partner-contact/partner_contact_birthdate/views/res_partner.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>

<record id="view_personal_information_birthdate_date" model="ir.ui.view">
    <field name="name">Birthdate Date field</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="partner_contact_personal_information_page.personal_information"/>
    <field name="arch" type="xml">
        <data>
            <xpath expr="//group[@name='personal_information_group']">
                <field name="birthdate_date"/>
            </xpath>
        </data>
  </field>
</record>

</data>
</openerp>