Updated March 2012.
For frame-filling images of your subject you will need a telephoto, a 300mm on a cropped sensor DSLR at the very least. For general views and group shots, something shorter is fine. A 70-300mm zoom is a good start.
Don't forget to take a pair of binoculars and pack your tripod as camera shake can be a problem for longer lenses and as you can end up waiting a while for shy ducks to swim back into frame, your arms will soon get tired supporting a camera with a larger lens. A light-weight model such as those available in Vanguard's Nivelo range
won't take up too much room and they're not too heavy so are easy to transport.
Late Or Early?
Getting out there early or staying out late has advantages. You will get much better looking light and the birds will be more active. If you can, getting into position before sunrise is recommended so you get a good location while it is still quite dark.
Where To Go?
There are ducks, geese and swans at the local park, duckpond, river or reservoir waiting to be photographed or you can try your nearest Wildfowl & Wetlands Trust venue. See WWT
for details. There are nine locations to consider but if they are not convenient, there are plenty of nature reserves that offer similar opportunities.
What's The Weather Going To Be Like?
Before going anywhere, it makes sense to check the weather forecast the night before. If it is going to be dull and very windy, you might want to postpone the shoot until there is the chance of good light and the wind is less fierce.
By the way, birds take off into the wind, so that is worth bearing in mind when you are getting into position. If the conditions are suitable, there is the chance of backlit take-off shots too.
Be Quiet And Patient
If you're taking photos at a local park where ducks are often fed by people you should be able to get close to them without much of a problem. However, if you're shooting at a location that's not as popular with families who feed them you'll have to be a bit more patient. Move around slowly and keep low so you are less threatening to the birds. If there are hides available go and set up in one and wait for the wildfowl to swim / fly back into frame once they no longer feel threatened by you.
Expose With Care
When the sky is bright the meter will be fooled into thinking there is a lot of light around, and underexpose the birds. That is fine if you want silhouettes but if you want bird detail you might be better off using manual metering and taking a reading from the ground.
If you find your subject's feathers are lacking in detail try adding a little fill-in flash.
For gloomy days, switch to a slightly higher ISO so you can use a quicker shutter speed.
In terms of focusing, your DSLR's autofocusing should be fine with birds providing the active sensor is kept trained on the bird itself. If the sensor moves onto an area of sky, the AF will keep searching and you will end up missing shots.
Panning or tracking a bird in flight with a telephoto lens is a knack, so do not expect to get it right first time. However, practice will make perfect.
While you are panning you will need to get the focus locked on your subject straight away and use continuous focus as you pan to keep your subject sharp.
Shutter speed choice depends on the effect you are after. For sharp shots, 1/500sec is fine but try slower speeds too for some deliberate blur.