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
Excellent article… Thanks for doing the research and posting the information.
Why not clean a nest box after fledgling leave? Use Clorox solution?
Hi John, You should never use harsh chemicals when cleaning out a nest box, as even small amounts can be harmful to future nestlings. It’s best to wash out with a mild detergent and warm water. That being said, mites do live on the body of the birds too, and there is always a chance for them to return even if the box is cleaned out each year.
I have a small trail of Western Blue Bird boxes, however there are also Tree Swallows and Wrens that use them. They are cleaned in September thru November after all the residents have flocked and headed south. After removing the old nest material I have experimented with adding a few sprigs of juniper in the bottom of the empty box. All three of the species mentioned above have built their new nests on top of the dried juniper come the next spring. Can’t recall running into too many boxes with mites.
I agree with everything except that I have used 5% Sevin dust in the nesting material of our Purple Martin Colony for several years with no adverse effects.We have a nest-box camera and can see the mites on the baby birds before we dust the nests, but none afterwards I check the nests from time to time and the absence of mites is thrilling.
Hi Van, While the Sevin dust you use may appear to have no short-term effects, long term effects are still possible. However, because this is not well studied, we recommend against using any chemicals in a wild birds nest.
This is an excellent article and I learned a lot. Is it ok to share with fellow bluebirders and is it possible to reprint in our Ohio Bluebird Society newsletter? Keep up the great work. Many of us are more hands on than others and we see a lot more. thanks! Lets ask for photos of what is left in nests once the birds fledge. That would be interesting too.
Hi Darlene, Great idea about collecting photos of nests after fledging. You’re certainly welcome to share the link to this article with anyone you wish, but please send us an email (email@example.com) about reprinting in the OBS newsletter. Thank you!
Thanks so much, Holly! This is super informative. I like the notion that the presence of mites may help “tune up” the nestlings’ immune systems. I would love to know if the presence of ants in the boxes negatively impacts mites, too.
I’ve found that propping the empty nest box open after the nest attempt with a stick to allow fresh air and sunshine in (and prevent another bird from nesting) for a few days helps eliminate them (at least to my naked eye). Of course, I had a bluebird that took the stick out and started building a nest anyway (they do love these nest boxes!)…
Thanks Chrisula. I actually looked into your theory about ants, but I couldn’t find any literature on it. It was quite hard to even find out the diets of some common North American ant species! However, since more research is needed on mites and their effects, I’m betting it’s simply not been formally studied yet.
Thank you for researching the ant-mite relationship! It seems there isn’t a lot of information that can be easily found about mites in nests and nest boxes. This is definitely an area with lots to study.
I appreciate this blog – it’s super informative! Thank you for taking the time to research and write it!
I would love to share this article and the comments as a basis for a talk for my local bird club. We have our planning meeting next week and I would like to suggest this topic. thanks for the work and research!
But what if you find all your nests have a ton of little red mites causing babies to jump out?
You can replace the nesting material with lawn clippings,(never use chemicals!) then place the nestling’s back in either the correct cavity, or if you have multiple birds, in near-aged nestling’s. if the parents have abandoned the nest call a wildlife rehab.
I have to put a layer of diatomaceous earth (DE) on the floor of the nest box to thwart ants and ear wigs. You’re saying don’t put DE on top of the nest right?
Our recommendation is to never add diatomaceous earth into nest boxes at all, even if the nests are not currently active, as even small residues could harm the delicate nestlings. Check out our Dealing with Predators page for an alternative method to deterring ants from nest boxes.
Does anyone know if Violet Green swallows nest in cedar tree cavities? If so wouldn’t it be OK to build nests out of cedar or putting some cedar shavings in the bottom of nest boxes to help with parasites? There is a particularly aromatic cedar called Port Orford cedar that could be helpful if it does no harm to the birds.
Hi! I am so grateful to Cornell for the wonderful plant and bird information you provide! 2018 was a disastrous year for baby barn swallows on my property in MA. It took more losses in 2019 before I realized the cause of death was a severe mite problem. After much research…but not seeing this article…I did use DE in and around a nest on my porch. I was able to monitor it daily and was very judicious. Mites were present but did not overtake the young and the entire nest fledged. A pair has returned to this nest and I expect babies are on the horizon. Please tell me what I should be doing for them if I see mites making a surge. (I think I have photos of some of the dead babies from 2019 if you are interested.) THANK YOU so much!
Hi Tracy, The best way to guard against mites is to clean nest boxes out between broods (be sure the nest is indeed “inactive” before doing so), however many mites arrive to nests on the birds themselves, so there is no 100% effective way to eliminate them. But, to reiterate our final paragraph in the above article, birds have evolved with the mites and have developed their own defenses, and 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 later in life. If mites become an extreme problem, then we recommend calling a local wildlife rehabilitator before taking any action.
Can these mites be seen with the naked eye? I thought I read that bird mites can also be passed onto humans? One bird blog I have been reading and posting to recommends putting DE under the nests. Some of the bloggers do that and recommend it. I always thought since DE is used as a form of insecticide, that it would kill birds. How can we protect Bluebirds and ourselves against mites?
Hi Cathryn, Mites range in sizes – there are thousands of species. Some are quite visible, others are harder to see. As we state in the article above, many mites are bird-specific, and there’s not much danger of them passing to humans, though “mite” is a diverse term and some, like ticks, can affect other species. The best way to protect yourself is to wash your hands after checking nests. For birds, it’s best to let the nests be – again, as we state in the article, birds have developed their own adaptations against mites, and we don’t know how eliminating such parasites may affect the immune strength of nestlings, who may need to rely on the immune responses they developed while in the nest box, later on in life. Diatomaceous earth is also not recommended as it can be potentially harmful to nestlings – especially when aerosolized (such as when it’s stirred up in the air when nestlings flap their wings later in the nestling stage) in confined spaces (such as a nest box) and inhaled.
I just discovered tiny red-brown mites today that originated from a robin nest shelf that I had placed on a post under the eaves of my house. The second brood of robins fledged today, so I planned to clean out the nest box and clean up my lawn furniture underneath. The furniture looked dirty, and then I noticed the dirt was moving!
I removed the nest, wrapped it up and disposed of it. I sprayed down the shelf, post, front of the house and the outdoor furniture with Begly’s spray cleaner. I left it sit for a while until there was no more movement, then hosed everything down.
We enjoyed watching 2 broods of robins – a total of 7 fledged – but the nest shelf will have to be moved somewhere else.
Thank you for the useful info on mites.
I am a retired NC Cooperative Extension Agent, but I still participate in the Extension’s Ask an Expert program. I received a question from a homeowner regarding a cardinal in their yard that is losing its head feathers. They are concerned that it is a mite problem. After reading your articles, I think it may be a molting issue. However, this article deals primarily with nest boxes, and I was wondering if there is anything homeowners can do to safely treat for mites that are on non-nesting birds? Thank you for your help. I refer many people who submit avian-related questions to your site for additional information. Thank you for providing this excellent resource!
I have had phoebe nestlings die two years in a row and it is always due to mites. The mother reuses a nest from prior years – there are a few of them in my barn. I bought a mite and lice spray for use on birds that I had planned to spray the nests with very early in the spring. The babies died in the nest and one even came out of the nest and died on the floor. They were crawling with mites.
Hi Linda, it may be a god idea to remove the old nests before the phoebes return to nest again – this is a legal practice as long as the nest is not active (i.e. does not contain eggs or young). The phoebes will be able to re-build easily and it may help remove mites that overwinter in the nesting materials. We don’t recommend spraying nests with anything that has intent to kill insects or mites as these chemicals may be dangerous for the delicate nestlings.
I am hoping you can help me. I have an active horse barn with several nests of barn swallows with infestations of small red mites that are literally dropping out of the rafters and all over me, the people in my barn and the horses. We are covered. Last year this condition existed in only one nest, and when we finally figured out where the mites were coming from and investigated, we found multiple babies dead and covered in mites. I am at my wits end. What can I do?
Hi Jessica, The best thing I can suggest is to, at the end of the breeding season around September, to take down the swallow nests so that the birds build brand new ones next year. Sometimes mites can overwinter in the nest materials, so you may want to wear gloves and a mask. Clearing the nests will help reduce mite load next spring, but keep in mind that some mites can live on the birds as well. Some mites can be helpful (read more here) but in any case, I’m afraid I don’t have relevant advice to solve the problem immediately, other than to avoid using pesticides anywhere near the nests. It may be a good idea to reach out to a local (state/province) wildlife agency for tips on techniques for this that do not violate federal laws, such as the Migratory Bird Treaty Act.
I started watching the birds in my yard on the security cameras last year and this year I decided to add some nest cams. I built three A frame style nest boxes and installed IP cams in the top of them. The birds fight over the nest boxes. And they started producing chicks right away. But I noticed some birds would abandon the nest after the fledglings left.
Closer inspection revealed mite infestations on two of my nest boxes. So after the the chicks fledged I removed and burned the nests. Then vacuumed out the nest boxes. Then placed a votive candle in the nest box and baked the little bugs that were left. I read somewhere they can’t handle heat above 125°F. So a butter warmer makes it nice and toasty in there without setting the place on fire.
I sited a new camera box last year so perfectly clean and mite free . During the season it was occupied by a brood of blue-tits . It was noticeable that the adult birds spent a lot of time inverted in the nest as though fussing about re-organising the bedding material . The nine eggs produced two fledglings which eventually flew the nest . On cleaning out the old nesting material there was an obvious red mite infestation which were minute . The only way I could clean out the box joints was with a gas blow lamp .