European Hornets in Tree in My Yard ....YIKES!!!!
European hornets are predatory wasps and considered beneficial for controlling pests in our yards. If possible, they should be conserved. They are not generally aggressive unless they are defending their nest.
There could be a hollow void in the tree that they are using for a nest, or they may be feeding on honeydew from scale insects, or other insects that are also there.
Here is our page on Predatory Wasps: http://extension.umd.edu/hgic/insects/predatory-wasps
and European Hornets in particular: http://extension.umd.edu/hgic/european-hornets
If they are in an area that you can just leave them alone, we suggest that you do so.
If there is an actual nest and you want to get rid of them, we suggest the help of a professional.