I am lucky enough to be able to monitor 2 nesting Kestrels sites, one is in a nest box and the other is a natural hole in the side of a quarry.
The Beckton Sewage works pair, have been in the box for 4 years to date if my memory serves me correctly, they are resident all year round and roost in/near the box outside of the breeding season also.
They do not face any challenges to the nest site, other than from Feral Pigeons, a couple of times I have seen one of the Kestrels, turf them out after the Ferals showed too much interest.
Crows/Magpies are a problem, especially on fledging if juveniles ground, thankfully there are a lot of Thameswater staff keeping an eye out for them.
Male with prey
Female clashing with a Crow
Female fluffing herself up to make herself look bigger
In contrast, the Quarry Kestrels have their hands full when it comes to approaching breeding; this is in the shape of Jackdaws.
The Beckton birds hold to the nest box all year round, in contrast the Quarry birds turn up when the breeding season starts, although I suspect they do roost there.
Jackdaws are feisty and can hold their own against the Kestrels, over the last 3 years I have seen some right scraps between them; a nest hole is worth fighting for.
I would turn up sometimes and find Jackdaws in the nest hole and then at other times Kestrels.
There are other Jackdaw pairs along the cliff face and bickering often occurs, if they get too close, especially from the male Kestrel.
Defending the nest hole
Jackdaw in nest hole
However on both sites they have again succeeded and have young with fledging not too far in the future, hopefully it will go smoothly.