Not the easiest question to answer really - depends on a few things. Most of all, it's going to depend on your budget if you're just starting out.
You say you've no idea, so let's break it down to the real basics. Take for example a lens like the Canon 70-200mm f4.
70-200mm is the focal length of the lens: a measurement of the zoom range. The higher the closer your subject will appear, so this particular lens will zoom from 70mm (kind of a good portrait range) to 200mm (moderate telephoto range - you'll be getting much closer to distant subjects). A lens of this sort of range is probably pretty good for what you're looking for, particularly as you'll be using a cropped frame sensor* and you'll get a bit more reach out of your zoom.
*A full Frame sensor is the same size as an exposure on an old-school 35mm film. Cameras with sensors that size are really expensive, and I'm assuming that if you're having to ask what lens to buy then you've probably not just spent £1.5k on a camera body. Most entry level digital SLRs have a smaller sensor, which essentially has the effect of only taking the centre part of what would be exposed on a 35mm frame. This means that you are, in a way, getting more reach out of a lens of a cropped frame sensor than you would have in the days of film. Or with a fecking expensive pro body.
f4 is the speed of the lens. The f number of a lens refers to the size of the aperture that lets the light through to the sensor. The larger the aperture, the more light gets in and the faster your shutter speed can be. f4 is kind of middle of the road: not very fast, but fine and dandy for shooting in daylight. Fast lenses both cost and weigh a heck of a lot more. The faster version of the 70-200mm weighs and costs over twice as much as the f4 version. Sometimes lenses have a rating that reads something like f3.5-5.6 - this means that when the lens is zoomed out the maximum aperture is f3.5, and when zoomed in the maximum aperture is f5.6 (quite slow).
Forgive me if I'm telling you what you already know. If you don't and you still don't understand, there's a fuller explanation here:
http://leadinglinesphotography.net/tuto ... re-and-iso (just started writing these tutorials - will try to get more online soon).
Using something about the 70-200mm range you'll get close enough to get some good detail with the ducks, but still be able to zoom out enough to take in a little more of the scene. You can of course go for a much wider zoom range - some lenses these days are giving you a range more like 18-200mm or more. Sounds great, but the trade off is that the image quality won't be the same. That said, if you're just getting started they have some serious advantages....
For a start, you don't have to change lenses all the time, cutting down the chance for dust to get into the camera. aside from the, it's often just more convenient and less kit to carry than taking a selection of lenses with you. If you can get one that does everything and you're not making large prints or selling your work then the compromise might be well wroth thinking about. And for the money, they might well be a lot more cost effective.
I don't know what system you're using, but most manufacturers have an equivalent of the 70-200mm f4. A few generic lenses you might be interested in researching would be:
Sigma 70-300mm f4-5.6
Sigma 50-200 f4-6.3
Sigma 18-200 f3.5-6.3
Sigma 18-250 f3.5-6.3
Tamron have equivalents of them all too, as will whatever camera manufacturer you've bought into. I've never used any of these (except the original 70-200mm f4 we talked about) so can't vouch for any of them, but there's plenty of reviews around the internet.
If you're wanting to take photos of birds in flight or further away then you're going to have to start looking at longer zooms with larger apertures: sheds more cash.
Hope that this helps.