LINKED PAPER
The Eurasian Treecreeper (Certhia familiaris) has an effective camouflage against mammalian but not avian vision in boreal forests. Nokelainen, O., Helle, H., Hartikka, J., Jolkkonen, J. & Valkonen, J.K. 2022Ibis. doi: 10.1111/ibi.13056 VIEW
Animal colouration is one of the key phenomena of biology when studying evolution, adaptation and behaviour (Endler & Mappes 2017). One major evolutionary driver for animal colouration is camouflage. Camouflage is an adaptation against the visual and cognitive skills of a receiver that helps prey species to conceal themselves from predators (and vice versa), playing an important role in determining survival (Endler & Mappes 2017, Merilaita et al. 2017). Camouflage is often generated through effective background matching where the animal’s colouration closely resembles its visual background and is thus difficult to detect (Merilaita et al. 2017). Although intuitively, this sounds straightforward, it has proven difficult to study camouflage in naturally variable environments.
The effectiveness of camouflage is often observer-dependent, as different animals perceive a variation in colour and brightness in different ways. For example, most birds have a tetrachromatic visual system, meaning that they are able to perceive ultraviolet wavelengths, in addition to visible light. Birds also have different receptor sensitivities to different wavelengths enabling them to perceive and discriminate colours differently from us (Vorobyev et al. 1998). Instead, most mammals have a dichromatic visual system, corresponding to red-green colour blindness (protanopia). Thus, colour perception and camouflage efficacy are subjective, depending on the receivers’ visual abilities.
Camouflage is often habitat-specific, and a certain appearance may not provide effective camouflage in different habitats where concealment requires matching different lightness, colour or pattern texture. Animals thus rely on different camouflage strategies in heterogeneous environments. Specialists typically conceal themselves against only certain visual backgrounds which, however, limits their habitat use (Merilaita et al. 2001). In contrast, camouflage generalists have adopted an imperfect compromise to moderately match different visual backgrounds, allowing habitat selection on a larger scale (Hughes et al. 2019).
If you ask a birdwatcher, the Eurasian Treecreeper (Certhia familiaris) is probably one of the classic examples of bird species with effective camouflage. In most cases, the Eurasian Treecreeper’s high-pitched call can be heard before spotting the bird climbing against the tree trunk. However, their visual camouflage has not been objectively quantified before, and the question of whether treecreepers are effectively camouflaged may be more complicated as 1) their breeding habitats vary within their wide distribution range from western Europe to Japan, and 2) their potential predators may have different visual skills to search for the target. If treecreeper camouflage is better in certain habitats or when climbing on specific tree species, could this affect their breeding habitat choice?
Figure 1 The Eurasian Treecreeper camouflaged against different backgrounds. Can you spot all three? © Juho Jolkkonen.
To answer these questions, we quantified Eurasian Treecreeper camouflage in its boreal forest habitat in Central Finland. We tested how the species’ different plumage features match against its typical background – a tree trunk. We also measured the background match using trichromatic human, dichromatic ferret and tetrachromatic avian vision modelling to better understand how effective their camouflage is against different observers. Then, we measured whether treecreeper colouration better matches the home (i.e., nesting tree), territory or habitat spatial scales to see whether camouflage could potentially influence their nest site selection. Finally, we explored which common tree species provide the best background match to explain treecreeper habitat use.
We found that the treecreeper’s wing and mantle provide the closest background match which helps them to hide when feeding predominantly against tree trunks. In contrast, the wing stripe and tail are more conspicuous, suggesting possible different adaptive functions such as intraspecific signalling or disruptive markings against a predator (wing stripe), whereas the tail could serve as a false target for the predators. Furthermore, vision modelling suggested that treecreeper camouflage provides better protection against mammalian than avian predators. Treecreepers have been shown to suffer from a high nest predation rate, and mammalian predators such as Red Squirrels (Sciurus vulgaris), Least Weasels (Mustela nivalis) and Stoats (Mustela erminea) are the most common nest predators, in addition to the Great Spotted Woodpecker (Dendrocopos major). Therefore, being camouflaged against these dichromatic mammalian predators could improve breeding survival.
Background matching was not found to be significantly different across spatial scales in local spruce forests, but the background match was better on conifers in comparison to deciduous trees. This may suggest that treecreeper cryptic colouration provides generalist camouflage within their own niche in boreal forests which are rich in coniferous trees, such as in our study site. However, the treecreeper appears to be relatively well-matched to several common tree species, which might support their habitat use in mixed forests, especially towards the south of their distribution range. Hence, generalist camouflage would plausibly provide flexibility for treecreeper habitat use across its range. Interestingly, a closely related southern relative of the Eurasian Treecreeper, the Short-toed Treecreeper (Certhia brachydactyla) inhabits deciduous forests, and their camouflage may thus rely on a different appearance to better match the local forest habitat.
To conclude, our results support the intuitive conjecture of treecreeper camouflage, but also suggest that their camouflage is an adaption particularly against mammalian predators with a dichromatic vision that are important nest predators of treecreepers. In our study site, treecreepers seem to favour boreal coniferous forest where their colouration appears to provide the best background match for the species. Although cryptic colouration in treecreepers has generally been assumed to provide effective camouflage, to our knowledge, this is the first study to demonstrate it objectively. It seems that birdwatchers still need patience when finding treecreepers on tree trunks – it is now proven to be difficult.
References
Endler, J.A. & Mappes, J. 2017. The current and future state of animal coloration research. How camouflage works. Philosophical Transactions of the Royal Society B: Biological Sciences 372: 20160352. VIEW
Hughes, A., Liggins, E. & Stevens, M. 2019. Imperfect camouflage: how to hide in a variable world? Proceedings of the Royal Society B 286: 20190646. VIEW
Merilaita, S., Lyytinen, A. & Mappes, J. 2001. Selection for cryptic coloration in a visually heterogeneous habitat Proceedings of the Royal Society of London. Series B: Biological Sciences 268: 1925-1929. VIEW
Merilaita, S., Scott-Samuel, N.E. & Cuthill, I.C. 2017. How camouflage works. Philosophical Transactions of the Royal Society B: Biological Sciences 372: 20160341. VIEW
Vorobyev, M., Osorio, D., Bennett, A.T., Marshall, N. J. & Cuthill, I.C. 1998. Tetrachromacy, oil droplets and bird plumage colours. Journal of Comparative Physiology A 183: 621-633. VIEW
Image credit
Top right: Eurasian Treecreeper © Juho Jolkkonen.
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