Jack Horner's argument states that cold-blooded creatures such as crocodiles grow about 30 centimeter per year. Even the fastest growing cold-blooded species would take a year to reach the size of the juvenile hadrosaurs discovered.

It would be very unlikely that these dinosaurs remained in their nests for such a long period of time. What is more probable according to Horner is that these creatures were warm-blooded. Their growth rate being compared to that of an ostrich, one of the slowest growing warm-blooded creatures. An ostrich achieves a length of about one meter in six to eight months. Since the young Maiasaurs seemed able to grow this fast or faster, Horner and many others believe it is very likely that they were warm-blooded creatures.