how did marlin,dory and nigel come face to face with seagulls that chased them and they got their beaks stuck at the sailboat?

It happened in the Sydney harbor scene in Finding Nemo : Nigel rescues Marlin and Dory, then the two fish end up fleeing with a flock of seagulls chasing them. As Nigel flies off with them, the birds lunge after the group and their beaks crash into the sail of a nearby sailboat.

What caused it

Marlin and Dory were with Nigel after he realized Nemo was the fish Marlin was looking for. When Marlin got excited, the seagulls reacted and rushed in, which kicked off the chase.

The sailboat moment

The seagulls kept pursuing Nigel until they collided with the sail of one of the sailboats, which is the part you’re asking about. In the summaries available, the key point is that the birds were distracted by the fish and smashed into the sail while trying to catch them.

In plain terms

• Nigel helps Marlin and Dory escape.
• The seagulls spot the fish and start chasing.
• The chase ends when the birds slam into a sailboat sail.

If you want, I can also give you a short scene-by-scene recap of that whole harbor sequence.