Fellow Travellers

Recently a friend lent me a book called “Fleas, Flukes and Cuckoos, a study of Bird Parasites”. The authors were Miriam Rothschild and Theresa Clay and it was published in 1957 by Collins as part of their New Naturalist Series.  As you may have guessed from the title it was not a lot of laughs and was quite heavy going being quite soporific at times.  However I stuck at it and the book gave me a glimpse into aspects of the life of birds to which I had previously given little consideration.

Birds have their share of “fellow travellers” both internal parasites like flukes and worms and external parasites like lice, mites and ticks.  While the latter are more easily seen, all weaken the host and can in severe cases kill them.  Lice eat feathers.  Mites and ticks suck blood.  Roundworms live in the gut and can cause weakness and weight loss.  These are but a small selection.  Many parasites are specific to a particular species of bird so you get, for example, House Martin mite and Sand Martin tick.

Birds most likely to be affected are those which have enclosed nests or nest chambers that are used year after year.  Many parasites have life cycles that correspondent to that of their host.  For example Common Swift louseflies produce eggs in the late summer which turn into larvae and then pupate. These survive over the winter and hatch out when the birds return in spring and then the parasites again infest both adults and nestlings.  

The life history of the bird fluke Leucochloridium paradoxum is remarkable. The definitive hosts, in which the parasite multiplies, are various woodland birds, while the hosts in which the parasite grows are various species of snail. The adult parasite in the bird's gut produces eggs and these eventually end up on the ground in the bird's faeces. Some very fortunate eggs get swallowed by a snail and here they hatch into tiny, transparent larva. These larvae grow and take on a sac-like appearance.  This forms a central body in the snail's digestive gland that extends into a brood sac in the snail's head, muscular foot and eye-stalks.  When the snails get eaten by birds, these embryo flukes get into the bird again.

Can bird parasites harm humans?  The answer in some cases is yes but that is another story.