The Angel Experiment ends in an open-ended way, with the whole flock flying off to Washington D.C. in order to find their parents. It's the sort of ending that makes perfect sense for the first book in a series, and Max sums up her complicated feelings about their next adventure the following way:
When we got to Washington DC, it would be either incredibly great or a totally heartbreaking disaster. (E.7)
She's setting the reader up for all of the ups and downs that are to come later in the series. Things could go great… but there will probably still be some obstacles and unexpected dangers thrown into the mix. That's okay, though, because this flock is more than capable of handling it.