A gull drinks seawater but drips brine from its nostrils. Why there?

Answer: Nasal duct plumbing

Nasal duct plumbing ✓ — Nasal duct plumbing is the mechanism. Seabird salt glands sit above the eyes, pull sodium chloride from blood, and send concentrated fluid toward the nasal cavity or bill. The runny-nose look is head plumbing that exports salt while preserving useful water.

Feather sweat glands — Feather sweat glands borrow a mammal-like idea. Feathers are not the outlet; paired skull-region glands do the filtering. The useful contrast is that seabirds are solving desalination, not cooling, by routing salt into a small head-plumbing system.

Food coming back up — Food coming back up is the wrong picture. Seabirds do something more selective: salt moves from blood into a glandular route near the nostrils. That is why the drip is not just swallowed seawater returning, but a small export stream for excess salt.