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How Do Nest Parasites Affect Purple Martins?

Photo © Adam Wilson

by 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

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

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

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16 responses to “How Do Nest Parasites Affect Purple Martins?”

  1. Bill Wenger says:

    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.

    • Mary Hollen says:

      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.

      • Jean Gauthier says:

        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?

  2. Wendy Williams says:

    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?

    • Holly Grant, Project Assistant says:

      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.

      • Robert McMillen says:

        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?

        • Holly Grant, Project Assistant says:

          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.

    • Holly Grant, Project Assistant says:

      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.

  3. Sue guarasci says:

    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?

    • Holly Grant, Project Assistant says:

      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.

  4. C. Rousseau says:

    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.

    • Holly Grant, Project Assistant says:

      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.

  5. W.Novitske says:

    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?

    • Holly Grant, Project Assistant says:

      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.

  6. Jace Stansbury says:

    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???

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Cornell Lab of Ornithology

Cornell Lab of Ornithology