No, this is a universal app and works on both phones and tablets. As long as the devices are on the same platform (e.g. iPhone and iPad both run iOS) and both devices are signed in to the app store with the same ID, you will be able to pay once and download and use the app on all of those devices.
Need to provide a spot for individuals to add a species observed not on the app. For example, a Spotted Rail now seen in TX, blown in surely by storms, is nowhere on the app list. Either we should be able to add it or Sibley should update the list to do so!