Grass quality.
Geese are great grazers, maybe you keep a few to stay on top of the mowing. Goslings spend most of their time eating when they aren't sleeping.
As the summer progresses, the nutritional content of the grass gets lower, so grazing birds will seek out the youngest growing shoots. Often you'll see the grass velvet-short close to the pond, but going to seed not far away.
You can help out by mowing the longer areas to encourage them to graze further afield.
