By Holly Faulkner, NestWatch Project Assistant
If you monitor nests, you’re probably pretty familiar with mites. Mites are a diverse group of arthropods in the class Arachnida within the same subclass as ticks (Acari). There are thousands of species of mites that are closely associated with birds, and they have been known to occupy all possible habitats in and around the nests and birds themselves. The mites that you find in your nest boxes are usually a type of ectoparasite falling into one of two categories: those that live in the nest, or those that live on the body of the host bird. While mites generally have a poor reputation, let’s explore a little further what mites do and how birds have evolved to fight back.
Out for Blood
The most well-studied avian mites are haematophagous, meaning that their primary food source is blood. Species in this group can live in the nest or on the body of the bird, and require these “blood meals” during some or all parts of their breeding cycle. The nest itself acts as a refuge, providing shelter from the elements and from predators, while also attracting a reliable food source during the breeding season. Some mites that live in nesting materials (like Ornithonyssus bursa) have even evolved so that they synchronize their breeding season to when birds are nesting (Powlesland 1977). During this time, the mites need a blood meal at least once every six weeks, but they can survive without food for the rest of the year while the nests are cold and empty.
Haematophagous mites have several strategies for feeding. Some skin mites burrow into the bird’s skin particularly around the legs, feet and bill, sometimes causing what is known as “scaly leg.” Other species are only parasitic during certain life stages, where the adult may be a peaceful nest-dweller, but the juvenile stage requires skin-burrowing parasitism. Most other species simply attach temporarily to the birds’ skin with their mouthparts to feed, and then return to the nest or feathers.
An in-home cleaning service
There is also a huge diversity of mites that live on or in feathers, dubbed “plumicolous” mites; however, little is known about them other than their diet. Contrary to what you might think, these mites do not eat the feathers (although there are some lice species that do). Rather, they feed on oil, pollen, and fungi that are found on the feathers. These mites may therefore be considered somewhat beneficial to the birds through their free cleaning services; however, studies on other species of feather mites that live within the feather quill show that they eat the pith, which could potentially weaken the feathers over time (Gaud and Atyeo 1996).
Mites may seem ominous, but rest assured, the birds they interact with have developed defenses of their own. The interlocking barbules of feathers act as a physical barrier against haematophagous mites, and the presence of melanin, a feather pigment, can help strengthen the feather structure. Behavioral adaptations have arisen as well; for example, studies on Barn Swallows show that they will avoid reusing nests that contained haematophagous mites (Barclay 1988), and others show that some bird species even use aromatic foliage in their nests, which act as a fumigant. Sunning and dust-bathing may also help birds rid themselves of mites. Immunologically, a bird’s skin becomes inflamed near the bite area which can prevent feeding, and in some cases, bird immune systems develop antibodies that bind with proteins in mite saliva, often preventing mites from feeding or inhibiting nutrient absorption (Wikel, 1996).
Interestingly, there are some species of birds (the genera Pitohui and Ifrita) which have evolved a particularly special defense against haematophagous mites: these birds store alkaloid neurotoxins acquired through their diet in their skin and feathers. These toxins, which are similar to those found in poison dart frogs, are effective against many of the most common arthropods they encounter and likely also make them unpalatable to larger predators. Similarly, there are certain species of auklets (a type of seabird) that produce a citrus-like aroma. The odor acts similarly to the lemony scent of Citronella candles, in that it helps repel ticks and mosquitoes! (Note: Citronella oil is sourced from a lemongrass species, not citrus.)
On the Home Front
It’s great to learn about the diversity of mites and how birds have learned to defend themselves, but what does this all mean in regards to monitoring your nest boxes? While it’s always a good idea to clean out a mite-infested nest box after the attempt has concluded (so birds won’t avoid it), be aware that mites are hard to eliminate entirely and that sometimes it’s best not to intervene. Current studies show both positive (Møller 1990) and negative (Darolová et al. 1997) effects of mites on nest survival. Mites are hard to study and more research is needed to help explore these conflicting results. While we understand it can be distressing to find mites in your nest box, keep in mind that birds have evolved with the mites and have developed their own defenses to help guard against infestations. Never add chemicals like insecticides or diatomaceous earth into nests, even if the nests are not currently active, as even small residues could harm the delicate nestlings. Furthermore, we don’t know how eliminating such parasites can affect the immune strength of nestlings, which may need to rely on the immune responses they developed while in the nest box.
- Barclay, R. M. 1988. Variation in the costs, benefits, and frequency of nest reuse by Barn Swallows (Hirundo rustica). The Auk 105(1): 53-60. https://doi.org/10.1093/auk/105.1.53
- Darolova, A., H. Hoi, and B. Schleicher. 1997. The effect of ectoparasite nest load on the breeding biology of the Penduline Tit Remiz pendulinus. Ibis 139(1): 115-120. https://doi.org/10.1111/j.1474-919X.1997.tb04510.x
- Gaud, J., and W. T. Atyeo. 1996. Feather mites of the world (Acarina, Astigmata): the supraspecific taxa. Part I: Text and Part II: Illustrations of feather mite taxa. Annales du Musée Royale de l’Afrique Centrale, Sciences Zoologiques, Tervuren, Belgique, 277: 3-193.
- López-Rull, I., and C. Macías Garcia. 2015. Control of invertebrate occupants of nests in Nests, eggs, and incubation: New ideas about avian reproduction. Eds. D. C. Deeming and S. J. Reynolds. Oxford University Press, Oxford, 82-96.
- Møller, A. P. 1990. Effects of parasitism by a haematophagous mite on reproduction in the barn swallow. Ecology 71(6): 2345-2357. https://doi.org/10.2307/1938645
- Powlesland, R. G. 1977. Effects of the haematophagous mite Ornithonyssus bursa on nestling starlings in New Zealand. New Zealand Journal of Zooology 4: 85-94.
- Proctor, H., and I. Owens. 2000. Mites and birds: diversity, parasitism and coevolution. Trends in Ecology and Evolution 15(9): 358-364.
Wikel, S. K. 1996. Host immunity to ticks. Annual Review of Entomology 41(1): 1-22. https://doi.org/10.1146/annurev.en.41.010196.000245