Disease and Parasitism

photograph of a Cliff Swallow gathering mud
for nest-building by Tom Grey
Many organisms obtain energy and nutrients by feeding on birds. If they are animals big enough to kill and devour a bird all at once, the consumers are called predators. If the consumers are small animals that live on or in birds indefinitely, they are called parasites. Parasites may or may not have a sufficient impact on a bird to make it sick. When a bird's tenants are microorganisms -- protozoans, bacteria, fungi, viruses, or rickettsiae (tiny bacteria-like organisms) -- that cause diseases, we call them pathogens.

There is a great variety of avian diseases. Birds get malaria (caused by close relatives of the protozoans that cause human malaria), aspergillosis (a fungal infection), tuberculosis (not the same pathogen that causes human tuberculosis),
Newcastle disease (viral), fowl plague (viral), avian pox (viral), avian influenza (viral), and hundreds of other infections. People cannot catch most avian diseases, but birds can serve as hosts of pathogens that cause serious human illnesses. The rickettsia that causes psittacosis is sometimes contracted by people from pets in the parrot family (Psittacidae), but the disease name is misleading, as the pathogen has been found in at least 140 species of 17 orders of birds. Psittacosis may be fatal in people. So may the equine encephalitis virus, for which some 80 species of North American birds can serve as reservoirs. The virus can be carried from bird to person by mosquitoes. Migrating birds have been implicated in the transport of virulent encephalitis strains between continents.

Birds usually do not suffer heavily from mosquito-transmitted viruses, but there are exceptions. The mosquito species, Culex pipiens, accidentally introduced into the Hawaiian Islands in 1826, was an agent (vector) for transmitting the avian pox virus, malarial protozoans, and other pathogens. Many lowland species of the wonderful Hawaiian honeyeaters proved extremely susceptible to the pathogens and were wiped out; highland forms were spared because the mosquito could not survive at altitudes above 2,000 feet.

Birds also are dined upon by a great variety of parasites. Chewing bird-lice (Mallophaga) invade plumage and live on accumulated "dandruff," on blood, or on other fluids. Fleas, louse flies (Hippoboscidae), small bugs (Hemiptera, relatives of bedbugs), ticks, and mites suck blood. Internally, birds host roundworms, tapeworms, flukes and so on, which live either in the digestive tract or in the blood vessels. Little is known about the impact of parasites on natural populations of birds, but there are indications that it can be considerable, especially in populations of colonially nesting species.

For instance, biologists Charles and Mary Brown of Princeton University found that the number of "swallow bugs" (bedbug-like parasites) per nest went up as the size of Cliff Swallow colonies increased. In addition, the weight of ten-day-old chicks declined as the number of bugs per nestling rose. By fumigating some nests and leaving others infested, they showed that the bugs also increased fledgling mortality in large colonies but not in small ones. In response to the threat of the parasites, the swallows apparently construct new nests (rather than reuse old ones) in large colonies more frequently than in the less heavily infested small colonies. The swallow may also switch sites in alternate years (or return to the same sites even less frequently), leaving its parasites to starve in empty nests in the meantime.
SEE: Coevolution; Coloniality; Nest Materials; Nest Sanitation; European Starlings.