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Changing colours in the dark

Changing colours in the dark
The phases of the moon affect the success of birds that hunt at night.
S.Ananthanarayanan
How species that hunt for prey during the night evolve and adapt to variations in moonlight has been an open question.  Animals have evolved to emerge in dim light, for the advantage of concealment, and others that prey on them,  have developed the ability to spot animals in the dark. But has the illumination during the night influenced the colours and plumage of nocturnal species?
Luis M. San-Jose , Robin Séchaud, Kim Schalcher, Clarisse Judes, Anastasia Questiaux,
Aymeric Oliveira-Xavier, Charlène Gémard, Bettina Almasi, Paul Béziers, Almut Kelber, Arjun Amar and Alexandre Roulin, from the University of Lausanne,  the Ornithological Institute, Sempach,
Switzerland, Lund University, Sweden and the University of Cape Town, South Africa, describe in the journal, Nature- Ecology and Evolution, their study of how the plumage affects the fitness of the white and brown barn-owls, over the course of the lunar month.

Most of the studies, so far, that seek connections between levels of light and the colouration of animals, the authors say, have been of animals that are active in the day-time, “and the consequences of variation in nocturnal light on the evolution of animal colouration are barely known”.  The authors make reference to an 1897 paper, Nocturnal Protective Coloration of Mammals, Birds, Fishes, Insects, etc., by A. E. Verrill,  in the journal, The American Naturalist, which says, “very little attention has been paid to their protective colors as seen by moonlight, twilight, and starlight, when large numbers of species of small mammals and fishes, and numerous insects are most active in search of food and most of the large carnivorous and insectivorous species are abroad in search of their prey. Moreover most birds and many fishes and insects sleep in exposed situations and are thus subject to the attacks of nocturnal predaceous species. The latter, in turn, need protective colors for the night-time, in order to avoid the notice of their prey. One of the most evident effects of moonlight or starlight is to give very black shadows. In the case of bright moonlight these black shadows of trees, etc., may be broken up by patches of white moonlight. Therefore, black or dark-brown animals are nearly invisible in such shadows. If black animals have patches of white or light yellow these will serve a useful purpose by breaking up and obscuring the outlines of bird or beast and look like patches of moonlight on a shadow.”

While studies of nocturnal animals have been less frequent, the authors say that recent work has suggested that animal colouration is related to the variation of light during the night. Apart from instances of animals actively changing colours at nightfall, the authors say that owls with feathers of different colours are seen more often in places where there tend to be variations in light levels.  Variation of light levels could hence be a factor that influences genetic selection in owls, but this has not been studied in a formal way, the paper says.

The authors, used twenty years of data of a population of barn owls that breed with feathers in the chest and belly ranging from white to dark red. Unlike A. E. Verrill, a century ago, the present study could mark individual birds and follow their movements the help of GPS, and use infra red cameras to observe their behavior.  The study hence recorded foraging behaviour, as well as success and timing of breeding, in a 1070 square km area in western Switzerland, using 360 nest boxes, since the year 1991.  

To investigate how the colouration of the owls affected their performance in foraging, the focus of experiments was turned to the response of the common vole (Microtus Arvalis), a rodent found in Europe, to owls of different colours, in different moonlight conditions.
  
In principle, owls should be more efficient in their predation in bright moonlight and the phase of the moon should affect hunting efficiency. And this would affect the success and timing of breeding. At the same time, brighter moonlight would make the owls more conspicuous to the rodents. The hunting efficiency of the owls should hence drop during full moon nights. Although rodents do not have full colour vision, the brown-red owls would be a little less visible than the white bellied owls. The brown-red owls should hence experience less of the predation-limiting effect of moonlight than white bellied owls. And the difference should narrow during the new moon nights, when the contrast is less apparent. 

 

The results of the trials were that the average hunting success was 4.78 prey per night and was significantly affected by the level of moonlight, and differently, according to the colour of the owl plumage. In the case of the reddest parents, the food provisioning fell from 5.67 prey per night, during the new moon, to 3.27 prey per night, at the time of the full moon. The drop, however, was not as much in the case of white owls, dropping from 4.94 to 4.61 prey per night, between the new moon and the full moon.

To investigate these differing effects of the level of moonlight on hunting success and food provisioning by red or white owls, the experiment shifted to the reaction of the vole, the staple prey of the owls being studied, to the sight of red and white owls. The study was in laboratory conditions, with voles that had become acclimatized to the experimental pen. And the study consisted of exposing the voles to models of the differently coloured owls, and with artificial light conditions that mimicked new moon and full moon nights. 

The response of the vole, on sighting the predator, was to flee, of course, but the first response was to freeze. This is a reaction of nocturnal animals, which shun bright light, that hunters who went on safaris regularly made use of. They would course through the jungle, and when they spotted an animal, they would shine a floodlight. The animal would freeze, for many seconds, long enough for the shikaris to take aim and shoot.

Similarly, the first response of the vole, on spotting the owl, is to freeze. The trials showed that the vole stayed frozen for 9.5 seconds, on the average. But there was great difference between red or white owls and in full or new moon conditions. During the full moon, the voles froze 5.6 seconds longer before a white owl than a red owl. And if the times between the full and new moons were compared, the difference was 9.6 seconds.

We can imagine that startling a vole with a bright image is a great advantage to the predator even if being white and visible declares the predator’s presence much before one with muted colours. 

While this finding indicates that the owl’s plumage is a driver for genetic selection, the group also looked for adaptation of timing of egg laying and the arrival of fledglings.  It was found that owls that mated with a male white owl were more likely to lay the first egg when the moon was at least 50% illuminated. Females that mated with red males, however, were more likely to lay the first egg before the moon was 50% illuminated.

The timing has to do with the phases of the moon and the prospects of ample food provisioning during the time of growth of fledglings. In the case of red feathered fathers, it was the chicks that were born in the first few days that had the best chance of survival and growth. But white feathered fathers were successful hunters throughout the lunar month and there was no difference.
[the writer can be contacted at response@gmail.com]


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