[annie123 (OP)]

Sitting by the ocean I often see crows and seagulls chasing an eagle. No nests nearby. The eagle ends up perching on a pole of some sort and the others just circle and then go away. Why do these smaller birds chase the eagle?

[alancalverd]

Because eagles, kites, buzzards and the like, prey on smaller birds. It's something of a "bomber v fighter" combat: if a bird of prey can catch a small bird unawares on the ground, it's an easy lunch, but once airborne a couple of more manouverable birds can confuse and attack a heavier beast with a large turning circle. Once the larger bird has perched it is no longer a threat as they generally need to dive to gain initial flying speed whilst the shorter-winged corvids can climb rapidly from a standing start.

Crows are particularly good at close combat in groups - highly intelligent and sociable, and they spend a lot of time practicing close formation flying in thermals. Seagulls I think are less aerobatic but very aggressive and fairly heavily armoured compared with crows. 

The unequal fight seems to be between sparrowhawks and pigeons - the hawk appears to single out its prey in flight and strikes with a slashing dive, too quickly for the pigeons to organise a defence. My garden is often littered with pigeon debris.


 

[pensador]

Eagles can take out bigger prey also. I witnessed a pair of sea eagles force a parasail to the ground in north wales of a few years ago. They hung from the shoot collapsing it, forcing the para sailor to land. Once landed they left him alone.

[cleanair]

I've wondered the same. I thought that crows may be jealous of bigger birds of prey. I've seen them copy the sound of a bird of prey above a young rabbit that was hiding in the grass (in a forest).

@alancalverd's answer may be more realistic.

[RD]

...  I often see crows and seagulls chasing an eagle ... Why do these smaller birds chase the eagle?

A free ride ? ... https://boingboing.net/2017/05/02/crows-atop-other-birds.html

[Monox D. I-Fly]

Seagulls I think are less aerobatic but very aggressive and fairly heavily armoured compared with crows. 

"Heavily armoured"? Do birds have thick hide or hard feathers?

[evan_au]

Around my way, Indian Mynas (an aggressive introduced species) attack other birds in the area.

[Bored chemist]

Seagulls I think are less aerobatic but very aggressive and fairly heavily armoured compared with crows. 

"Heavily armoured"? Do birds have thick hide or hard feathers?

Feathers are made of the same stuff as rhino horn...
I understand that "feather armour" was a  real thing.

[alancalverd]

A seagull's beak is the nearest the avian world has got to a Browning 303.

[Monox D. I-Fly]

Seagulls I think are less aerobatic but very aggressive and fairly heavily armoured compared with crows. 

"Heavily armoured"? Do birds have thick hide or hard feathers?

Feathers are made of the same stuff as rhino horn...
I understand that "feather armour" was a  real thing.

I thought only peacocks have sharp feathers...

[Colin2B]

A seagull's beak is the nearest the avian world has got to a Browning 303.

So that’s what nearly took a pasty out of my hand on St Ives prom! An avian sharpshooter.

@alancalverd's answer may be more realistic.

I agree. The mobbing is to take a way the element of surprise and drive the predator away. Sometimes the mobbers will vomit of defecate on the predator. I’ve also seen corvids mobbing cats and foxes.

[cleanair]

There are also birds of prey that behave like crows:

Flying Devils

At a later moment in the video the NGC reporters video their behaviour in their camp. The camp is destroyed in minutes.