Geese help great egrets catch voles

This video says about itself:

A flock of White-Fronted Geese (Anser albifrons) landing on a small lake in a part of the Ooijpolder called ‘Groenlanden’, near Nijmegen in the Netherlands.

Translated from Dutch daily NRC:

Great egrets prefer to be among flocks of greater white-fronted geese. Because that improves their chances of catching common voles; probably because the noisy geese make the egrets harder to detect. The biologist Dirk Prop writes this in the April issue of the Dutch ornithological review Limosa.

During visits to the Eempolder near Baarn, Prop discovered that conspicuously many great egrets (Egretta alba) kept close to flocks of grazing greater white-fronted geese (Anser albifrons). Sometimes, over twenty of those striking white herons were between the geese, though in all of the Eempolder there were probably not more than forty great egrets. Over half of the great egrets which Prop observed through his binoculars during January were close to geese.

Prop observed groups of egrets both with geese and without geese, and he concluded that egrets with geese catch many more common voles than those hunting dispersed across the Eempolder: on the average 3.3 vole an hour versus 1.8 vole an hour. In the winter, the common vole is important prey for the great egret.

Prop does not know exactly why the egrets catch more if they are close to geese. He suspects that the voles have more trouble detecting stalking egrets because of the noise made by the white-fronted geese, sometimes over a thousand of them. A strong argument for this is that the egrets are hardly attracted by groups of mute swans and Bewick’s swans, which are far less noisy.

Egrets do not always follow the flocks of geese, Prop discovered. If the geese go grazing in the central parts of the meadows, then the egrets stay away. That is very efficient, Prop writes, as far less common voles are there because of more water in those parts.

The egrets’ preference for hunting in between geese is also recorded elsewhere in the Netherlands, like the Ooijpolder near Nijmegen. In southern countries, egrets prefer to be among cattle, looking for insects disturbed by the mammals. The grey heron (Ardea cinerea) seems to have discovered the ‘goose effect’ as well.