Add Suface Modifier & Meshsmooth Modifier:

Now is good time to add some modifier to see how the final surface looks like:|

1. Make reference copy of your spline.

2. Move the copy to some other location so that it doesn't bother you while modeling.

3. Add Surface modifier to the copy. Make the following settings to the Surface modifier:
Threshold: 0,01 (always use very small value here and create the model accurately)
Flip Normals: if the surface is upside down use this
Steps: 2 (I have found that 2 is usually the best value. If you use larger values here it's very difficult to model without visible seams.)

4. Add Meshsmooth modifier to the copy. Make the following settings to the Meshsmooth modifier:
Iterations: 2 (2 is often enough but sometimes you need 3)
Now the model should look like in the picture below. (If you wanted you could use Symmetry modifier to see also the other half of the model)

The Wings

Start modeling the wings of the duck by creating two circles and attaching them to the duck. Modify the new splines according to the picture below.


Model the lower part of the wing by creating new splines according to the picture below.


Finalize the wing by creating six new splines according to the picture below.



Now the model should look like in the picture below.

The Head

Now the lower body of the duck is complete. Start modeling the head by creating three new splines according to the picture below.


Add some more splines into the head according to the picture below. It should be pretty easy to modify these lines according to the picture if the eye is in the right place.


Now the model should look like in the picture below.