MY local supermarket was crowded during the recent cold spell; people were building up stores of food.
Animals protect themselves against freeze-ups by putting on weight.
Hedgehogs fatten up in late autumn but this is not an option for all creatures. Red squirrels, for example, must remain slim and trim; being heavy when flitting about in thin branches at the tops of trees, is not recommended. Contrary to popular belief, however, squirrels don’t hibernate. They rely on food stores instead.
Birds can’t put on weight; they must remain light for flying. Small ones carry enough fat to get through a cold night but not much more. They can eat their own weight of food in a day but locating items is difficult when the ground is covered in snow. Some species solve the problem by caching.
Woodpeckers, squirrels and supermarket customers are what zoologists call ‘larder hoarders’. They keep their supplies tucked away in a safe place. It’s a good strategy; when the weather deteriorates and the going gets tough, food is readily available. But there’s a risk. ‘Lay not up for yourself treasures on earth, where moth and rust corrupt and thieves break in and steal’, said Matthew. Your nest-egg may be pilfered. But there’s a burglar-proof way to store food. It’s known as ‘scatter hoarding’.
One of our woodland birds, the jay, heads the scatter hoard league-table. This member of the crow family eats insects and fruit but oak seeds are its particular favourite. From early September, jays begin carrying them off. Beech and hazel nuts are also taken. The nearest oak stand may be several kilometres from a bird’s home. Otto Wadewitz, studying jays during the 1970’s, found that over 1,000 birds arrived at a German forest every hour. About 250 individuals were involved, each travelling an average of 4km per visit.
He estimated that three tonnes of acorns were removed from the wood in 20 days. British researcher MR Chettleburgh found caching activity peaked in mid-October, with birds working up to 10 hours per day. It took each jay 10 minutes or so to collect, carry and cache a load of acorns, travelling an average of 1.5km per excursion. Each bird stored about 5,000 acorns during the season.
The jay’s shopping basket is called a crop. This pouch in the throat can hold up to nine acorns but, normally, only two or three are transported at a time. Another acorn may be carried in the bill. Arriving back at its ‘home range’, an area up to 13ha in extent, the bird seeks out cracks and holes, mostly in the ground. One acorn, sometimes two, is placed in each hole or pushed into the soil with the bill and covered up. A bird will bury food if other birds are busy doing the same thing. When depositing food on its own, however, it won’t proceed while other jays are watching. When a bird thinks it has been observed, it may dig up its acorns and move them to another location.
Thieving mammals and birds may stumble on a cache or two, but they won’t be able to find many of them. Most of the hidden acorns will remain undiscovered. Scatter hoarding, therefore, is much more secure than larder hoarding.
However, its effectiveness depends on a crucial faculty; memory. Remembering where thousands of acorns are hidden, months after they were deposited, is an extraordinary feat. After snowfalls, the appearance of a landscape is transformed but, researchers claim, birds can find and retrieve items from under 40cm of snow.
But bird memory seems to differ somewhat from ours. British ornithologist David Sherry placed patches over the eyes of captive marsh tits in an aviary.
The tits were able to hide food items and retrieve them hours later without difficulty, despite having one eye covered. But if, after hiding food, the patch was moved to the other eye, the bird could not longer find the caches. Tits and jays don’t have binocular vision. Their eyes, located at the sides of the head, are focused in opposite directions so that a bird sees two completely different images at the same time.
Each eye seems to have its own separate memory.
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, December 13, 2010
