There is a bit of a paradox about volunteering. You give your time and get no pay, yet so often you get more out of it than you give.
I work full time, so my nights and weekends are precious free time. Yet I am happy to give up that time for volunteer work; in most years, it averages out to 8 hours worth of volunteer time each and every week.
It's no secret that we're screwing up the planet pretty badly, and the wildlife of the world has even less say in the matter than we ordinary people do. But while we may not be able to do as much as we'd like to stop the despoilers and developers, we can do more to help birds and other wildlife.
While it helps to have some experience as a birder to get involved, you do not need to be an ace; most programs provide training to make sure that everyone is working consistently. What's most important is your willingness to make a commitment and stick to it. If they are going to invest effort in training you, they need to get something back. That said, there are plenty of opportunities for one-day activities (especially in habitat restoration and citizen science), so even those of us with less time to spare can still make a difference.
Wildlife Monitoring Long term monitoring is crucial to understanding what is happening in response to habitat loss and other environmental factors. It's not all bad news - monitoring can also show the growth of populations after restoration efforts. Monitoring takes many forms: Breeding bird surveys, beach surveys, banding and mistnetting operations, migration count sites and individual species monitoring (e.g., Snowy Plovers on western beaches).
Citizen Science Maybe you don't have the time to commit to a regular volunteer schedule, but you still want to help with monitoring. There are many other opportunities, like the Great Backyard Birdcount (happening this weekend), Project Feederwatch, and the Christmas Birdcount among others. These allow you to contribute on a more flexible schedule and even from home (great for those with limited mobility). And of course, you can turn every birding trip into data collection by recording your sightings through eBird.
Habitat Restoration One of the best things you can do to help birds is to give them a place to live. You can do it on a small scale by planting local natives in your yard, but the real impact comes with restoration of larger areas. Restoration efforts involve a lot of labor - removal of structures and debris, weeding, digging and soil prep, weeding, collecting seed stock, growing new stock, weeding, planting, tending until plants are established, weeding, and more weeding. Schools and corporations like to do the planting part of it, because it feels great to put a little tiny thing in the ground and watch it grow over the years. It's also a good one-day activity. But none of that happens without the people who make the long term commitment to pull these projects together. Be one of those people if you can.
Outreach and Docenting Sharing your love of birds or wildlife or a favorite wild place is something you can do to help them survive in the future. Leading walks and tours, talking to school groups, serving as a docent at a refuge or working at the local nature center - all of these help open people's eyes to the world around them and let them know that there's something here worth fighting for. It's even more helpful if you are fluent in other languages and can reach out to different groups in your community.
Rehab This is one of the most direct ways to help wildlife, but it's also the most demanding and heartbreaking. Helping a hawk who's been hit by a car, a songbird that smacked into a window, a clutch of baby birds whose nest was blown from a tree - it all takes special training. After an oil spill, people want to help wash ducks. That's great, if you already know how to wash ducks. But organizations don't have time to teach duck-washing during a crisis. If you want to learn to wash ducks (or rehab hawks or feed baby birds), contact an organization now and go through their training programs so that you'll be ready for the next crisis. Also important to know: Many of the patients don't make it. A bird that you can catch to bring in for treatment is already in pretty bad shape. The success rate continues to improve, but if you can't bear to watch an animal die, this is definitely not the volunteer opportunity for you. Which brings me to...
Grunt work You want to help those oily ducks and injured hawks, but you haven't had the chance to attend a training? They still need a lot of help at wildlife facilities. Cleaning ducks means a lot of filthy towels that need washing, a lot of feeding tubes that need to be filled and cleaned and filled again; floors need to be mopped, perches need to be scraped. It's not just wildlife care facilities that need grunt work - parks need trail maintenance, habitat projects need dirt shoveled and compost turned, visitor centers need boardwalks repaired and floors swept. Get physical. Or don't....
Office Work. Someone needs to answer phones when everyone is volunteering to wash ducks. Someone needs to coordinate all of the local bird walks and monthly talks for the chapter meetings. Someone needs to put together the spreadsheet tallying the Christmas Count. Someone needs to write up the results from the Beach Cleanup. Maybe that someone is you.
A few resources to suggest:
National Audubon Society/Local Audubon Chapters. Your first stop to find out about volunteer opportunities in your area should be your local Audubon chapter. They may sponsor some activiites, and they are likely to be aware of other organizations in your area. They are a great resource. Find your local chapter here.
Volunteer.gov is a great portal to all sorts of volunteer opportunities.
UPDATE: One other place to volunteer - except you're not doing it for free, you're actually paying to take part: Earthwatch. You serve as volunteer labor on scientific projects all over the world; Earthwatch handles travel logistics (other than you arranging to get to the starting destination) and much of your cost is actually tax deductible as a contribution. In return, you are working up close with knowledgable researchers and amazing animals in incredible places. I studied macaws in the Peruvian Amazon (see above), but there are projects monitoring penguins, loons in the Canadian north and lots of big cats to follow. Check out their current list of wildlife projects.