Birds are among the most intricately patterned and ornate creatures on the planet and to get the most from your birding experience you will undoubtedly want to see them as well as possible. To this end, go with a binocular with sufficient magnification (first number) and aperture (second number) to render bright images that reveal sharp detail and accurate color.
While compact binoculars are small and portable, they're not generally recommended as a primary birding binocular; they don't gather enough light to discern plumage subtleties and feather color. Because of this, the overwhelming majority of birders agree that the best combination of magnification and aperture for watching birds is 8x42.
What does this mean? Objects you see through the binocular are 8 times larger (or closer) versus the naked eye. Dividing the aperture by magnification (42 divided by 8) gives an exit pupil of 5.25mm. This is the ideal number. While a 10x42 increases magnification it also decreases the exit pupil (4.2mm) and therefore won't be as bright in low light or render color as well.
To make a 10x binocular as bright as an 8x42 requires an increase in aperture size, but a 10x50 adds both size and weight. This underscores why 8x42s have become so popular for birdwatching; they're the best combination between magnification, physical size, and light gathering, while making virtually no compromises.
Today, most binoculars are waterproof and nitrogen purged for fogproofing. If you plan on birding around wetlands or intend to watch birds in diverse weather conditions and climates, you want to make absolutely sure your binocular has this feature. Because birds are often in difficult-to-reach locations, accidents can happen, so durability is critical. Even so, binoculars can break regardless of how expensive they are. Therefore, make sure you find out what type of warranty coverage the manufacturer provides. The best plan is a life-time warranty with no-fault coverage that protects your investment even from accidental damage.
Another feature of a good bird watching binocular is a relatively close focus. While all binoculars can focus to the horizon, the closest point to which they can focus will vary from model to model. An ideal birding and nature binocular should be able to focus on an object within 10 feet. If you enjoy looking at butterflies and other tiny critters and plants, consider one of the many models available that can focus down to 5 feet or even closer.
There is really no such thing as beginner, intermediate, or advanced binoculars. The price you pay will have a direct correlation to how well you're able to see birds. Binoculars with high density glass that have quality multiple anti-reflective optical coatings are more expensive, but shouldn't be thought of as advanced. In truth, the beginner birder would also benefit from being able to see birds better, which is exactly what high quality binoculars provide.
Link: Recommended Bird Watching binoculars at Eagle Optics.