The quick answer is twice a year; in the spring and again in the autumn

The reason the deer need to moult is to physically prepare themselves for the season ahead of them, hence why the coat changes occur the spring and summer.

Autumn moult

This occurs around September/October and is to prepare the deer for the cold, winter months ahead. The variability in the timing is due to the age of the deer: the youngest individuals (e.g. the current year’s fawns) moult before the adults. Generally speaking, the older the animal, the later he/she will moult; the master buck and the oldest females in the area could easily be about a month behind the fawns.

However, as with many things related to roe deer, there are no hard and fast rules: I have known twin buck yearlings change from summer to winter coat three weeks apart. And with that particular pair, the same occurrence happened the preceding spring, but interestingly, the order of the moult was reversed. In other words, the buck that moulted initially in the autumn, was the last to jettison his winter coat in the spring, but that time, the difference was two weeks.

Although in general, a particularly late moulting in any age group, is likely to be due to health problems.

Technically speaking, the autumnal moult for the fawns is actually their second coat change of the year; since their pelage changes significantly over the course of the summer. They are born with a covering of dark brown hairs, with an attractive pattern of light-coloured spots, spread across their flanks and back. This very effectively camouflages them in the dappled light of the rough undergrowth in which they hide away in during their early weeks.

The first signs of the autumnal moult are dark brown/grey hairs appearing around the head and neck of the deer. Over the course of a month, the thick hairs (twice as thick as the summer’s), up to 30mm long, push through and over the shorter, finer hairs of the summer’s coat.

These ‘winter’ hairs are stouter than the ones they’re replacing and contain an air pocket, which greatly increases the insulative properties of the hair. Studies by Parker and Robbins with other mammals that grow a similar coat to that of a roe deer, suggest that roe’s winter pelage is likely to be twice as good as an insulator, compared the summer coat.

Spring moult

This occurs during March, April and May, and is a far more dramatic and obvious event in the life of a roe deer, compared the relatively subtle transition that occurs in September and October.

As with the autumnal moult, it’s the youngest deer than change first. Again, the oldest deer could easily be a month behind the (now) yearlings. [Traditionally, last year’s fawns become ‘yearlings’ in the spring, even though they’re not actually 12 months old.]

And just like in the autumn change, the first signs of the spring moult appear on the head and neck. But this time, large clumps of hair fall out, revealing the first of the light brown summer pelage. Sometimes, particularly on the neck, large bare patches of skin appear, where bundles of the old, winter hairs have come away, but their replacement hairs have yet to appear.

The weather can still be rather inclement in this time of year too, so this is very difficult time for the deer, particularly for the youngest individuals. Not only are they the first to moult (when the weather is most likely at its worst), but their surface area to volume ratio means they will lose a higher percentage of their body heat during cold conditions, compared to the adults. And on top of that, the level of their body fat is going to be around 1.6(±0.4)%, which is the lowest it would have been since the autumn.

As a result, this is the season when an underweight youngster is most susceptible to a natural premature death.

Roe deer can look rather bedraggled in the spring, so much so that observers who see one of these unkempt deer midway through his or hers moult, often think there’s something wrong with it. Ironically, these are probably the healthiest of individuals; it’s the deer that are especially late (for their age group), that are most likely going to be the ones that are suffering ill health.

Another difference between this moult and the one in autumn is its duration: the spring coat change, starting on the head and neck, and finishing on the back and loins, often can take 2 months. By mid-June, though, even the oldest roe should be entirely in their summer pelage.

There is a latitudinal difference in the timings: the deer in the south normally moult about a month before those in the cooler north.