I recently cleaned out the nest boxes in my garden. I tenderly removed a dead chick and gently scooped out the mossy blue tits nest. Then, I dipped the box in boiling water to kill any parasites. As the box was drying, I stopped to wonder if I was doing the right thing.
Natural processes are normally best left alone. The natural world contains systems of checks and balances that have been around for a very long time, and it’s not usually wise for us to go wading in deciding that some species or processes are a problem. In most cases, our knowledge barely scratches the surface of the complex interconnections at play, so it’s best to only intervene based on clear science or experience.
Nature doesn’t turf out nests from cavities or bushes at the end of the year and then douse the area with boiling water – so why do we?
Both the RSPB and the Wildlife Trusts recommend cleaning nest boxes each year, but neither offer much explanation as to why, so I decided to do a deeper dive. Here’s what I found.
Before we delve in, I should point out that this all centres on the presence of ectoparasites. These are parasites that live on the skin of their host, and they occur in nesting material. The key documented reason I’ve found for suggesting nest box cleaning is to reduce the prevalence of ectoparasites.
A secondary documented reason is preventing the build-up of nesting material to a height that makes the nest vulnerable to predators (as it ends up nearer to the opening).
First thing’s first, we need to face the fact that the nest boxes we provide aren’t the same as natural cavities. A custom-made box, made from timber with no living tissue, hung on a brick wall is not the same as a tree cavity, surrounded by the (often living) tree. The difference is particularly stark when it comes to nest boxes that aren’t in woodland or farmland – like the ones on our fences or buildings.
Whilst we should try to allow our artificial installations to mimic nature so far as possible, there are some elements of natural cavities that can’t be replicated. For instance, natural cavities and the surrounding wood contain a wealth of decomposers – fungi, bacteria, insects – that break down old nesting material, meaning a quicker rate of decomposition than in nest boxes.
We also need to think about the extent to which birds’ natural behaviour is impacted by our human environment. Whilst cavity-nesting birds (like tits) will often nest in cavities with old nesting material due to the limited availability of suitable cavities, birds that don’t usually nest in cavities (like robins and blackbirds) don’t usually reuse nests.
Birds that aren’t cavity nesters tend to build new nests to avoid harbouring lots of ectoparasites – even if the prior year’s nest was successful (and therefore perfectly placed for a second year). These birds will often use nest boxes (particularly the half-open ones), and it might be that they have little choice in the surrounding area and so are forced to re-use existing nests, with associated ectoparasites.
This would mean we’re forcing modification of the birds’ natural preferences in a way that is potentially harmful to them, and nobody wants to accidentally cause more harm than good. Whether this is really the case is extremely site-specific, as well as species-specific (as some birds will remove old nesting material themselves).
Does removing old nests mean fewer ectoparasites?
It might not matter if you clean out your nest box if birds choose to use it because some have adapted to live with ectoparasites. For example, male house wrens tidy away old nesting material in between clutches, effectively doing the work for you.
GETTING THE BALANCE RIGHT
Recall that getting rid of ectoparasites from bird nests shouldn’t be the goal. Because birds are accustomed to coexisting with ectoparasites, it is rarely a good idea to intentionally decrease a species’ natural prevalence. Instead, the goal should be to prevent human-caused harm, such as higher densities of ectoparasites due to the differences between natural cavities and nest boxes.
There isn’t a middle ground, which is the problem with that logic. The populations of ectoparasites in nest boxes may not even be harmfully higher (see below), and it is not possible to return the natural balance by simply removing the “extra” ectoparasites.
We also have to consider knock-on effects of removing ectoparasites. Some ectoparasites are killed over the winter by parasitic wasps, meaning that removing the old nesting material makes little difference to their population numbers and deprives the wasps of suitable hosts, creating ripples within the ecosystem that are poorly understood.
Additionally, clearing out nest boxes lowers the amount of ectoparasites below what is naturally occurring, which may favor species that use nest boxes over those that don’t (e.g., favoring blue tits over willow tits). This could contribute towards reductions in biodiversity and increased homogenisation.
By Anita Tendler, Cornell Class of 2019
We make sure our nest boxes are prepared for their future occupants before the breeding season begins. We observe the naked hatchlings grow into fully-feathered fledglings as the season goes on. The question of what to do with old nests arises because our nest boxes are abandoned after the breeding season, leaving behind nesting material.
FAQ
Should you remove old nests from bird boxes?
What happens if you don’t clean out a birdhouse?
Are you supposed to clean out bird houses every year?
What time of year do you clean out bird boxes?