That is indeed very frustrating! Sometimes plums need a spraying program to combat various worms (insect larvae) that attack the fruit. If you are interested in pursuing that, you can check with your county’s Cooperative Extension as to recommendations. To get started, visit this page to find contact information for your area:
https://www.almanac.com/content/cooperative-extension-services
For birds, you might try bird netting, available at many garden centers. Install this over the entire tree (easier with dwarf fruit trees) and tie shut at the trunk below the leaves, so that birds can’t get in underneath the netting. Be sure no fruit is right against the edge of the netting.
A few gardeners just place mesh bags over small fruit clusters that are developing, if it is a small garden. Some people also hang among the branches strips of special metallic-looking tape made for this purpose, or similar items, with mixed success. Others use items made to look like owls or owl eyes–also with mixed success.
Hope this helps!
