Is iOS a natural choice for a startup to release their first app?

Depends on the nature of the startup and your development lifecycle. If you do rapid development, I would go with Android first, iterate very fast based on the feedback from your first batches of users, make decisions based on any usability/feature complaints, and then release an iOS app which has maximized chances to do the best it can on the AppStore.

If your app is highly social, a facebook app would probably be even more appropriate, since the first users don't need to update your app to get the new changes.

There's also the fact that most developers are already in the Apple ecosystem - more likely to have an iphone, ipad and most common, a mac laptop.