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What Should You Do About Bird Blow Flies in the Nest?

Photo © George Petras

By Robyn Bailey, NestWatch Project Leader

If you monitor nest boxes, you have probably noticed that birds and their nests host a number of invertebrates, including (but not limited to) ticks, fleas, and blow fly larvae. These unwelcome blood-feeding parasites are harmful to their hosts, particularly when their populations build up in large numbers (Sabrosky et al. 1989). However, birds have evolved physical, behavioral, and immunological adaptations to parasites, and in healthy birds, these defenses help them withstand the attacks (López-Rull and Garcia 2015). As a result, death from ectoparasites alone is rare and other factors such as malnutrition and inclement weather are more likely to compound the stresses of parasitism.

Bird blow flies in particular (Protocalliphora spp.) have a bad reputation because their larvae are visible in the nest material when cleaning out nest boxes, and sometimes visible on the nestlings themselves. If nestlings die in the nest and there is no other obvious cause, there are usually blow fly larvae available to serve as convenient scapegoats. The potential confounding factors, such as poor nutrition and disease, are usually not visible and not available as evidence. Therefore, blow flies may receive an unfair portion of the blame whenever nestlings die without apparent cause (Sabrosky et al. 1989).

A Novel Idea

Because adult female blow flies lay eggs directly on nestlings or the nest material immediately next to them, it is not possible to prevent them from hatching and taking their first meal. However, with the rationale that blow fly larvae fall to the bottom of the nest when not intermittently feeding and then climb back up to the nestlings when another blood meal was sought, Ira Campbell devised a raised platform made of hardware cloth which was intended to prevent the legless larvae from climbing back up to the nestlings once they had fallen through (Campbell 1982, 1984). Forty years later, Ira’s blow fly trap remains a popular and passive tool for reducing the impacts of some blow fly species on nestlings. 

The Blow Fly Platform Trap

The Blow Fly Platform Trap

A wire mesh platform used as a blow fly trap can be seen integrated into the bottom of this nest box. A Carolina Chickadee has built its nest over the top.

However, when asked about whether the wire platforms were likely to be helpful, North America’s bird blow fly expert Dr. Terry Whitworth was doubtful, explaining that, “Larvae are not inclined to fall from nests until they are ready to pupate.” By that time, they have finished feeding and thus no longer pose any threat. He points out that, “They live happily in open bird nests with no box surrounding them, and if they fall, they die” (Berger 2001, pg. 86). So it’s likely that the traps are facilitating the removal of pupae that will soon become adult flies, which at best would prevent a few adults from reaching maturity; this probably has minimal impact on the individual nest or the blow fly population. 

The Risks

There are some risks associated with using blow fly traps that nest monitors should be aware of, the main one being entanglement. Recently we have seen several NestWatchers submit photos and records of nestlings becoming entangled in the holes of the hardware cloth. They seem to be especially vulnerable when the nest is thin and the wire is visible through the floor of the nest. In such circumstances, the nestlings can become entangled and die. Even if found alive, disentangling them could result in injuries, or at the least, major disturbance to the other nestlings. Needless to say, none of us want to find that on our watch, and so we advise the utmost caution when deploying blow fly traps in nest boxes. If you notice a thin nest on top of your trap, it is probably best to remove the trap before eggs hatch and allow the nestlings to cope with any potential parasites on their own. 

Be Extra Cautious With Thin Nests

Be Extra Cautious With Thin Nests

This Eastern Bluebird has built an especially thin nest on top of a blow fly platform, causing the eggs to be in contact with the hardware cloth.

I confirmed with two NestWatchers who reported entanglements that one was using ½” hardware cloth and the other was using ¼”. The original design called for ⅜” hardware cloth (also called #3 wire mesh), but this is difficult to find in major retailers. Mr. Campbell does not explain the reasoning behind this choice in either of his articles. This leads me to suspect that there is nothing fool-proof about the ⅜” size cloth—which is intermediate between the two problematic cases—even if you were able to find it. Another risk is that of the eggs falling through the screen, which can be prevented by avoiding ½” hardware cloth (Smith 2023). The eggs of small birds such as chickadees and wrens are small enough to pass through these holes, especially if the nest is thin, but should not pass through a screen of ⅜” or smaller.

Why Not Insecticides?

Insecticidal powders applied in nest boxes before eggs are laid are ineffective because they only last a short while (from a few hours up to a week) and would be inert by the time nestlings hatched some weeks later. Moreover, blow fly larvae are extremely tough to kill even with “very strong insect powder,” and indeed soaking them in solutions of 90% alcohol may still take days to kill them (Sabrosky et al. 1989). These chemicals may also indiscriminately kill the beneficial insects in the nest, such as the tiny wasps that parasitize the blow flies.

What To Look For

What To Look For

Blow fly pupae can often be found by looking at the floor of the nest box once the nest has been cleaned out. Circled in green above is what looks like a blow fly pupa which are dark and inactive (active larvae look like a maggot and should be moving).


Current research, combined with the risks highlighted here, underpin our recommendations surrounding blow fly management. Importantly, there are some species that cannot be controlled at all because the larvae burrow under the skin or into the ear openings or nostrils of nestlings where they stay until mature enough to leave (e.g., Trypocalliphora braueri). None of these control methods would work against embedded larvae, which have the most potential to impair nestling body condition or cause mortality. However, keep in mind that these larvae are only a temporary problem for birds—their larval period being 10 days long—and birds do not get reinfested once they leave the nest. 

The information presented here is intended to help NestWatchers make informed decisions about their nest boxes. To recap, our recommendations at this time are to:

  1. Evaluate your boxes for evidence of heavy infestations (i.e., if you’re noticing more than about 10 larvae per nestling). If your area is subject to widespread infestation, consider placing traps before the nesting season begins.
  2. If you do use traps, monitor for thin nests, and remove the trap in these cases. If removing it would disturb the nest unduly, you could place something in the gap underneath it to block the holes.
  3. Avoid using traps made of ½” hardware cloth to prevent eggs from slipping through the holes, especially for chickadees and other very small birds.
  4. Always avoid handling nestlings and eggs except as a last resort (e.g., entanglement). Handling risks injury to the eggs and young, and triggers stress responses that are detrimental to their development.
  5. Do not apply insecticides to nests. They are ineffective against blow fly larvae and may kill beneficial predators of blow flies.
  6. Consider supplemental feeding (e.g., of mealworms) as an alternative. Supplemental feeding has been experimentally demonstrated to increase parasite resistance in Eastern Bluebirds (Knutie 2020).
  7. Consider removing heavily-infested old nests between breeding attempts to reduce loads of ectoparasites, but be aware that some birds prefer sites with old nests in them and may initiate a second clutch faster if they don’t need to build a new nest. The scant literature on this practice points to mixed preferences even within a species (read more here).
  8. Know that many studies have shown little impact of blow flies on fledging success, and birds can quickly replace lost blood under normal circumstances (Berger 2001). You may choose to allow nature to take its course and provide the healthiest habitat possible to buffer against immunological challenges. Adults with strong immune systems will pass on their genes to their offspring, promoting a parasite-resistant population.


  • Berger, C. 2001. We can also advise you about… Pages 69-88 in C. Berger, K. Kridler, and J. Griggs, editors. The bluebird monitor’s guide to bluebirds and other small cavity-nesters. HarperResource, New York, New York, USA.
  • Campbell, I. L. 1982. Experimental nesting box designed to reduce blowfly parasitism. Sialia 4(2): 49-51. Available here.
  • Campbell, I. L. 1984. Improved design for experimental nesting box. Sialia 6(2): 70. Available here.
  • Knutie, S. 2020. Food supplementation affects gut microbiota and immunological resistance to parasites in a wild bird species. Journal of Applied Ecology 57: 536-547.
  • López-Rull, I., and C. M. Garcia. 2015. Control of invertebrate occupants of nests. Pages 82-96 in D. C. Deeming and S. J. Reynolds, editors. Nests, eggs, & incubation: new ideas about avian reproduction. Oxford University Press, Oxford, United Kingdom.
  • Sabrosky, C. W., G. F. Bennett, and T. L. Whitworth. 1989. Bird blow flies (Protocalliphora) in North America (Diptera: Calliphoridae) with notes on the Palearctic species. Smithsonian Institution Press, Washington, DC, USA.
  • Smith, E. Z. 2023. <>. Accessed 27 Mar 2023.

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5 responses to “What Should You Do About Bird Blow Flies in the Nest?”

  1. Peter M Lehnert says:

    does anyone have any photographs of blow flies and their larvae. i have no idea what these insects look like. what is the ecosystem range of blow flies, where do they live. thanks for any response.

  2. I found the article on NestWatch about bird blow flies to be really informative. It’s interesting to learn about the different types of blow flies and their impact on fledgling birds. The tips for preventing or minimizing infestations were also helpful. As someone who loves bird watching, it’s important to be aware of the potential dangers that baby birds may face and take steps to protect them. Overall, great article with practical advice!

  3. Kathleen Shea Kettel says:

    We’ve had a blue bird house for several years. This was the first year we’ve seen activity and heard babies. I never saw babies out of the nest. We have to take down the tree they were in. We haven’t seen much activity, so we opened it and checked in the nest to find dead birds.

    Is this the reason they died? How can I confirm that?

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Cornell Lab of Ornithology

Cornell Lab of Ornithology