By Anita Tendler, Cornell Class of 2019
Prior to the breeding season, we make sure that our nest boxes are ready for their future occupants. As the season progresses, we watch the naked hatchlings grow and develop into fully-feathered fledglings. Once the breeding season comes to a close, our nest boxes are abandoned and the leftover nesting material remains, which leaves the question, “What should be done with old nests?”
Nest boxes vs natural cavities
Nest selection of natural cavities is quite different in comparison to our handy nest boxes. With a finite number of natural cavities to choose from, most birds can’t afford to be too picky. Birds often choose to reuse successful natural cavities because constructing a nest from scratch requires critical and limited time and energy. However, the threat of ectoparasites (e.g. mites, blowfly larvae) from the old nesting material can also act as a strong deterrent for cavity selection. With most birds not having the option to be choosy with their nest site selection, birds like the Eastern Bluebird simply build atop old nesting material if alternative cavities aren’t available.
Nest boxes provide cavity-nesting species the option to choose among several nest sites. As well-constructed as some nest boxes might be, they are not immune to ectoparasites, the presence of which can deter some birds from occupying a nest box.
Does removing old nests mean fewer ectoparasites?
Some birds have adapted to cope with ectoparasites, so cleaning out your nest box may not have any impact on whether they occupy it. Male House Wrens, for example, clean out the old nesting material between clutches, essentially doing the job for you.
To measure whether human intervention was helpful, researchers in Illinois removed old nesting material from some nest boxes that they knew successfully reared fledglings in the prior breeding season (Pacejka and Thompson 1996). With the other boxes left for the House Wrens to clean, the researchers conducted a mite count to determine if there was a perceptible difference. They found that there was no real difference, so regardless of who, or what, cleans out your nest box, mites will still be there.
Not all birds clean house
Bluebirds do not remove old nesting material, rather they simply build over an existing nest. If you do not clean out your nest box, it may become filled to the brim with old nesting material. This can potentially leave the new nest dangerously close to the entrance hole, where predators can easily reach it.
To learn whether removing old nests influenced Eastern Bluebird nest box occupancy, a team of researchers in North Carolina erected 100 nest boxes. After a successful first clutch, they cleaned out half and left the others as is. When the bluebirds were left to make a choice to re-nest in a box with a positive association or to avoid ectoparasites, a whopping 71% of them them chose to move to a clean nest box (Stanback and Dervan 2001).
So that means you should clean your nest boxes, right? As compelling as these results are, it’s important to remember that this is situation-dependent. Interestingly, opposite conclusions were reached in a Kentucky study that found that Eastern Bluebirds in that state preferred nest boxes with old nests in them (Davis et al. 1994). There, parasitic wasps kill blowfly pupae over the winter; therefore, removing old nesting material may actually compromise this natural process.
To clean or not to clean? It depends…
Cleaning out your nest box is your choice, as nest site selection varies among cavity-nesting species. When making your decision, feel free to weigh the pros and cons, taking into consideration individual species preference and ectoparasite abundance. If you’re hoping to attract House Wrens to your nest box, don’t worry, they’ve got it covered. But, Eastern Bluebirds are a bit tricky. Depending on where you are, cleaning out your nest box may either invite or deter them.
Whether you decide to clean out your nest box at the end of the breeding season or not, don’t forget that leftover nesting materials make the perfect home for small mammals. If mice occupy nest boxes, you should definitely clean the boxes in the spring by removing nest material and washing with a soapy solution. Take precaution and wear gloves and a mask when removing rodent nests; they are far less fastidious than birds.