuity of the parasites upon a single 

 host species. Among gregarious birds the parasites may mi- 

 grate from individual to individual of the same species, thus 

 breaking up too much close breeding. But several species of 

 birds of gregarious habits may roost or perch together : so the 

 parasite may spread among several Gull species or Duck species 

 or Alcine species, conditions which actually exist as shown by our 

 records of occurrence. And there may be other rare opportuni- 

 ties for migration from host species to distinct host species, as in 

 cases of hybridisation, or where the Mallophaga might be carried 

 by winged parasites of birds, like the Hippoboscida;. After all, 

 however, the majority of instances in which a parasitic species 

 is common to two or more host species or host genera cannot be 

 so simply explained. The instances in which actual contact and 

 hence direct migration is possible are few. 



There are certain data at hand which should be known to any- 

 one attempting a solution of this problem in distribution. Most 

 important is this fact : where a single Mallophagan species is 

 recorded from two or more North American host species, the host 

 species are, in almost all cases, closely related. That is, the hosts 

 may represent two or more species of a single genus, or, as less 

 frequently is the case, of two or more allied genera. • Only in a 

 few cases do we find a parasite common to genera representing 

 different families of birds. Similarly, in those numerous in- 

 stances of commonness of parasite to European and American 

 hosts, the hosts are always or nearly always allied forms. As 



