Key behaviours / activities to look out for this month are:
Fawns become weaned
Reduced activity overall, particularly from adult bucks, but younger males still very active
Mother and fawns frequently seen together
Compared to July and August, there's a noticeable decrease in roe deer activity in September, particularly in relation to the older bucks.
The males' testosterone level is in a downwards trajectory at this time of year and the main function of their territory (which is to maximise the chances of mating) has passed.
The older males will still frequent their core areas though, but territorial behaviours such as scraping the ground and scent marking by rubbing their head along prominent pieces of vegetation, are far less likely to be observed than they were before the rut.
This does not necessarily apply to the younger males though, including the yearlings. These bucks are still quite likely to be aggressive, and can occasionally be seen behaving in a manner similar to the time before the rut. But their 'fights' tend to be more tussle like, and lack the ferocity of the adults' confrontations.
Once again, early morning and late evening are the best times to observe this behaviour, because this is when the bucks are generally at their most antagonistic.
When it comes to the does and their fawns, they are a much easier proposition to photograph than they were in May, June and early July, when the does were especially flighty as a result of her youngsters heightened vulnerability.
Although nearly all the fawns will be weaned by this time, I have seen twins suckling from their mother in early September, so there's still a chance of seeing nursing behaviour - but it's not the norm.
Instead, what you commonly observe is an eager fawn reaching under its mother searching for a milk giving teat, only for the parental doe to step away because she has run dry.
With the youngsters now relatively comfortable being out in the open, it possible to photograph close interactions between the doe and her youngster.
Towards the middle to the end of the month, the deer's coat will start to take on a darker appearance, as longer, thicker hairs grow through their summer pelage; the youngsters change first. For more information on this, you may like to have a look at the article called, "When do deer moult?", in the FAQ section.