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
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).
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.
- 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