Observation at Night
Night observation is very different from observation during daylight. At night we rely on our senses of hearing and sight in conjunction with artificial light to locate an animal. Nocturnal animals have very acute senses compared to ours and they are usually aware of our presence long before we detect them. Often I find an animal by its call or noise, then locate it by it's eye-shine.
Vision is a complex sense which involves the brain interpreting signals from the light sensitive rod and cone cells in the retina at the back of the eye. Cones are fully active in bright light and rods only work in very dim light. Diurnal animals including humans have cone dominated vision which enables excellent colour perception in daylight with high visual acuity. Nocturnal species have eyes with a greater proportion of rods, which makes for enhanced vision in low light. Nocturnal eyes also have a number of physical adaptations to collect more light, including larger eyes having a short focal length, wide aperture and often a highly reflective tapetum lucidium which allows light to pass twice through the retina.
In most vertebrates, rod cells which have a peak spectral sensitivity at approximately 500nm (dotted curve in graph below) are completely insensitive to red light. Consequently, you can observe many nocturnal species at night by using dim red light without causing disturbance. Different species see red light to varying degrees and nearly all can see a red light if they look at it because their retinas do contain red sensitive L-cones.
The graph below shows the spectral sensitivity of the rods and cones in the human eye. The cones types are referred to as 'L', 'M, and 'S' , referring to long, medium, and short wavelengths, respectively..
Human perception of colour under widely different levels of illumination is summarised below.
Night-time vision (Scotopic vision) - In very dim light cone cells don't function and the brain processes the signals it receives from the rod cells. As there is only one type of rod cell, the brain cannot use the rods to perceive colour, which is the reason why we can't see the colours of objects illuminated by starlight or the partial moon. The image below of children's crayons photographed in starlight (15 seconds, f1.8, ISO 9000), shows that unlike us, the digital camera sees colour perfectly even when we perceive only shades of grey.
Twilight vision (Mesopic vision) - Twilight is the period between night and day when the sun is below the horizon and during this time both the rods and cones are active. Colours appear muted compared to the colours we see during the day. If you sit in your garden after sunset you will notice, the colours around you start to disappear as the cones slowly switch off. The longer wavelengths such as red disappear faster than the shorter wavelengths and this is known as the 'Purkinje effect'.
The fact that the brain interprets the signals from our eyes and sees colour can result in optical illusions and incorrect colour perception. Moonlight is reflected sunlight having a colour temperature of approximately 4000K, yet our eyes perceive shades of grey or blue as depicted in cinema movies. Moonlit night scenes photographed with a digital camera with a white-balance set to daylight, show a full range of colour with a strong yellow cast
Daylight vision (Photopic vision) - The eye contains several cone types, each having a peak sensitivity to a different light wavelength. The brain 'sees' colour; not the eyes and it does so by interpreting the combined signals from the different cone types, analogous to mixing the primary colours to achieve the colours of the rainbow. In bright light it is generally believed that the more cone types a species has, the better is its ability to distinguish colours. Mammals have either two or three cone types whilst birds, reptiles and fish have at least four, which probably enables superior colour perception and the ability to see into the ultraviolet part of the spectrum.
High intensity flashlights are great for finding animals at night but are not well suited for studying their behaviour. White light has the advantage that you can see the true colours including the colour of an animal's eye-shine, which can be useful for rapid identification. The photo below shows the distinctive blue eye-shine of a Mouse deer in my hotel grounds in Sabah, Borneo.
Bright white light directed into the eyes of an animal may cause temporary night blindness lasting at least 10 minutes, during which time the animal could become susceptible to predation or injury. Rods don't see red, which suggests that rod dominated nocturnal eyes have poor sensitivity to red light. Consequently red lights such as my headlamp below are ideal for observing the behaviour of nocturnal animals. The main disadvantage in using a red light is that true colours cannot be discerned.
The photo below of a Powerful owl bringing a freshly killed Ringtail possum to it's nest was taken using the headlamp above (1/3 second exposure, f6.3, ISO 6400). I observed this nest for many years using red light and the owls were always aware of my presence but were unconcerned. If a white light was used I may not have been able to observe their behaviour and in the worst case, the owls may have abandoned their nest.
Many birds and mammals can be attracted by playing their calls and sometimes by playing the call of another species. Playing of calls is often used in owl surveys, by bird watchers (twitchers) and photographers. When searching for owls at new locations, I often use recordings of their calls. Call playback can be highly disruptive if not used judiciously and I do not encourage indiscriminate usage. Playing calls near nest sites during the breeding season is strongly discouraged. You should resist the temptation of divulging owl locations to photographers and twitchers because owls are strongly territorial and are vulnerable to disturbance.
WALKING at NIGHT
Long-sleeved shirt, long pants and sturdy boots afford protection from cuts, scratches and bites. I prefer to walk on well defined tracks because you can move quietly and are likely to see more animals. Depending on the weather and location, I may take extra clothing, insect repellent and something to sit on. Being alone can be very rewarding and you usually see more animals because your senses are heightened, however you need to be more careful. At night your vision and sense of direction is impaired and it is easy to become disoriented and trip on something you didn't see. Before I venture alone I familiarise my route and tell someone where I am going.