Do Parasites Lower the Nesting
Success of Purple Martins?

Reprinted from: Purple Martin Update 5(1): 28-29

James R. Hill, III
Purple Martin Conservation Association

 Untreated House
Treated House
# of nesting attempts
% of nesting attempts
Total # of eggs lais
Total # of eggs hatched
Total # of young fledged*
Average # of young hatched/nest
Average # of young fledged/nest*
% of eggs laid that hatched
% of young hatched that fledged*
% of eggs laid that fledged young*
*Statistically-significant differences

This chart shows the results of a 4-year experiment to determine the effects of nest parasites on the reproductive success of Purple Martins. The study was carried out during the summers of 1985, 1986, 1987, and 1988, at PMCA headquarters in Edinboro, Pennsylvania. Our procedure was to place a 1/2" by 1/2" chunk of "No-Pest" strip in each of 24 compartments of a Trio™ Castle aluminum martin house, just prior to martin arrival (see Fig. #1). A small hole was drilled in each chunk and they were hung by a paper clip from the ventilation hole in each compartment. An identical, untreated house stood just 15 feet away as a scientific control (see Fig. #2). Both houses had been active since 1981 and had the nests removed between seasons. Each year we alternated which house received the "No-Pest" chunks and which was left untreated as the control.

Fig. 1. The pesticide, generically known as a "No-Pest" strip, that was used in this study to control the parasites of Purple Martins. It has been shown to be toxic to birds and we no longer recommend its use in birdhouses.

Fig. 2. The two Trio Castles that were part of our 4-year investigation into what effect nest parasites have on the reproductive success of Purple Martins.


As expected, the pesticide vapors emitted from the chunks eliminated all blowfly, nest mite, and flea populations from each nest. The vapors also appeared to eliminate the louse living on the martins themselves, and likely kept mosquitoes and blackflies from entering the compartments for a blood meal. Since the only variable between the control and test houses was the presence or absence of parasites, the observed reproductive differences can be attributed entirely to parasitism.

As can be seen from the chart, the presence or absence of "No-Pest" chunks had no effect on the acceptance of a compartment by martins - 51% of the nesting attempts were in the control houses versus 49% in the test houses - these are statistically indistinguishable. Nor did the presence or absence of the chunks (or parasites) affect egg hatchability - 62% of the eggs hatched in both the test houses and the control houses. Apparently, the vapors from the chunks did not affect the incubation attentiveness of the females, nor did the vapors kill the developing embryos. In contrast, profound differences were observed after the young hatched. This is when nestlings first become exposed to any parasites living in the nests. Only 44% of martin nestlings in the control (i.e., the parasite-infested) nests survived to fledging age, compared to an amazing 84% in the nests with "No-Pest" chunks (i.e., in the parasite-free nests.) This difference is statistically very significant. Nearly twice as many martins fledged per nest from the parasite-free houses (1.98/nest) as from the parasite-infested houses (1.08/nest).

One could argue that the differences might actually be greater than what we measured. The young raised in the absence of parasites seemed fat and healthy, and probably had a higher-than-average first-year survival rate, whereas the young subjected to parasites seemed thin and often sickly. Their exposure to disease and internal parasites was certainly higher, no doubt lowering their post-fledging survival.

During two of the four summers these tests were conducted, prolonged foul weather made food gathering difficult for parent martins and greatly lowered their reproductive success. This is why only 1-2 young fledged per nest instead of the 4-5 that is more typical. In any event, I believe the poor weather conditions exacerbated the deleterious effects the parasites had on the nestlings exposed to them in the control nests. If this 4-year sample of 96 nests is indicative, then fleas, blowflies and nest mites, when present in combination, kill about half of all martin nestlings, at least in northwestern Pennsylvania during foul weather! It's uncertain whether the observed reproductive disparity would have been as extreme had the weather been better.

Despite the effectiveness of "No-Pest" strips at controlling martin parasites, we now strongly recommend against their use. The active ingredient in this product (2, 2-Dichlorovinyl dimethyl phosphate) has been shown to be toxic to birds. And although the vapors may not be killing the martins outright, they likely are having sublethal effects that could shorten their lives. Besides, it is illegal to use any pesticide in a manner inconsistent with its labeling. Currently, there are no pesticides licensed for use in Purple Martin nests.

So, what's a landlord to do? Depending on their personal philosophy, a landlord can either do nothing and just let nature take its course (after all, martins coevolved with their parasites, insuring that their clutch sizes are perfectly adapted to compensate for the losses wrought by their predators, parasites, and native nest-site competitors). Or, if a landlord wants to help increase martin numbers, they can instigate a pesticide-free parasite-control program. The safest and most effective control method is total nest replacement.

What is nest replacement? It's when a landlord removes a parasite-infested nest from a martin house or gourd and replaces it with a bed of clean, dry material. Pine straw (brown pine needles) can be used, as can dried lawn clippings, cedar shavings (gerbil bedding), soft straw, or soft wood shavings. Saw dust should be avoided as it does not dry rapidly when wet. Using this method, martin nests should be replaced when the young reach about 8-12 days of age, then again when about 18-22 days of age. The second replacement eliminates all the blowflies, fleas, and nest mites that hatch from eggs laid after the first replacement. Don't do changes earlier or later than the 8-22 day range unless it's to replace a wet nest. Nestlings may need the thermoregulatory integrity of their natural nest bowls when tiny, and after 22 days of age they can be more difficult to keep in the nest after handling. And never do nest replacements during rainy or cold weather, as this will further stress the birds. And since parasites have no effect on egg hatchability, there is no benefit from doing a nest replacement during the incubation stage - more importantly, altering the nest at this early stage may cause abandonment.

Obviously, only folks with easily-accessible housing (those on telescoping poles, or winch-operated systems) who do weekly nest checks can effectively use the nest-replacement method. Landlords with inaccessible housing, or those unwilling to do weekly nest checks, should consider placing an inch or two of cedar shavings in each martin compartment just prior to martin arrival. Tests have shown that martins prefer these preexisting nests over empty compartments (see Update 4(4)), and cedar shavings have well-known parasite-repelling tendencies.

Recently, some concern has been voiced that aromatic cedar shavings may be harmful to birds. According to Dr. Richard H. Evans, DVM, MS, associate editor of the journal Wildlife Rehabilitation Today, there is documentation of problems associated with cedar shavings in cage birds and poultry, especially in poorly ventilated areas. The volatile compounds in cedar may cause irritation to cells lining the respiratory tract, allowing secondary respiratory infections to occur. To be safe, landlords who plan to do regular nest replacements may wish to use one preseason layer of cedar shavings, and then switch to another material, like soft straw or pine straw, for their nest replacements after the young have hatched.

While we are continuing to look into these concerns, we have not concluded that cedar vapors are necessarily harmful to Purple Martins. First of all, one cannot extrapolate test results from one species of bird to all species of bird. Second, many woodpeckers excavate nesting chambers in cedar trees, and martins still use such cavities in western North America. In addition, several cavity-nesting birds deliberately bring cedar chips and aromatic green plants into their nests for the purpose of parasite control. In fact, the green leaves martins bring into their nests give off cyanide gases.

Some evidence that cedar shavings are not harmful to martins comes from the tests of Andrew Troyer, 1993 PMCA Landlord of the Year. While practicing nest replacement with cedar shavings, Andy recorded some of the highest reproductive success in martins we've ever seen (126 fledged from 132 hatched (95.5%)). He also had twice the published norm of banded nestlings returning to their natal colony site to breed as subadults (10.7%). Finally, Andy gave his returning martins an equal choice between empty compartments, and compartments with a 2" bed of aromatic cedar shavings. Eighty percent of his pairs chose the pre-made cedar nests (see Fig. #3). We will continue to monitor Andy's results with cedar shavings and are testing it here ourselves for the second consecutive season. We will keep you informed.

Fig. 3. To eliminate martin parasites, remove the martin's nest after their young reach 8-12 days of age, and replace it with a bowl of wood shavings or other material.


But remember, it's the act of nest replacement/parasite removal itself that provides the benefit to martins. To safely provide their martins with pest-free quarters in which to raise their young, landlords can use any of the materials listed previously. Whatever you do, do not to dump pesticides on the nest. Most of the pesticides that have been used by martin landlords in the past (e. g., Sevin, sulphur, rotenone, etc.) have unknown, long-term effects and may actually do more harm to martins than the parasites do. We no longer recommend the use of any chemical pesticide.


Copyright 1994 by Purple Martin Conservation Association. All Rights Reserved.

Our members benefit from 4 issues annually, packed full of helpful and fascinating information like the article above. You can become a member and support the work of the PMCA by making a tax-deductable donation.