Recently I read the publication by Ruczyński, I. et al., 2007, that attempted to begin to discuss roost locating mechanisms (i.e., finding new cavities in forest trees that have not been formerly colonized) in a forest bat species, noctule bat (Nyctalus noctula).
It was discovered that species such as noctule bats which do not demonstrate high maneuverability (inability to hover) might find it very difficult to locate new suitable roosting spots.
In the particular species, the individual used echolocation but the use of this skill was limited due to the flight pattern.
Also, social cues were utilized (echolocation calls by conspecifics) and, possibly, temperature-related cues were utilized (temperature on the bark surface should be different than the temperature in the cavity and bats are able to perceive such differences; however, it is not known if temperature is perceived directly or the temperature rather serves as an intermediate to perceiving, e.g., changes in olfactory cues or air flows caused by heat).
In noctules, visual cues (light) and direct odour cues of conspecifics did not appear to play part in the locating of the cavity in noctules.
Mostly, noctule bats seemed to fly until they perhaps perceived a patch on the trunk (log in the experimental conditions) that could offer suitable roosts which was then examined closer by crawling along the trunk (log) and probably reading more subtle nearby-perceived cues.
Social cues (calls by conspecifics) were the most significant aid.
However, the use of social cues is only possible once at least one bat has discovered the new roost.
Meanwhile, as reported in the publication through referencing unpublished data, other bats such as brown long-eared bats (Plecotus auritus) are capable of discovering a new cavity while in flight due to their ability to hover and to echolocate from one spot in the air.
The publication also referred to other research that attests to associative learning in bats.
Associative learning enables the individuals to learn echo roughness of different surfaces (i.e., different types of bark) which can assist in detecting the more suitable tree species that can offer roosting cavities or the likeliest parts of trees (e.g., dead branches without bark or the height of the trunk above ground etc.) where suitable cavities could be found.
As it is apparently difficult for bats (especially, non-hovering species) to find new roosts and the availability of roosts in the contemporary, relatively even-aged forests has decreased considerably, it might be important to take these findings into consideration when manufacturing and installing roost boxes for particular bat species.
This would reduce the time and energy necessary for an individual to locate a new suitable roost.
Time and energy invested in finding of new roosts is a trade-off between the needs for shelter and the needs to perform foraging or other crucial activities.
When constructing roost boxes, the needs and the detection abilities of the particular species should be considered (possibly through tests in experimental conditions where bats have to locate cavities in different types of nest boxes).
For example, if bats can tell apart different tree species (e.g., in Białowieża Forest bats tend to roost in large old oaks, ashes, alders high above the ground; 19 m), in hovering species that use echolocation in flight to survey the trunk surface, it might be better to cover the manufactured roost boxes with pieces of bark of the same tree species that the box has been made of.
This could assist bat species in choosing appropriate microclimate.
If bats prognosticate microclimate conditions based on associative learning of the insulatory and conductive qualities of different tree species, these qualities are probably discerned through inspecting the bark surface.
Falsely-deduced microlimate conditions might lead to higher mortality (especially, on occasion bats choose to hibernate in aboveground tree roosts).
If species choose roosts according to tree species, they might become misled if a roost box that has been constructed, e.g., of ash wood has been attached to another tree species (they might choose the box although it is not suitable for the species because they consider it to be of another tree species according to the surface of the tree that the box has been installed on).
Alternatively, bats might avoid roost boxes that are attached to the ‘wrong tree’ if they determine the qualities of the roost by the qualities of the tree surface that the box has been attached to.
Smooth boxes might not provide the respective cues to determine the qualities of the wood used in construction of the box and bats might not be able to prognosticate the microclimate conditions in such boxes.
On the other hand, if the bark is too smooth (if the nest box has been manufactured out of younger trees or parts of trees growing lower to the ground level and if bark has been obtained from such material), bats might become confounded into thinking that the nest box is not suitable because it does not resemble an old tree or the properties of bark high above the ground.
While hovering bats might need these cues, non-hovering bats (or species that roost in barkless parts of trees) might utilize other types of cues and the smoothness of the box might be better perceptible to such species as well as more inviting.
Thereby, the locating mechanisms (skills, senses, associative cues) of bat boxes might be different in different species.
The construction of bat roosting boxes therefore might take into account the needs of the target species (as determined in prior research and tests).
Alternatively, the slight protrusion of the bat box away from the trunk might itself assist detection through echolocation.
While preserving veteran trees is the best pathway to ensure forest bat conservation, in some places, roost boxes are used during remedial stages (e.g., to assist bats through a stage in forest development where there is a paucity of old trees due to former forestry methods) or as supplementary resources.
Frequently, it is known which species inhabit the forest because bat species are also known for niche differentiation and respective association with specific forest types.
If slightly adjusting the construction (e.g., through attaching bark to the outer surface of the bat box) could facilitate the discovery of the roost box by vulnerable bat species, it might be worth including in the manufacturing and conservation effort.
Further studies would be needed to determine how bat species locate new roosts and what cues can assist them in finding the roost and determining its suitability.
Notes by Ieva 