Good eyesight helps not only choose the right types of food, but also keep an eye out for competitors and predators while eating it. It appears that the risk of predation can influence what foods a squirrel takes, while the food item itself may dictate how vigilant the animal is while eating. In a 1986 paper to the journal Animal Biology University of Rochester biologists Steve Lima and Tom Valone found that Grey squirrels foraging in Genesee Valley Park, in New York, rejected more energetically profitable but smaller food items in favour of less nutritious larger ones that could be carried off to eat under cover. The researchers suggested that the squirrels picked their food items based on their proximity to cover and, thus, that the potential of being spotted by predators was a key factor in deciding what and where to forage.
A similar study by Jonathan Newman and colleagues at the State University of New York found that, although distance from cover didn't affect the number of seeds their subjects ate, it did influence the speed at which the seeds were eaten. Newman and his co-workers studied the Grey squirrels on their University's campus from January to May 1987 and observed that the risk of predation influenced not only the 'patch residence time' (i.e. how long the squirrels sat in this particular spot), but also the so-called 'foraging process constraints' (i.e. how long they spent getting to the spot and handling the food). Basically, what the researchers found was that when eating 200 seeds at five metres (16.5 ft.) from cover, they handled each seed for just over five seconds; when they were 15m (50 ft.) from cover, however, they handled each seed for just under two seconds. Now, this may not sound like much, but it's a saving of just over 10 minutes to the squirrel while it's out in the open.
It seems probable that the impact predation has on feeding behaviour or choice of food is likely to vary widely with habitat. Where squirrels are at greater risk from birds of prey, they may be safer if they feed under cover, while this populations where predation comes from stalking mammals, being out in the open where they can see a predator coming may be advantageous. Indeed, in my experience of squirrels in gardens and parks, many can be seen out in the middle of lawns or flowerbeds eating and, in almost all cases where I have watched Greys recover cached food, they have proceeded to sit down (in the open) and eat it there and then, rather than retreating to cover. Once a squirrel has chosen its food and where to sit and eat it, the energetics of that food (i.e. how energy-rich it is) may influence feeding vigilance.
In an interesting series of experiments on Greys in Canada, Joanna Makowska and Donald Kramer at McGill University looked at how the size of a food item affected a squirrel's vigilance behaviour (looking out for predators and competitors) while feeding. They obstructed the squirrels' vision and offered them sunflower seeds and crackers with peanut butter. The results, published in the Animal Behaviour during 2007, showed that when handling the sunflower seeds, the squirrels didn’t move their position to get a better view of their surroundings, while those feeding on crackers did. The biologists concluded that with small food items, the cost of moving out-weighs the value of the food and so they are willing to be less vigilant while handling them. In essence, the greater the reward, the greater the risk the animal is willing to accept to get it.
The feeding kinematics of Red and Grey squirrels are similar – food items are held and rotated by the front paws as the squirrel either sits or hangs. Squirrels, indeed all rodents, have four long, curved, powerful incisors (one pair in the top jaw, one pair in the bottom) that grow continuously during the animal’s life and are constantly ground down by gnawing. The incisors consist of a central pulp cavity containing nutritive blood vessels surrounded by a layer of dentine and with enamel on the outer face. The incisors develop a sharp, chisel-like profile because the dentine, which comprises the bulk of the tooth, is softer than the enamel and worn down more quickly. Squirrels have a split upper lip (hare lip) through which the upper incisors protrude when feeding and lack the canine and most of the pre-molars, resulting in a gap called a diastema between the incisors and the molars at the back of the mouth.
Feeding methodologies can help differentiate squirrel-chewed nuts from those gnawed by other small mammals (e.g. voles and mice), although distinguishing those chewed by Reds from those chewed by Greys by visual inspection alone is impossible. Squirrels use their incisors something akin to a crowbar; they gnaw a small hole in the top of the nut into which they insert their incisors and ‘jimmy’ it open from the top, causing it to split roughly in half with clean edges. This is a fairly rapid process and, depending on circumstances and experience, takes less than a couple of minutes. According to an article on the BBC’s Autumnwatch, Red squirrels took, on average, 22 seconds to open and consume the contents of a hazel nut, and Helen Butler gives the same figure in her film about Reds on the Isle of Wight. Peter Trimming, however, filmed an adult Red squirrel at Forest How in Cumbria take about one minute to break into a hazel nut, tearing chunks off the shell of what appeared to be a particularly tough nut, while other squirrels in the same location have split the nuts open in 10 seconds of less.
In their 2002 paper, Luc Waters and his colleagues found their subjects took, on average, almost five minutes to eat a hazel nut. The biologists also observed that their Reds took 7 min 45 sec to eat a chestnut, just over 15 mins to eat a walnut and half an hour to break into and eat a black walnut. In The Eurasian Red Squirrel, Bosch and Lurz note how opening these nuts is a skill that must be learnt and perfected. They discuss some early experiments with Red squirrels by the eminent German ethologist Irenäus Eibl-Eibesfeldt during the 1940s. Eibl-Eibesfeldt gave some of his captive 60-day-old squirrels a walnut and noted that they took 15 minutes trying to get into the nut, the shell exhibiting the scratch marks of many failed attempts. He also found that it took the squirrels about 20 days to master the technique for breaking into a hazel nut, being able to open them as quickly as adults by about 80 days old.
Peter Lurz has observed something similar in squirrels living in spruce plantations that don’t contain hazel – when presented with a hazel nut even adult squirrels failed to open them up initially. In their book, Bosch and Lurz note how the nuts “remained intact in the traps but were covered with lots of scratch marks”. Preliminary observations by Peter Trimming on the Forest How Red squirrels have found that offspring often handle nuts in the same way as their mother, suggesting that they may learn how to open a nut by watching her.
Fir cones are processed by gnawing the scales off to expose the seeds; the cone seeds are eaten and the seed wings are discarded along with the cone core. The husks of acorns and chestnuts are peeled off.