Key behaviours / activities to look out for this month are:
Buck fawns becoming increasingly restless and separate from their mother
Adult bucks becoming very aggressive
Mothering females still with their doe fawns, but their bond weakening
The most significant behavioural change in the roe deer's community during March is the onset of more aggressive behaviour in the oldest bucks, but now in April, there will be major changes afoot in all the deer in the area - male, female, young and old.
I would say that April is the best month to observe and photograph roe deer behaviour, particularly if you’re looking for variety.
First of all, the youngest bucks will start to break the bond with their mother; preferring to spend time in solitude and exercising territorial tendencies, by chasing other similar sized (or smaller) males away from their preferred patch.
Just as it was with the oldest males back in March, the critical day is when their antlers are fully grown and the velvet starts to peel away.
When this happens, the strong bond between him and his mother (which would have already been weakened) will be decisively broken. And just as it was with the adults, he will be particularly aggressive, possibly engaging in head-to-head combat, even with a larger individual, who will invariably win the contest.
The coats of the deer can look particularly untidy right now. This is the time of their major, annual moult. It's not uncommon to see patches of bare skin exposed, where a clump of hair has fallen out and the new growth has yet to replace it. The deer moult in age order, youngest first.
If you would like to know more about this, you may like to the read the article called, "When do deer moult?", found in the FAQ section.
Adult bucks will see elevated levels of aggression, too. Spring is the time when the roe deer's range will be parcelled up into territories, and therefore boundary disputes will be common between similarly matched males. Early morning and late evening are the most reliable times to observe these interactions.
Once more, I would say that if you want to maximise the likelihood of photographing these behaviours, it helps to be able to interpret what's going on around you, which will improve your field craft and allow you to more accurately predict how a given scenario is likely to unfold.
Again, the article "Roe deer mating behaviour and the rut, part 1" could prove useful to you, in this respect.
As I'm sure you've realised, the ability to photograph wild deer will depend to a large extent on your field craft; if you take the time to learn about your subject, both in general terms (i.e. learning about their behaviours and biology, etc) and specifically (i.e. getting to know the animals at your site(s)), your success rate will increase, I promise you that.
Another behaviour that will occur with all the bucks that still have loose skin hanging around their antlers, is fraying.
Fraying is where a male will rub his head up and down a whippy piece of vegetation in an effort to remove the annoying remnants of the velvet. This action has the unfortunate effect of removing the bark around the stem, which damages the plant, much to the chagrin of gardeners and foresters.
Fraying and rubbing their head along the stem also anoints the plant with the scent of the deer, therefore acting as a territorial marker, which all male deer will detect, and often overwrite with their own.
With the elevated level of testosterone coursing through his veins, a male will often appear 'frustrated', and he will take out his aggression by violently thrashing his head on surrounding vegetation, or engage in mock battles with low-lying tree branches.
If carried out in the presence of another male, these actions act as 'displays', or ‘shows of strength’ to the potential rival, warning him of the wrath he will be subjected to if he doesn't retreat.
In addition, such behaviours allow the buck to strengthen his physique, by building muscle in key areas, like the neck and legs, which are essential to successful buck-on-buck combat.
When it comes to the parental does, they will normally still be with any female fawns that they may have, and will still show frequent signs of affection by mutual grooming.
However, their bond will show signs of weakening, not so much from the youngster, but from the adult; as she sometimes appears to allow herself to be distanced from her daughter, something that she would have been reluctant to let happen earlier in the year.
By the end of the month, as fawning time approaches in May, the distance between the mother and a persistently clingy doe fawn (which could now be called a yearling), will increase, as she becomes increasingly cantankerous.
Non-parental females, if they are pregnant, will start to seek out suitably secluded fawning grounds.
Those who are not pregnant, will often maintain a loose relationship that they may have formed with an adult buck during the more social time in the deer community, during the winter.
But whether the 'relationship' is allowed to continue depends on the character of the male; some individuals are more solitudinal than others.