Until now, the mites that inhabit the feathers of birds used to be observed by scientists during the day. A group of researchers has analyzed these organisms at different times, especially from dusk to dawn. This study has been published in Ecology. The results that they harbor is that their habits are mainly nocturnal, since there is an increase in their abundance, they move, feed and reproduce during these hours. Another of the conclusions they have drawn is that, possibly, mites are not parasites, as was thought, but can even be beneficial for birds because they feed on fungi and bacteria, among other things, that damage and degrade feathers. Therefore, they would be mutualists. “They are like feather sweepers,” explains Roger Jovani, scientist at the CSIC’s Doñana Biological Station and co-author of the study. The results are based on a large sample of feather mites, but on a very small number of birds, so the scientists emphasize the importance of checking that difference between day and night in the mite count and their habits in different species .

The researchers captured two warblers at 5:00 p.m. In order to quantify the number of feather mites and study their ecology, they expanded the two flight wings and the tail with a light source behind and photographed them. They repeated this process at 8:00 p.m., already at night, after having left the birds in the open air and then every three or four hours until 9:00 a.m., when it was already daylight. The total number of mites increased on the wings and on the tail until midnight and then began to descend towards dawn. In addition, depending on the species of the mites and their stages of development, they had different dynamics of abundance and were distributed in different feathers and areas of the feathers.

Due to the abundance of mites and their progressive increase until reaching a peak at midnight, the scientific team – whose members belong to the Doñana Biological Station, the University of Granada and the University of Illinois – decided to take a large sample of mites of two blackbirds and another blackcap to investigate the contents of the intestine through microscopes and thus be able to check if they were feeding at night. Of these members, who were collected both at sunrise and sunset for comparison, they selected 120 females to photograph and study the amount of food they had. By dusk most mites had not eaten recently, while by dawn most had the ventricle (the first intestinal compartment that food reaches when ingested) filled with food. This pattern was followed in the mites of the three birds observed.

When they were assembling the article they realized that what they saw in an image they had taken of the mites was an egg, “an egg that is almost as big as half a female,” recalls Jovani. From that moment they reviewed the 120 photographs and discovered that 21 females had an egg inside. The highlight is that 20 of the 60 females that were collected at dusk carried an egg, while only one of 60 of those that were counted at dawn carried an egg. “The most plausible thing is to think that they put them on overnight. Everything is stable: at night it is safer to move and they lay eggs in places where they cannot be during the day ”, the scientist details.

From the data, the team provides an estimate of the amount of organic material that feather mites could clean from the surface of these feathers. Each night they could remove an average of 0.17 square millimeters of fungi, bacteria and other organic particles from a bird’s wing. If the estimate is made annually, it would reach 80,000 square meters only in European passerines. “This is important because they are microscopic individuals who eat even more microscopic things, but cumulatively – because it is how they act as there are many – they end up having macroscopic consequences that can have consequences in the life of birds,” explains Jovani. Heather Proctor, a mite specialist who works at the University of Alberta (Canada), and who, although she has not participated in the research has helped in the identification of mite species, argues that the study strongly supports the idea that Feather mites benefit their hosts by cleaning feathers of “accumulated debris” and therefore treatments to remove these mites from birds in captivity (such as when they are in zoos or rehabilitation centers) may “not always be the right thing to do.” better for the birds ”.

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