In foster cow-calf contact systems, a dairy cow nurses 2-4 calves for several months after birth and is usually not milked. If the nurse cows care for both their own calves and alien calves, the latter may not gain enough milk from the cows that may prefer their own calves. On a commercial farm with foster cows, the calves’ suckling behaviour of, namely udder contact duration, frequency and position, was investigated in the first days after 2-3 foster calves (AK) were allocated to foster cows in addition to their own calf (EK), in order to determine possible differences between EK and AK and effects of regular checks by farm staff. The extent of udder contact with alien foster cows was also considered. Additionially, we hypothised that foster calves preferentially use those times for udder contact when EK are at the udder or are licked by their mother. Using video-based continuous focal animal observation of six Holstein-Friesian cows, their EK, and 13 AK during the second week of life, no differences were found between EK and AK in terms of udder contact duration per 24 h, number of udder contacts and duration per udder contact. Overall, however, udder contact duration was at a low level of approximately 32 min/24 h (median) and the number of udder contacts (approximately 30/24 h) suggests that many interruptions of udder contact occurred. A significantly increased proportion of udder contact in lateral position of EK vs. AK indicated a preference of the mother for her own calf. Udder contact of AK was increased during the control phases compared to EK; but except for 2 AK  that were regularly assisted during drinking at the udder (12 and 22 min/30 min control), udder contact duration during this phase was under three minutes. Eight of nine AKs that had the opportunity for udder contact with an alien foster cow used this (median=4.3 min/24 h), with two having more udder contact with the alien than with their own foster cow (<21 min/24 h). It could not be confirmed that a greater part of udder contact time of AK occurred during udder contacts of EK or during licking of the EK by their mother, but the duration per udder contact was longer when EK were simultaneously licked. In conclusion, during the introduction phase of the AK to the foster cows, great restlessness with subsequent rather low udder contact duration of both EK and AK could be observed. Both achieved udder contact times in the lower range of what is reported as suckling times in the literature. Except for the calves which were assisted to suckle, the controls by the farm staff contributed to a lesser extent to increased udder contact of AK than the possibility to suckle alien foster cows.