The mild degree of swelling on her middle toe could be correlated with simply soft tissue damage that occurred after tripping over an object or getting her foot caught in something which resulted in damage to some of the soft tissue.
I would examine the area thoroughly to ensure the issue is not more severe such as a broken digit. This can be done by gently bending the joints listening for abnormal popping sounds, cracking, or simply comparing the affected digit to the opposing ones for abnormalities.
Considering what you find does not need immediate veterinary attention, I would suggest starting her on an antiinflammatory medication, specifically those of the NSAID class to help relieve some of the inflammation that is occurring. Asprin can be used at a dose of 5 milligrams per kilogram of weight, orally, once a day. If available, Meloxicam, or carprofen would also prove useful. Additionally, restricting movement to soft grounds, increasing time spent in water, and simply giving the injury time to heal will be conducive to healing.
These injuries are very common with waterfowl and generally resolve themselves quite well.
