Many sources offer guidance on artificial nest box building for birds, small mammals and medium-sized mammals.
In some situations, nest boxes are useful and even crucial.
For example, they can be used in places where species are recolonizing habitats that offer food resources but that cannot sustain populations with respect to denning/nesting resources, e.g., due to even forest stand age, reductions of forest ground cover etc.
It would be advisable to combine nest box use with habitat restoration (e.g., introducing more age-diverse stands, encouraging structural diversity etc.) but nest boxes can provide important resources during the initial stages.
It has been discussed how nest boxes might prove less efficient than natural cavities (poorer thermoregulation, lack of microclimate diversity inside of the nest box etc.).
I have also been wondering about two effects that I have not seen mentioned in the usual accounts of artificial nest box construction and use considerations (with respect to species that ordinarily use tree cavities):
- lack of natural interactions between saprophytic/decaying wood communities that would be present in natural cavities;
- lack of structural interconnection between the interior of the nest/den and its exterior environment.
Reduced interactions
Natural cavities would form in wood that is subject to decay processes (enlarged, in many instances, by woodpecker activities).
Thus, the cavity walls would create a continuum with the living wood and they would likely consist of dead wood, decaying wood and living wood as well as the respective organisms.
Artificial boxes are made of deadwood and while the deadwood is normally not treated chemically (to avoid toxic hazard and pollution), is it also not an integral part of the natural death/decay/life community surrounding and forming the natural cavity.
In fact, the material might be selected for durability and it could be obtained from an entirely different tree species relative to the tree that the nest box has been attached to.
I have been considering the interactions between the organisms involved in the forming of the cavity as well as inhabiting the cavity and its vicinity.
Unfortunately, I yet lack the necessary knowledge to refer to particular organisms or particular interactions (and, in fact, I cannot claim with certainty that the isolation is undesirable rather than beneficial) but I am concerned that some of these organisms might have also evolved to participate in pathogen/bacteria/fungi/virus/parasite cycles of the birds and animals using the den/nest.
For example, a bird or an animal residing in the nest/den would be a host to certain types of microorganisms or parasites that either live in the nest/den or that are brought in from the outside habitat.
It is conceivable that some of the natural cavity organisms are agents that ensure control (predation, competition etc.) of the denning/nesting animal’s pathogens or parasites (although it is also possible that some of the pathogens and parasites are acquired specifically in the nest/den cavity).
Generally, a greater diversity of organisms is beneficial to pathogen/parasite control and exposure to this diversity or to specific beneficial organisms might be an important part of the denning/nesting animal’s pathogen/parasite control mechanisms.
While the concept of an artificial nest/den box might appeal to us as a ‘cleaner option’ (because we tend to associate human-created objects and environments as ‘disinfected’ and ‘pathogen free’ in many founded or unfounded respects), the artificial box can be entered by some of the organisms (such as beetles, spiders etc.) but not by other organisms (such as fungi, bacteria and other microorganisms to which the wall of the box is a formidable barrier).
Similarly, some other factors (such as water flows, freezing events etc. that affect decaying wood) could be of significance here.
It would be important to study these effects of the isolation of an artificial nest/den box from the continuum of its environment (compared to natural cavities and their integrity within the rest of the tree).
It could bear consequences on the health of the hosted species through the reduction of natural interactions between predators and prey, parasites and hosts as well as competitors.
Artifical den/nest boxes might prove less hygienic than natural cavities.
Reduced structural connectivity
Reduced structural connectivity might result, for example, in increased stress levels in the denning/nesting individuals (especially, in natal nest/den situations) as well as inability to adequately follow the diel cycles or predict and consider (extreme) weather events.
A natural cavity is subject to the same structural impacts as the tree surrounding it.
The animal resident in a natural cavity can hear, see and otherwise sense (through scent, vibration, changes in activity by other organisms such as invertebrates) changes in the outside environment.
These changes could be related to the weather (rising wind, rising temperature, precipitation) or to the presence of a predator (vibrations or subtle sounds as the predator is moving nearby or the activities by other organisms that are subject to predation risk by the same predator).
Even the ability to follow daily (regular) fluctuations in temperature or photoperiod might be impaired upon disruption of this structural connectivity with the outside environment that, in natural cavities, in mediated through the tree and the rest of its organisms.
For example, if a nesting female or male cannot rely on alternative methods of detecting predator risk or following other changes in the environment, they might be forced to leave the clutch/brood more often in order to use the nest/den entrance as a lookout (applying visual detection or enhancing the use of other sensory channels).
This could cause stress in the adult individual as well as energy losses (increased activity) and it could reduced time spent brooding the eggs or the chicks.
Ultimately, constantly increased stress levels might lead to compromised immunity and disease.
Also, e.g., if the presence of danger can only be detected when it is already very near (rather than perceiving more subtle cues of gradually heightened risk), the detection of a threat might always result in a shock reaction that has not been mitigated by gradual awareness (first, perhaps even instinctual and only then growingly cognitive).
For example, a nesting bird, in such situation, might only detect the presence of a predator (or a nest competitor) when they are already entering the nest (compared to hearing them land on a branch outside or to the quiet scraping of claws as the predator is moving up the trunk or some other cues of more progressive nature).
Structural disconnectedness thus might result in energy losses, reduced resources allocated to the young as well as high stress levels and lack of preparedness that can limit the response effectiveness to danger.
Additionally, the impairment of perceptibility of the external environment could cause a cognitive overload and unhealthy constant vigilance because many of the environmental cues would be processed awarely rather than on physical (instinctual) level.
I would assume that instinctual vigilance is less resource-demanding than cognitive vigilance and the lack of signals sent by the animal’s body (detachment from senses and what they are telling the individual) could create a state of perpetual anxiety.
It is possible that the body produces a constant flow of ‘status updates’ by processing the external stimuli or lack thereof.
If there is a lack of external stimuli that surpasses a certain threshold, the animal might experience a psychological condition of ‘self-doubt’ (reduced self-reliance).
A low level of ‘constant worry’ (instinctive vigilance and background sensory processing of external cues) might be normal and healthy and promoting of psychological wellbeing.
If such information (regarding risk or other changes in environment, including changes that allow for gaps between bad weather events etc.) is not processed as a background ‘intel’, the animal might be forced to exert itself physically (to move around) and on a cognitive level (to keep active rather than passive vigilance at all times).
Notes by Ieva 