The easiest and most reasonably priced gift is a hat. Not only would this gift can help prevent the glare of the sun but also protect them from its rays.
Another idea could be to find a pair of 'combat' pants which has a lot of pockets as they can accommodate many items conveniently.
Another cool item which has even more storage is a bird watching vest. For easy access to a cellular phone, water and map etc. these vests cannot be bettered.
When they've a backpack that's worn out, it's a simple matter buying a new one for them. There are plenty of sizes available in backpacks; a heavy duty one is not really required as these people should be traveling light.
The gift which costs most for a birdwatcher is a pair of decent binoculars. This is a costlier present because the quality lens which enables the birdwatcher see farther is expensive. Current technology has meant huge improvements in binocular design so it is possible to buy models that can be used at night and others where they behave like digital cameras and take photos.
Ornithologists normally document what varieties of birds they've found, in what numbers and particularly where they were seen; so a good gift idea could be a bird watching journal.
If none of these ideas work for you, have you ever thought of buying an ornithologist a one year subscription to a bird watching periodical? These are beautiful magazines published monthly (or quarter) and bear information and pictures of birds which live in remote and exotic places.
How about making a collection and buying a bird watching holiday for your friend to send them someplace they have not been like South America or Canada for instance; providing you've that kinda money! It is important to remember that there are thousands of species of birds in this world; several do not find the climate of US a suitable.
Unless you talk to the recipient first and interpret their needs, you can never determine which gifts would be most appreciated. A short conversation will already give you a lot of ideas, and then you barely have to pick the right one that's within your budget and meaningful to the person.
After the time spent finding the right gift, it may light a desire in you to start bird watching in the future. Rather than just supporting that person, you could soon join them on excursions!