For the first few years that I went Birding during spring migration I would get a condition known as warbler neck. This is when muscles in the upper shoulders and neck stiffen up from looking up in the trees with binoculars trying to find warblers and other returning migrant birds. It's no joke. My fingers actually started to go numb one year from a pinched nerve. That's when I decided to rely more on my ears to find birds.
I'm now more selective about which birds I search for with binoculars. For example, I know you can get nice views of Ovenbirds (photo) without having to look in the tops of trees since they stay closer to the ground.
It also helps when you know the songs of some birds so you can zero in on birds that you may be eager to see like an Indigo Bunting.
Birding by ear is more efficient because you don't have to chase down every bird to know what it is. I probably heard a dozen Louisianna Waterthrushes before I actually got a good look at one. Instead of chasing after waterthrushes I spent time searching for other species which I had not yet seen.
I had some time off in May so I tried my best to find as many as the incoming migrants that I could as quickly as possible and was able to find most of what I was looking for. Now I can do my birding at a more relaxing pace. There will be plenty time to take photos, check out different places, and take in the beauty without the urgency of trying to find species I haven't seen yet.
(county list is at 165).