You have to have a field that contains the month. This field does not have to be visible on your layout and you don't have to fill it in manually. It can be calculated automatically using a calculation field.
I just helped someone else with a similar question. She wanted bills listed per month. So you might want to check out my example file in this thread:
The ideay is to go to "File" - "Manage" - "Database"
There you add a new field that's of the type "Calculation". Let's call it "c_MonthName". That way we know it's a calculation field.
When you create that field a dialogue opens up to specify the calculation.
MonthName ( )
And then put your cursor in between the brackets and double click your Birthday field in the top. It will show up in between the brackets and you should end up with something like:
Make sure the calculation result is set to "Text" and you're done.
Now you can create a dropdown based on this c_MonthField. And create a find script that finds you only birthdays in that month.
If you want to do that do be warned that you will need a text field set to global storage to be the field that you use to select a month to search.
But you can see that in my example on the other thread.
Unless you really need to have the birth date split into three fields, create a single date field for Birthday and you do a find for birthdays in June: (assuming American dates) go into Find Mode and enter the following:
The asterisks are wild cards for day and year, so you get all the birthdays in June.
You can make a custom value list of 1 for each month
ANd FM has built in functions to split the date out into separate fields if you ever need it. Month (date) , etc.
But if you really need the three fields, then just do a find in the month field,