GDataEntryContact returned without birthday

[KukrCZ]

Hello,
I have pretty bad problem with my semestral project app. Everything works just fine, but [feed entries] just don't have birthday value that is set for all contacts in my test account for Google Contacts. I receive every and all field values (even custom ones). But no luck with birthdays...

Here is an example entry:

GDataEntryContact 0x6080003815f0: {v:3.1 title:Test3 content:notes3 etag:"I deleted for security" categories:1 links:photo,self,edit edited:2017-01-13T09:54:38Z id:I deleted for security org:(
"GDataOrganization 0x608000524920: {rel:http://schemas.google.com/g/2005#other name:Company3 title:JobTitle3 rel:http://schemas.google.com/g/2005#other}")
email:(
"GDataEmail 0x608000524560: {address:test3@seznam.cz rel:http://schemas.google.com/g/2005#other primary:true}")
phone:(
"GDataPhoneNumber 0x608000524740: {rel:http://schemas.google.com/g/2005#other uri:tel:+49-33333-3333 content:333333333}")
name:
GDataName 0x6080005249c0: {given:Test3 full:Test3}
structPostal:(
"GDataStructuredPostalAddress 0x6080005241a0: {rel:http://schemas.google.com/g/2005#other city:Mlad\U00e1 Boleslav fmtAddr:Jana Palacha 1001\nMlad\U00e1 Boleslav 29301\nCzech Republic street:Jana Palacha 1015\n29301\nCzech Republic}")
group:(
"GDataGroupMembershipInfo 0x6080005242e0: {href:http://www.google.com/m8/feeds/groups/I deleted for security/base/284fa98b0d06ebc3 deleted:false}",
"GDataGroupMembershipInfo 0x608000524060: {href:http://www.google.com/m8/feeds/groups/I deleted for security/base/6 deleted:false}"
)}

I would be really happy if somebody could point me to probable problem :)

Thanks and have a nice day