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Estimates greater than zero show that the zone nearer the clutch matches the eggs or adult better than the zone further away.

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Estimates below zero show the opposite effect, matching the more distant zone better than the nearer zone. Colour matching for plover and courser eggs did not differ between zones, so is not included. An exception was nightjar egg luminance, which was better matched to non-chosen backgrounds 5 m away than to their chosen nest site.

However, nightjar eggs were less well matched to the chosen background under trichromatic and dichromatic mammalian vision, whereas they were better matched under avian tetrachromatic vision. Finally, since nightjars flee from the nest only when a predator is nearby our data show a mean flush distance across all three nightjar species of 1.

The plumage pattern of Mozambique and pennant-winged nightjars matched their chosen nest backgrounds better than the adjacent background a few cm away, whereas fiery-necked nightjars did not; individuals of all nightjar species matched their chosen background for pattern better than other potential backgrounds 5 m away.

For luminance, nightjars were better matched to chosen backgrounds than to adjacent backgrounds both a few cm away and 5 m away. Plumage colour was a better match to chosen nest sites at the metre scale, but not the cm scale. First, they were able to choose a suitable nesting patch in the general habitat at a scale of approximately five metres, and second, they refined their nest site selection within that patch at a very fine scale within a few cm. Moreover, because females selected sites that matched their own eggs and plumage better than those of their conspecifics, decisions were made with reference to their own individual phenotype rather than following a general species-wide strategy.

Our findings are consistent with those of a recent laboratory study on substrate selection in nesting quail 27 , and also tie in with our recent study of escape distances in the present study system. The latter showed that individuals modulate escape behaviour based on their level of camouflage, providing further evidence that nesting birds can modify their behaviour in response to perceived levels of concealment These findings also concur with a study of island populations of wall lizards, which found that individuals were more likely to be found sitting on backgrounds that provide better camouflage than on other potential sites Our work here shows that the benefits of microhabitat choice and behavioural changes based on assessment of individual camouflage extend across a wide range of avian species, several spatial scales, and two life history stages adults and eggs.

The above results demonstrate that behavioural choice of substrates and backgrounds may offer a major route to enhancing camouflage, and suggest that studies that simply compare the camouflage of individuals against random background samples may sometimes yield inaccurate findings if individuals vary both in appearance and substrate preference. Many camouflaged species show either discrete polymorphisms or high levels of continuous phenotypic variation 3 , 9 , 19 , 42 , In such cases it may be particularly beneficial for individuals to have corresponding substrate preferences, both to improve their level of camouflage and perhaps to increase the range of microhabitats exploited.

Microhabitat choice may also have important broader evolutionary consequences. For example, some insect species comprise several morphs that occur on different host plants, and disruptive selection against intermediates may potentially drive speciation through reproductive isolation of populations in sympatry We cannot entirely rule out the possibility that our results could be partially explained by predation having eliminated poorly-camouflaged nests from our dataset before we could record them.

While this would itself be an important finding given that individual variation in camouflage matching in wild animals has rarely been directly demonstrated to affect predation risk , we feel that this explanation is unlikely to fully explain our results. Second, a proximate mechanism of background selection based on individual egg coloration has already been established in controlled laboratory experiments with ground-nesting birds, implying that this is a more parsimonious explanation for our results They do not preclude other factors, beyond the scope of this study, influencing nest site selection in birds; these may include habitat visibility for detecting predator approaches, thermal considerations, and vicinity to other nesting birds.

This is, however, highly unlikely to have affected the majority of our findings see Supplementary Information. These factors may add to a rich complexity of factors influencing background selection in birds and other animals.

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Beyond the question of how widespread background choice for camouflage might be, there is much to be gained from trying to disentangle the mechanisms involved. In birds, egg coloration appears to be strongly heritable, with relatively little environmental influence Exactly how birds make appropriate decisions is not yet clear but we suggest it could arise through two not mutually exclusive mechanisms.

First, if background choice is also heritable, a genetic correlation could allow individuals with a given egg phenotype to also inherit the appropriate substrate preference. Alternatively, behavioural preferences could develop with experience as birds learn what their eggs look like, and so to make appropriate decisions. The latter mechanism seems more likely because inherited behavioural choice would offer little flexibility, and also because there is good evidence that birds learn their egg appearances in other contexts.

For example, hosts of brood parasites appear to learn what their own eggs look like in initial breeding attempts, and then reject any subsequent parasitic eggs that deviate from this template of appearance A further and related potential mechanism could involve chicks imprinting on specific backgrounds after hatching, and basing nest site choice on this when they later become breeders. Overall, camouflage can be enhanced not only through genetic or developmental changes in individual appearance, but also through individual behavioural choices.

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Thus, in many species the value and tuning of animal camouflage may result from a complex mixture of morphology, behaviour, and environment. More broadly, our study underlines that animals possess sensory and potentially cognitive mechanisms that allow them to improve the adaptive value of their own individual phenotype by choosing appropriate backgrounds. We should further look for individual background choice in the many other contexts where signalling success is affected by aspects of the environment, such as conspicuous warning coloration and sexual signals 48 — The study system, general methods, and quantification of camouflage closely followed our past work including that demonstrating how our camouflage metrics predict survival of the nests of the birds we study here and a range of past and recent methodological approaches 37 , 38 , Our dataset here overlaps with our previous work with respect to the individual birds recorded and measured, and to some of the images of the natural backgrounds used to assess camouflage, with the addition of further comparison background images taken at 5 m scales used only for this study.

The study site comprised c. Fieldwork was undertaken during the hot dry season, when an open understorey affords nesting habitat for the ground-nesting bird species we studied. The field sites are in an agricultural region, but the cultivated areas primarily maize and tobacco crops are comparatively small and occur within a greater area of natural habitat deciduous miombo woodland and grassland.

As such, predator communities should not differ greatly from conditions occurring before than human impact on the region. Most nests were found by local farm workers, detected when the birds flushed on approach, or through nocturnal eye-shine from torchlight. Our sample of nests may lack the extremes of camouflage matching if we were unable to find the most camouflaged nests, and if some of the least camouflaged nests were attacked by predators first.

However, our resulting sample should remain ecologically representative of the surviving nests, and indeed there was considerable variation in survival and camouflage among them We took digital images with Nikon D cameras, fitted with mm Micro-Nikkor lenses, which transmit ultraviolet UV light. The cameras had undergone a quartz conversion Advanced Camera Services Limited, Norfolk, UK to allow sensitivity to both human-visible and ultraviolet wavelenghts, involving replacing the UV and IR blocking filter with a quartz sheet to allow visual analysis throughout the avian-visible spectrum 51 , For photographs in the human-visible part of the spectrum, the lens was fitted with a Baader UV-IR blocking filter transmitting to nm.

UV photographs were taken using a Baader UV pass filter transmitting to nm.

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During the brief crepuscular periods at our study site, there would be changes in ambient light spectra, background contrast, and shadows, but we cannot test those effects with our current dataset. To quantify adult nightjar camouflage, we closely followed past work on the same system 37 , Images of nightjars sitting on their nests were taken from a standing position from 5 m distance and the flank least obscured by vegetation.

If both sides were clearly visible, images were taken so as to avoid directly facing the sun.

Acquiring images of adult plovers and coursers was not possible because these birds frequently flush at long distances. This enabled us to control for lighting conditions in the adult bird images without the standard needing to be in the same photograph the sequential method Images of plover, courser, and nightjar eggs were acquired in situ from 1. We chose 5 m as the photography distance for the meter scale because adult nightjars could reliably be photographed at this distance without fleeing their nests, and because control photographs taken 5 m on either side of the nest did not overlap with one another.

For the fine cm scale, we photographed from directly above the clutch because this included both the largest clutches and surrounding nest site area.

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To calibrate the images, all photos were linearized to control for the non-linear response of the camera to light intensity, and then standardized against the grey standard to remove effects of the light conditions Visible and UV photographs were aligned and scaled using an automated script, minimizing the absolute spatial difference between pixels.

This accounted for focal length changes when re-focusing in UV, and minor shifts in camera position. To model predator vision, we chose appropriate visual systems based on recorded predation events at a subset of nests from custom built motion-triggered cameras The diurnal predators we recorded included animals with three different visual systems: We used the ferret Mustela putorius as the closest available visual system for modelling banded mongoose vision.

Ferret cone sensitivities absorptance data were obtained from electroretinogram flicker photometry-based data 54 and used to model visual pigment absorbance 55 , corrected for light transmission through the ocular media Vervet monkey cone sensitivities are very similar to those of humans 57 , and so we used human vision models here For birds, the grey-headed bushshrike likely has a violet sensitive VS visual system 59 , and so we used representative peafowl Pavo cristatus sensitivity data for this visual system For each vision model, predicted cone catch values were obtained by transforming the images from camera to animal colour space with a widely used mapping technique 39 , 51 , We used a dataset of natural reflectance spectra to model predicted camera and visual system responses.

All calculations were based on data from to nm in 1 nm increments, under D65 illumination. Models were calculated using custom code in ImageJ 63 and R As has been demonstrated in multiple previous studies, mapping from camera to animal colour space is highly accurate and with very low error rates compared to modelling of photon catch data with spectrometry e. In fact, images much more accurately account for illuminating conditions and angles, and measure larger areas of the focal object or scene, than is possible with spectrometry, meaning that using image analysis is likely even more accurate than purely based on its high correspondence with spectrometry data.

Our aim was to compare how closely the eggs of all species and adult plumage of nightjars matched their chosen nest site compared to other potential sites at different spatial scales. For the analyses of egg coloration, our overhead photographs were segregated automatically into two zones corresponding to the clutches' immediate surroundings an area with a radius of pixels, or approximately 4. We chose an area of pixels because this reliably encompassed the nest area including any potentially modified substrates around the nest and centre of the image.

Eggs were selected using an egg-shape selection tool For the adult nightjars, we also followed the above area selections and comparisons, with individuals selected using the freehand selection tool in ImageJ. Adult nightjars could not be caught and photographed under controlled diffuse lighting conditions. As such, colour and pattern metrics for adult nightjars were based on in situ images, and any local lighting effects such as dappled shadows would have been cast on both the adult bird and its surrounds. Although the normalisation would control for overall colour and luminance differences, pattern could have been affected by dappled shadows.

This effect should be most pronounced in fiery-necked nightjars, which often nested under dappled light, whereas Mozambique and pennant-winged nightjars tended to nest in open sites. However, in our previous study 37 , we found that the pattern match between adult nightjars and their surroundings was the best predictor of nest survival, suggesting that our in situ measures of pattern were at least ecologically relevant.

We calculated three metrics for camouflage matching: They also relate to other aspects of behaviour linked to camouflage, such as escape behaviour by incubating adults Therefore, we can be highly confident that the metrics used here provide ecologically relevant measurements of camouflage. Pattern and luminance metrics used the luminance-channel image following past work 40 because pattern is widely thought to be encoded principally by achromatic vision Ferret luminance was based on the LW cone sensitivity since these are more abundant than the SW cones by Natural backgrounds have luminance levels that are spatially correlated due to a mix of light and dark objects, with a roughly log-normal distribution of intensities.

However, while animal patterns also demonstrate this spatial non-independence of intensities, they often have two or more main levels such as dark spots on a pale background in eggs. Parametric approaches are therefore not suitable for analysing these multi-modal distributions.

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Luminance diff values qunatify to what extent the egg or nightjar luminance values, to each visual system, match the values of their surrounds 37 , This metric overcomes the problems of relying on mean values in data that are not normally distributed, and significantly predicts human detection times of camouflaged objects Pattern differences were generated with Fast Fourier Transform bandpass filters at 17 levels from 2 pixels, increasing exponentially with the square root of 2, up to pixels , using the standard deviation of the luminance values at each spatial scale to derive the 'energy' at that spatial scale.

Next, we calculated overall energy differences across all spatial frequencies Pattern diff in a similar manner to Luminance diff , by summing the absolute differences in energy between the target and the background at each spatial scale s 37 , 38 , Differences in pattern energy between the samples over the spatial scales results in increased Pattern diff values.

As such, Pattern diff quantifies how closely egg and plumage patterns match the size and contrast of those background features 37 , This metric should provide several advantages over past approaches that derive multiple descriptive statistics from granularity spectra 40 , For example, granularity spectra are often multi-peaked, such that selecting only the main peak in the spectrum discards potentially important information at other scales.

As with luminance intensities above , the pattern energy in adjacent scale bands is correlated, resulting in smooth energy spectra. Utilising just one metric of pattern match also simplifies the statistical analysis and interpretation, and is well supported by behavioural evidence as it predicted the likelihood of nightjar nest predation in the same study system 37 and human detection times of hidden targets Our approach here is most relevant to the concept of background-matching, where we expect the irregular patterns of the target to match the size and contrast of those in the irregular background across a range of spatial scales.

It does not test a masquerade hypothesis where one would predict that the sample should be recognized as a different class of background object i. Versions of this model are commonplace in studies of animal coloration reviewed by As is convention, JNDs describe colour differences between two objects in predicted discrimination values, whereby values less than 1. Colour differences for both adult nightjars and the eggs of all bird groups was the mean difference in JNDs between the most abundant colour in the camouflaged object and all the colours found in its surrounds, weighted by coverage, as in our previous work 37 , This approach is in principle, therefore, fairly straightforward: Guidance for caravan and campsites owners and operators Flooding - Who can help?

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