United Airlines blocked a passenger from bringing her emotional support peacock onboard a flight departing from Newark Liberty International Airport this weekend. The airline explained that the peacock “did not meet guidelines for a number of reasons, including its weight and size.”

Read: Delta’s New Emotional Support Animal Policy Will Make It Harder to Travel With Your Pet

While United expressed an understanding that some passengers need assistance from an animal while traveling, the airline noted that passengers are required to “provide documentation from a medical professional and at least 48 hours advance notice.”

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This incident follows an announcement by Delta Airlines earlier this month that it would begin implementing stricter requirements for emotional support animals. Delta had seen a 150% increase in animals onboard flights since 2015, while the number of animal incidents in-flight has increased by 84% since 2016.

With more and more passengers trying to bring a range of animals onboard flights, other airlines may follow Delta’s lead. For its part, United noted that it had explained to the peacock owner three times prior to her arrival at the airport that she would not be allowed to fly with the bird.