iPad Unable to delete individual events on iPad calendar

I wonder if these birthdays you wish to delete are in the Birthdays calendar. You will be able to see this quickly by looking for the little red birthday present icon to the left of the birthday person's name in the calendar event.

You can see if you have the Birthday calendar active in the Calendar app by touching on the Calendars label at the foot of the screen. This will reveal of list of your calendar names, including those in the category called 'Other' at the bottom of the page.

The Birthdays calendar in this category is generated from the field in your Contacts app. Because it is a generated calendar you cannot edit the events it adds to your Calendar app. The only way to remove it is to delete the birthday for that person in the Contacts entry.