The 100-foot abseil, down Yeovil District Hospital, in Somerset, was due to take place tomorrow to raise funds for a new special care baby unit.
But organisers were forced to cancel the event when a seagulls' nest containing baby chicks was discovered yards from the launch site during final preparations.
Health and safety inspectors and conservation experts who visited the site were subjected to dive-bombing from the seagull parents.
A spokeswoman for the Flying Colours campaign, which aims to raise £500,000 for the unit, said the event would take place once the birds had moved on.
"Unfortunately, we have had to make the decision to postpone the abseil planned for Sunday," the spokeswoman said.
"On inspection of the roof ready for Sunday, a seagulls' nest was discovered, with baby chicks inside. The seagulls are fiercely protective of their offspring, and even during the inspection the inspectors were dive-bombed by the seagulls.
"After lots of phone calls, with relevant bodies and health and safety experts, it has been decided to reschedule the abseil for a later date.
"Legally we are not allowed to move or remove the nest, and have to wait for the birds to fly the nest.
"It is a little ironic how we are raising money for small babies, yet it is baby chicks that are stopping the fundraising from happening on Sunday."