I can see that it is a birthday due to the candle-symbol (yes, that is not a battery symbol) and the colour of the item (calendar colour). (I think the english version is "%name's birthday" where "birthday" is cut away when the text is too long.)
So I filed a bugreport but it seems to be more difficult to get a real good solution for that. However the text is changed (thank you Frederik) in the next version.
I didn't want to wait that long and Burkhard was so kind to give me a howto so I could change it on my local machine. I want to share that with anyone who wants to change that, too (it seems to be a problem in other languages as well). This howto is for openSUSE but you should change that easily to your distro of choise.
0) install package gettext-tools
1) msgunfmt /usr/share/locale/de/LC_MESSAGES/akonadi_birthdays_resource.mo >/tmp/akonadi_birthdays_resource.po
2) edit /tmp/akonadi_birthdays_resource.po
Change "Geburtstag von %1" to "%1 (Geburtstag)"
[shortcuts: i = edit esc = exit edit :wq = save and exit] or use "kwrite" instead of "edit"
3) sudo msgfmt /tmp/akonadi_birthdays_resource.po -o /usr/share/locale/de/LC_MESSAGES /akonadi_birthdays_resource.mo
It takes a view minutes until the titles will be changed.