I upload a lot of recordings to my eBird checklists. With the recent Crossbill irruption in the Adirondacks and elsewhere I really got hooked on recording pairs for Matt Young at The Cornell Lab of Ornithology's Macaulay Library. eBird has made it easy to upload our recordings.
Recently I was contacted by Martha Fischer, also at the Macaulay Library, who shared some thoughts on recording bird calls/songs with me. She shared this information... "We recommend .wav format. This format is very different from .mp3; .mp3 format was designed for human hearing, and therefore has much less information (the algorithm removes sounds that are outside the range of our hearing) than .wav.
I am using my iPhone 7 with an app called RODE recorder. It seems to serve the purpose but there are likely other apps that use .wav format that would also do the trick. Hope this helps.