Binoculars are notated as y times (x) z where y is the magnification and z is the diameter in millimetres of the objective (front) lens. For general bird watching 8-10 is a good number for magnification, this will give you a reasonable balance between field of view (being able to find the bird in the binocular view) and being able to see more detail. A larger number in the 'z' portion of the description will enable better use in low light e.g dawn or dusk but will increase the size of the bins and their price. Zeiss, Leica and Swarovski are all good (but pricey) makes, check the RSPB for their deals as well.