They aren't eggs. Those are spindle galls and it is the tree that has produced them. Spindle galls are most often created by the tree due to Eriophid mites, a type of arachnid mite that sucks on leaf juices. See picture below of spindle galls.
Spindle galls are formed by the leaves of the tree in response to the chemical irritation caused by the insects when they're feeding on the juices in the leaves. Usually once you notice the galls the insects and/or their larvae have already done their thing and completed their cycle or have moved on to another location - leaving behind cosmetically unsightly galls formed by the trees.
Since the insect damage has already occurred before the gall formed treatment is rarely recommended. To be effective, sprays must be timed to coincide with initial insect/mite activity before gall formation begins. Once galls start to form, they conceal the causal organism and it is too late for treatment. For insects/mites that overwinter on the host plant, if you have a serious reoccurring problem you can spray your tree in early spring next year to lessen the severity of the damage. Contact your local extension office for specific guidelines and recommendations in your area.
Spindle galls on cherry and plum.
