To add birthdays to your iPhone calendar without adding them to your contacts, follow these steps:

1. Open your iPhone’s Calendar app.
2. Tap on the plus sign (+) located at the top-right corner of the screen to create a new event.
3. Type in the person’s name along with ‘Birthday’ as the event title. For example, “John’s Birthday.”
4. Set the start and end times for the all-day event to mark it as a full-day event.
5. Scroll down and tap on “Repeat” to set the event to repeat annually, ensuring that you never miss this birthday in future years.
6. You can add an alert reminder by tapping on ‘Add Alert’ if you want to be reminded a day before or on the day of the birthday.
7. Once done, tap ‘Add’ at the top-right corner of the screen to save the birthday to your calendar.

Remember that since these birthdays are not linked to your contacts, they won’t automatically update if there are changes. You’ll need to edit them manually if needed.