New arrivals still continue to show up.
While some adult birds are still tending young,
… others are starting to moult. Bird feathers get damaged and the process of shedding the old to make way for the new is called moulting. Not all feathers are replaced at once and this can occur more than once during the year.
In the case of this Mallard you can see the new primary and secondary flight feathers growing in. This adds some literal meaning to the idiom “sitting duck” when you cannot fly. For this reason some water birds will stay in large groups for safety while moulting.