Hi Niels,
when they achieve surprise is to me the point, flocks of healthy geese should be able to avoid most such surprises.
Admittedly, I have no idea how often the eagles' attempts fail
My impression from observing their flight patterns is that eagles apparently like to check out interesting birding sites from some height and a fair distance, and then come back later, approaching at a fairly low altitude using ground cover (like dykes) to break the potential prey's line of sight.
I'm not quite sure about the height of the dyke in the case of the attack I witnessed, but it might have been 5 m above the geese's eye level, and the geese were about 150 m from the dyke. That equates to a glide ratio of 30, so the eagle could have stayed below the horizon from the geese's point of view only in powered flight, but the gradual altitude loss would still have helped it to keep the speed up.
The time it would have taken the eagle from appearing over the horizon to getting into striking distance to the geese might have been as little as 10 seconds, in which the geese had to recognize the eagle, launch themselves into the air, and gain enough speed and altitude to be able to evade the eagle, who would have come in with enough speed to be quite manoeuvrable.
(There's a biological law that leads to smaller organisms having a better power-to-mass ratio than larger organisms, everything else being equal. I'm pretty sure geese can out-"perform" eagles in the long run due to this, but in the short term, the eagle can use its momentum from the fast approach in lieu of muscular power.)
If the geese manage to evade the initial attack and the eagle expends its momentum, I'm pretty sure they will be able to evade any further attacks easily.
However, while birding, I've more than once been surprised by an eagle "out of nowhere" pretty close by. Good moment not to be a goose! ;-)
Regards,
Henning
