The logo should be rectangular with the image of the bird on the left and the text, "US Birds", to the right. It should be easy to remove the text so that the image of the bird on the left can be used in isolation for the app launch icon and in the Apple and Google Play app stores. The bird picture needs to look good as an app icon, shrinking effectively without losing meaning, and filling the available square space to a reasonable degree.
The bird image needs to be abstract, modern and dynamic, showing motion and grace. It should not be clearly identifiable as a specific bird species but should hint at ambiguity and the need for an identification to be made.
Note that the example fish logo is from the Lizard Island Fish Guide and meets the requirements of being abstract and modern and presenting well in a square format against white and black backgrounds at any scale.
The web version of the field guide can be accessed at
http://www.gaiaguide.info/Group.html?groupId=WnY7uHKh&hierarchyId=1ZWb4Oay
The hummingbird image files are intended to be an inspiration for form and colour.
The hummingbirds pictures in the field guide can be seen at:
http://www.gaiaguide.info/QuickComparisons.html?groupId=Q4qbZK2u
While the hummingbirds have great form and colour, do not feel that they are the only bird species that the bird form can be based on. They are just for inspiration.