Birds here are not much of a problem on lychees except for when they get super ripe. So just by picking them before that stage can avoid losing most of the fruits to birds. They often split also when super ripe and then attract fruit flies and other insects.
BTW the Sweetheart seems to be a real winner here. Has fruited 3 years in a row. Most lychees don't perform consistently here due to warm and wet winters. Sweetheart is good, but not nearly as good in my opinion as a Kaimana. But so far it seems to be more consistent bearing here than even the Kaimana.