The Limits of Flight

The African Goliath Heron, Ardea goliath, is one of the world’s largest flying birds. It weighs up to 5Kg (11 lbs), is 1.5m (5′) tall, and has a wingspan of 230cm, or 7′ 7″.

This one stood in the Gambian mangroves as we drifted along, looking at something overhead:

They hunt passively, waiting for an unwary fish to swim by, and then they pounce with that lethal beak.

They only eat 2 or 3 fish a day, but big ones, over a pound each, so they gobble about 25% of their body weight a day. And they can live for 22 years.

The full glory of this huge bird shows best in flight, I think:

How on earth do they do it? Rayner (1988) is a fascinating article on the aerodynamics of bird flight. Herons have large wings in comparison to their load (weight divided by wing area), which puts them in the same group as thermal soaring birds like hawks, storks, and eagles. They have similarly shaped wings too, broad with squarish tips, and separate primary feathers.

The surprise is that the Goliath Heron can fly at all. It is distinguished by having a very slow wing beat of 1.8 beats per second, and this puts it at the outer limits of all birds and near the theoretical limit of possible flight (apologies for the poor quality of the graphic, I couldn’t find a better one; the vertical axis is wing-beat rate, and the horizontal axis is mass):

PS That very slow wing beat is why I was able to get sharp flight photos even from a moving boat. Here’s another one, just taking off on a different flight, though quite possibly the same heron:

As you can see, gaining height takes time! The jumbo jet of the avian world.