How Do Nest Parasites Affect Purple Martins?
January 22, 2021By Dr. Heather Williams, Department of Biological Sciences, SUNY at Buffalo
The eastern subspecies of Purple Martin is one of a very select list of wild birds that is totally dependent on humans for nesting—the species exclusively breeds in human-made structures, mostly nest boxes. That dependence gives martin ‘landlords’ an extra responsibility to make sure they are doing everything they can to optimize breeding conditions for ‘their’ birds. One feature that both scientists and landlords have noted about Purple Martin nests is that they often contain a lot of parasites. Lift up the surface material of many Purple Martin nests and you will realize that it is not only inhabited by nestling birds, but also houses large populations of blood-feeding mites, fleas and fly larvae. These parasites spend most of their time in the nest material, but surface to get a meal from the unsuspecting nestlings.
This realization has led many Purple Martin landlords to try to reduce the number of nest parasites to help the nestlings. One common method of doing this is through nest replacements. In a nest replacement, nesting material with its associated parasites, is removed at regular intervals by landlords during the nesting period and replaced with parasite-free material such as dried pine needles. Although this practice makes intuitive sense, the practice had never been scientifically tested to be sure that it is really effective in reducing parasite load, and that this made a measurable improvement to nestling success.
Measuring Nestling Health
A Thriving Colony
Purple Martins depend heavily on human-provided nesting sites. This example shows a common setup attractive to the colony-nesting birds.
To test this practice, my students at the State University of New York at Buffalo and I worked with the Purple Martin Conservation Association. Our goals were to measure the reduction in parasite load associated with nest replacements, and to see if there were any improvements in nestling health due to the reduction. We also wanted to know if we could detect any negative impact of nest replacements, due, for example, to handling stress in the nestlings.
In our study, we subjected half of the nests in the martin colonies at our field site (Iroquois National Wildlife Refuge) to weekly nest replacements, and left the other half unchanged. We then collected samples of the parasites in nesting material from all nests and made measurements of nestling success. We counted how many nestlings fledged from each nest, recorded their body mass and took blood samples to determine their hematocrit level (which can tell us whether or not nestlings are anemic), the level of their plasma metabolites (which can tell us how well fed nestlings are), and their ratio of their two main kinds of white blood cells called heterophils and lymphocytes (a high ratio indicates a bird under immune or physiological stress).
Handle With Care
Handling nestlings unnecessarily can increase their stress while offering no clear benefits to birds. Nestlings should only be handled for humane or scientific reasons (e.g., untangling a foot from nesting material, leg banding under a permit).
No Clear Benefit
We found that Purple Martins certainly do have a lot of parasites to deal with in their nests. A nest might house as many as 1,000 fleas, 6,000 mites and 100 blowfly larvae, all of which feed exclusively on blood. Nest replacements were effective at reducing the populations of fleas and blowfly larvae in the nests, but mite populations rebounded extremely quickly after nest changes, returning to their original population size in less than one week. We didn’t find any evidence for decreased fledging rate, body mass or plasma metabolite level in the nestlings with the higher parasite load, but we did see these nestlings had a slightly lower hematocrit level, implying they might be struggling to replace the red blood cells taken by the parasites. Against our expectations, we also saw that the nestlings with the higher parasite load actually had a lower ratio of heterophils to lymphocytes than their neighbors which had nest replacements. This might indicate that nest changes are actually increasing nestling stress due to additional handling, negating any benefit associated with the reduction in parasite load.
“Our results show that, at least for routine infestations of parasites, nest replacements are not clearly conferring a benefit to Purple Martin nestlings.”
The level of contact that people should have with wild birds in the name of conservation can be a contentious issue. Without careful research, it can be tough to decide what is helping wildlife and what might be inadvertently causing them stress. Our results show, that at least for routine infestations of parasites, nest replacements are not clearly conferring a benefit to Purple Martin nestlings.
Reference:
- Williams, H. M., K. Dittmar, and S. Smith Pagano. 2020. A parasite reduction conservation intervention does not improve fledging success or most condition metrics for Purple Martins. The Condor: Ornithological Applications 122(4): 1–14. https://doi.org/10.1093/condor/duaa051
21 comments on “How Do Nest Parasites Affect Purple Martins?”
It appears that you have done an in-depth study on nest replacements. Why not continue this study with Sevin being used as a parasite control agent.
At Lagoon Point on Whidbey Island us Western Purple Martin (PUMA) landlords use food-save diatomaceous earth to control the mites. Also when we do nest changes (which are necessary if the material gets wet among other reasons) we add a few green leaves of the PUMA’s choice on top. In this residential community there are a lot of House Sparrows, and during nest change you will often find the body of a PUMA killed by a House Sparrow pecking its eyes out. The body is buried by subsequent PUMA nesters, so it should be removed for the health of the second nest.
Hello Mary! I live on Whidbey Island and would like to become a Purple Martin landlord on my Greenbank Farm plot. How do I contact you for tips?
Diatomaceous Earth has been proven to be hazardous to birds lungs. That is why the PMCA recommends not using it.
Proven by whom? Name and state your resources…because I have used it for years in all sorts of nest situations and have not seen what you are referring to…
Think about it. Gritty dust particles inhaled regularly during the extreme growth period as a nestling. HY bird heads south for migration and because it’s lungs have been damaged, it can’t keep up and falls into the ocean. Or, during stressful time when chased by a hawk, it loses energy due to poorly oxygenated blood because it’s lungs are damaged and falls prey to the hawk. I got no proof, I just know that dust cloud created everytime they exercise their wings inside the cavity is causing trouble.
What about adding some form of insecticide to the nesting material to deter the mites, etc.? If that is done while there are eggs in the nest, before nestlings hatch, would that be a wise thing to do?
Hi Wendy, We do not recommend the use of any insecticides, pesticides, nor even Diatomaceous earth in or near a nest, no matter the stage.
Your nest replacement study is commendable. Can you cite a similar study to back your rejection of insecticides like Sevin for control of PM nest parasites?
Hi Robert, Sevin is a pesticide; though many pesticides are targeted at insects, they often harm other organisms. While we’re not aware of any studies showing sevin dust specifically is harmful to birds when applied to nest materials, it is considered mildly toxic to birds and this factsheet from the National Pesticide Information Center shows carbaryl (Sevin dust) can reduce the number of eggs laid. When it’s applied to treat insects in the areas surrounding a nest, ingestion of treated insects can kill nestlings. Additionally, this study suggests that use of carbaryl on smaller-bodied birds needs more research. Without evidence that pesticides benefit or at least have no effect on birds, we cannot recommend adding them to nesting material.
The study featured in the blog above is robust and took place over 3 years, during which time the parasite level in untreated nests never reached a tipping point of causing mortality of the eggs/nestlings, which suggests that treatment for mites is unnecessary. One of the things about the study that was really interesting, which didn’t make it into the blog post, is that the authors speculate about the function of the leafy green materials that martins bring into the nest. Several species do this, including European Starlings (also a colony nester). In the starlings, it has been demonstrated that the green leafy material has a medicinal effect that may promote nestling health. The authors of this study speculated that perhaps when landlords change out the nest material, they may be getting rid of some beneficial component that helps nestlings. Lastly, the Purple Martin Conservation Association also recommends that you avoid using any pesticides in or around martin nests.
What about using herbs to repel parasites?
https://www.prevention.com/life/g32982866/plants-that-repel-bugs/
Hi William, In the paper, the authors discussed the function of the leafy green materials that martins bring into the nest. Several species do this, including European Starlings. In the starlings, it has been demonstrated that the green leafy material has a medicinal effect that may promote nestling health. The authors speculated that perhaps when landlords change out the nest material, they may be getting rid of some beneficial component that helps nestlings.
Great information. QUESTIONS: 1. During the study, when changing the nest with a mite infestation, was the nest box wiped down well with damp cloths until all traces of mites are gone? If so, if there is a difference in the population rebound to the original size between wiping it down and not. 2. Do you have a suggestion for nests that have clear mite infestations? 3. Similarly, Do you have suggestions for what to do when prefledglings are found on the ground with mite infestations?
Hi Sue, I’m not sure whether the nest cavities were wiped down, but this study took place over 3 years, during which time the parasite level in untreated nests never reached a tipping point of causing mortality of the eggs/nestlings, suggesting that treatment for mites is unnecessary. In the paper, the authors discussed the function of the leafy green materials that martins bring into the nest. Several species do this, including European Starlings. In the starlings, it has been demonstrated that the green leafy material has a medicinal effect that may promote nestling health. The authors speculated that perhaps when landlords change out the nest material, they may be getting rid of some beneficial component that helps nestlings.
I find 2 possible flaws with this study. One of them is that the nest change interval was done at 7 days instead of 10 days (10 is recommended by the PMCA), which probably increased the stress level of the nestlings. The second flaw was that this study was only done in one state and in one area. Parasite loads can be increased in areas with hot, humid summer weather. Martins do not face the same parasite loads in all areas of the country – some areas are plagued by blackflies (not controlled by nest changes, but can still kill nestlings from blood loss), and others only by nest mites (and not blowflies). I suggest a more widespread study, and also to incorporate the use of sevin dust under the nest to lower the stress levels of the martins from frequent nest changes, as well as lower the parasite load in general. Also consider nestling losses from those that jump out of the nest due to extreme uncontrolled mite infestations. It can and does happen, and if your study size was increased, this factor might have been discovered. Large colonies where nests are close together, especially in houses (versus gourds) are prone to large mite blooms.
Hello, The study featured in the blog above is robust and took place over 3 years, during which time the parasite level in untreated nests never reached a tipping point of causing mortality of the eggs/nestlings. One of the things about the study that was really interesting, which didn’t make it into the blog post, is that the authors speculate about the function of the leafy green materials that martins bring into the nest. Several species do this, including European Starlings (also a colony nester). In the starlings, it has been demonstrated that the green leafy material has a medicinal effect that may promote nestling health. The authors of this study speculated that perhaps when landlords change out the nest material, they may be getting rid of some beneficial component that helps nestlings. I agree that it would be interesting to replicate this study in other parts of the martin’s range, and for more studies to be done on all of these interconnecting variables.
What about tobacco leaves
I heard years ago that they worked against parasites
Hi John, Yes some birds have been shown to weave the fibers of cigarette butts into their nests with success at reducing the parasite load, but it’s unknown whether there is a net positive or negative effect as birds in nests with these fibers turned out to have more “genotoxicity” in their red blood cells (in those cells, the DNA was more damaged). You can read a fascinating article about it here, skip to section 3d for relevant information. The last paragraph in that section talks specifically about tobacco, but other plant species containing aromatic compounds have been used by several birds in their nests. This older blog post of ours about use of aromatic herbs may also be of interest.
As a Purple Martin Landlord this information is going to create food for thought and no doubt some controversy. Is there a more detailed paper written on this study available?
Hello, The original scientific paper is linked at the bottom of the article. Click on the DOI link to open it up in a webpage.
I’ve been an active landlord for 27 years and perform nest change-outs when needed (I’m totally against the use of pesticides) but isn’t it funny how purple martins have survived ectoparasites for thousands of years before humans got involved in their lives???
I feel that human actions have influenced the birds. Your research is so useful, and I think everyone needs to know in order to protect birds from noise and light pollution. Thanks for sharing
How can I stop parasites in my purple marten houses ?
2 years ago I had a bad infestation of blow flies and mites. I removed the babies and wiped the interior down with rubbing alcohol and let it air dry until the smell had dissipated, it seemed to work. This year someone told me to use food grade diatomaceous earth and know I’m seeing it’s not recommended. So, what is the best thing to do when you have a really bad infestation? I just can’t stand to do nothing to help them. Thank you.
Hi Cheri, The best way to prevent mite infestation is to clean the nest box out at least once per year. If you notice mites in the box while the birds are nesting, do not interfere – in most cases the birds will survive just fine. However, if it gets to a point where you’re particularly worried, the first thing to do is to call a wildlife rehabilitator who is specially trained to help in crisis situations and can offer advice on how to proceed.
I have S&K BO-11P gourds which have a unique feature that I’ve found super helpful for keeping parasite loads minimal. The gourd has an inner gourd and outer gourd- 2-layers with an air-gap, mainly for insulation to control heat, which works great. Before the season I spray the inner surfaces between the layers with delta dust dissolved in water and assemble the gourd. This coats the surfaces that DO NOT come into contact with birds, ever. Once dry it’s immobile and very long lasting as it’s not subject to the weather. Mites crawl all over but once they venture into the later between the inner and outer gourd, they’re toast! Works great and I don’t have to mess around with nesting material and worrying about parasites or harming the birds inadvertently.
When cleaning out my Martin houses after the martins left, I found I am getting (bird mite) rash all over my body!!! How do I remove the nesting material and wash out the houses without getting the bird mites all over me?!? I have suffered for two weeks with what the doctor calls “bird mite rash”
Hello Keith, It may help if you wait to clean the boxes out, at least a few weeks after the birds leave at minimum. The mites should disperse once their hosts are no longer available, so it may be more beneficial to come back in late fall, or winter, and clean the houses out. Gloves and long sleeves would likely help protect your skin as well, and especially so when you’re conducting your nest checks. Always be sure to wash thoroughly when you come inside after visiting bird nests.