Spring is most definitely in the air.
Looks a little rough, but trust me, this is nothing unusual and its natural, in more extreme circumstances i've seen as many as eight males on one female. The females seem to test the males by making them work for it very hard in competition, from chasing her around to test their fitness (ie : if you can't keep up you don't get mating rights) to inspiring combat in other males. Its very easy to assume this is all purely about male aggression, but TBH ive been watching these guys long enough to know the females are submitting by and large, in straight one-on-one fights with males they often win, and they put in formidable shows of strength and intimidation on other species where room for nesting and raising ducklings might be at a premium.
When it comes to birds fighting, the will to win seems to be instinctive, even perhaps dictated by hormonal state, much as does submission, its very rarely if ever more than a transient state of being. Birds arent really what you'd call generally ill tempered, it all goes round by date and ebbs and flows, even in a mixed species community of birds. The balance of power seems dictated largely not by size, or even strength, but conviction. One day you will see swans being belligerent and ruling all, another day it will be common shelducks, another day a tiny little brown duck or a pint-sized moorhen will scatter all before it, including swans, and the sex of the animal seems almost irrelevant! I've seen days where herons and gulls , and the usual corvids put the fears into all the other birds, but then swans start heron bashing, ducks beat up gulls, and another wader like lapwings suddenly grow a severe temper and bully whole groups of crows or jackdaw all on their own.
The female mallards seem to encourage some of the chasings and mating/beatings they do get, and even seem to shoot off in an uncharacteristically manic waywhen not persued, as if to stimulate the males to actually give chase. Many is the time ive seen a male just feeding near the female, being generally a bit ducky, paying her remarkably little attention, and then she shoots off like a whippet with a bumful of mustard, he looks around confused and is thinking,, "not again...!". lol.
Occassionally it does get out of hand though,especially for young first timer females, with females becoming exhausted, and very occassionally drowned by the mob of males that harrass them. 99.9 percent of the time that doesn't happen though.
By the end of the season despite usually surviving in reasonable condition, quite a few females will be sporting bald spots on the back of their heads from all the multiple mating grips.
But all in all, thats how mallards stay vibrant and strong as a species, and of course strong parents mean strong ducklings, and evolution is served, no matter how brutal it can look to us, and trust me compared to some lizard courtship, its not even violent.
I've seen people interrupt mating ducks before, and when you see the true scope of their behaviour you quickly realise what a stupid idea it is to do so. Sometimes nature does get a little rough, but never without purpose. By interrupting a mating and thinking your "saving a female from bullying" you might have just sabotaged her days of work tempting in the strongest male or co-operative team of males to provide her offspring with the strongest genetics, and as unkind as it may seem, those who cannot withstand male attention with guile and strength are not meant to necessarily breed or survive, for some a beating is the learning process in its infancy.
We have to learn that sometimes our human principles of being a gentle lover do not apply to other species, and not every fight is a persecution, but a fully intended and gratefully recieved genetic reward for many days of effort, even if it is very rough. Truth is you may even prolong the suffering of the female, for if the mating is unsuccessful, she might have to go through it all again and take more beatings over the coming days, and you might literally have put her in a life threatening position, for you cannot be sure of the level of her energy reserves and how battered she may be under those feathers.
Her sexual imperative will drive her to encourage matings however battered she may be and that will only cease when she is fertilised and she stops giving out hormones and behaving like she wants to mate, and thats not a conscious decision but an organic state. The mating you interrupt could have been her last one, and the day after if not interrupted, she might be have been safe, all rough courtship over, and nesting. Her behaviour will have changed, not running, not encouraging chase, not giving out courtship signals or even mixed signals, but directly confrontational, defensive, unsuboordinate and directly aggressive, and as I mentioned before, thats really all it takes to win with birds, and few males would have bothered her.
Female mallards as with many other birds might act either independant or monogamous in raising the young, but even if an attendant male hangs around, it wont necessarily be the father, female mallards hedge their bets with multiple partners and matings, that way brains is served as well as brawn, they know what they are doing. Its easy to pity a female duck taking a rough mating, but much of it is by the design of herself and her forebears. The mallard is one of the most common ducks around, its superbly designed for survival.
As always with nature, its better just to watch quietly, document if you wish in whatever format you like, and not interfere. Apart from preserving wetlands and the animals foodsources, and protecting them from chemical spills and invasive alien species, and all the other artificial harms we humans inflict on them they are best left to govern themselves as they always have, they have been doing it for millions of years prior to our race even evolving. Mallards do not need protection from mallards.