This is the time of year when ewe hoggs return from their winter sojourn on low-ground farms to the hills of their birth. Lambs are called hoggets or hoggs after New Year's Day, when they are about 8-months-old, and then become gimmers in late summer when their first two incisor teeth have fully emerged.

Ewe hoggs are the flock's breeding replacements, and are traditionally sent away on October 1st to better pasture, usually a dairy farm, where they stay until April 1st. Nowadays dairy farmers prefer to send hoggs back to their hill farms earlier, some as early as mid-February. That allows their grass to recover from being grazed by the sheep and grow into lush pasture for their cows to be turned out onto mid-April. Those fields destined for growing silage also benefit from having sheep removed early by growing heavy crops of highly-nutritious grass ready to be mowed in May.

The practice of sending hoggs away to better pasture for the winter allows them to grow out to their full potential, as well as conserving precious, scarce grazing on the hill for pregnant ewes and gimmers. A typical flock of 1,000 breeding ewes and gimmers needs at least 250 hoggs to replace those that die during the year, and the 5-year-old "draft" ewes that are sold in the autumn.

On returning home, hoggs undergo routine medication such as vaccinations or dosing for stomach worms or liver fluke. Many farmers also apply insecticide to their fleece either by pouring small quantities of a concentrated form on their back, or by plunge-dipping them in a solution. That insecticide controls pests like sheep scab mite, lice and ticks. It is customary to gather the rest of the flock at this time of year to undergo similar treatment as part of the run-up to lambing.

Once treated, the pen gates are opened to allow the hoggs, gimmers and ewes to return to their hefts, just as homing pigeons return to their lofts. Hefting is a phenomenon that allows hill farmers and their shepherds to control grazing and manage a breeding programme in the absence of fences. If sheep wandered the hills freely, rams could end up breeding with their mother, daughters and other near relatives.

A heft is a family group of females that can range from as few as fifty or sixty to hundreds. They graze a clearly defined piece of land that may be bounded by a road, burn, skyline, loch or trees, and may extend to hundreds of acres.

The secret to success of this traditional system of grazing is to have the right number of sheep in each heft, so that the hills aren't overgrazed during the summer leaving too little food for the sheep to over-winter on, nor too few, as that would allow the grass to become rough and less digestible.

Over the years shepherds have worked out the optimum number of sheep for each heft and vigorously maintain the system by chasing any strays back to where they belong. It's amazing how sheep rarely stray from their heft.

A hirsel is a group of hefts and was traditionally the ideal number for a shepherd to look after - anything between 450 and 700 - although nowadays its more likely to be more than 1,000. After a hirsel has been gathered for routine tasks the sheep will instinctively return to their hefts, as they are all keen to be reunited with their mothers, sisters, aunts and cousins.

Different hefts carry different marks to help the shepherd identify strays and know where they belong. It may be a certain colour of keel - marking fluid that can be scoured from the wool when processed - applied in different ways to different parts of the body, such as the back of the head, "far" or "near" shoulders, ribs or hips.

Sheep carry other distinctive marks branded or sawn on their horns to record ownership or age. Then there are different types of nicks in different parts of their ears that tell you what hirsel or heft they belong to, and also their age in the case of hornless breeds.

Those marks are recorded in the Register of Sheep Marks that were compiled for each county, and my copy was printed in 1907.

Traditionally, shepherds used to held meetings when strays were returned to their owners. Those without marks were sold, and the proceeds given to the poor.