Feather holes
Encyclopedia
Feather holes often characteristically occur on wing and tail feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...

s of some small-bodied species of passerines. In the case of Barn Swallow
Barn Swallow
The Barn Swallow is the most widespread species of swallow in the world. It is a distinctive passerine bird with blue upperparts, a long, deeply forked tail and curved, pointed wings. It is found in Europe, Asia, Africa and the Americas...

s, it was suggested that the holes were feeding traces of avian lice, either Machaerilaemus malleus and/or Myrsidea rustica (both Phthiraptera: Amblycera
Amblycera
The Amblycera are a large suborder of chewing lice, parasitic on both birds and mammals. The Amblycera are considered the most primitive suborder of lice.-Description:...

).

Hole counts were shown to be highly repeatable, and thus counts appeared to be useful measures to quantify the intensity of infestation
Quantitative parasitology
-Counting parasites:Quantifying parasites in a sample of hosts or comparing measures of infection across two or more samples can be challenging.The parasitic infection of a sample of hosts inherently exhibits a complex pattern that cannot be adequately quantified by a single statistical measure...

. Since then, a number of influential papers have been published on the evolutionary, ecological, and behavioral aspects of host-parasite interactions based on the assumption that holes were chewed by Machaerilaemus malleus. More specifically, host sexual selection, feather breakage, flight performance, immunity levels, arrival dates, and even song characteristics were shown to covary with the number of holes. Cross-fostering experiments showed that infestation levels were heritable.

Recently, however, it was shown that Machaerilaemus malleus is apparently absent from Europe, where all these studies were carried out. Correlational evidence supports the hypothesis that feather holes are feeding traces of lice, however, the occurrence of Brueelia
Brueelia
Brueelia is a genus of lice in the family Philopteridae, containing the following species:* Brueelia amandavae Rekasi & Saxena, 2005* Brueelia astrildae Tendeiro & Mendes, 1994* Bureelia cantans Sychra, 2010...

spp. lice (Phthiraptera: Ischnocera
Ischnocera
The Ischnocera is a large suborder of lice mostly parasitic on birds but including a large family parasitic on mammals. The genus Trichophilopterus is also found on mammals but probably belongs to the "avian Ischnocera" and represents a host switch from birds to mammals...

) provides the best fit to the distribution and abundance of feather holes both in barn swallows and across several small passerines.
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