Bird flight altitude

[Tinkerbell]

I wonder if there is much info. on the height birds may reach when flying here in the UK. I looked up skyward yesterday and saw two birds (quite large) which appeared to be flying very high. Is there an optimum for different species?

[nightshade]

I often see buzzards and gulls at little dot heights swifts swallows are also high flyers
swans have been seen at 29000ft. migrating birds seem to use thermals to get very high then descend again to their destination so using less energy each species seems to have a ceiling size would play a part as it is cooler higher up and only large bodies could stay warm perhaps

[Mark43]

Just as an aside the highest recorded is 37000 ft by a Vulture. but as Nightshade says Swans and Geese have been recorded at 28-29k

[Tinkerbell]

Wow!! The two birds I saw weren't flying in a straight line which would possibly be explained by the thermals. I just wish I could have seen them clearly enough to identify. How incredible though that they can reach such heights - surely it must be much colder up there.