In 2015, we were fortunate to tag and track two birds that formed a mated pair: A765 (male) and F367 (female). We captured them in eastern Washington state at Crescent Island prior to the 2015 breeding season. Following capture, tagging, and release, they quickly moved to the Columbia River estuary and nested at East Sand Island. Monitoring crews there observed them nesting through the 2015 season and confirmed that they raised a chick to fledging age.
It has been interesting to track their movements since completing that nesting effort! This breeding season they apparently have not resumed nesting together. F367 arrived back to the Columbia River estuary in early May and was tracked in the vicinity of East Sand Island consistently for almost the entire next month. On 30 May she was tracked heading north to the Salish Sea. The male, A765, stayed south this spring for much longer than most terns. It didn’t arrive in the Columbia River estuary this year until 08 June. Coincidentally, F367 returned to the Columbia River estuary from the Salish Sea on about the same day and they both were present in the area until 13 June, when F367 headed north to the Salish Sea again. So far this breeding season, the period from 08-13 June is the only time they have overlapped near any colony.
As of 24 June, A765 appears to be heading south again. He doesn’t appear likely to attempt to nest this year. It is difficult to say if F367 attempted nesting in May before wandering off to the Salish Sea. Her tracking behavior is consistent with nesting, but could also represent her using the colony as a safe roosting site while she foraged in convenient and productive nearby areas.
This is just one interesting anecdote of Caspian tern mating behavior, but probably represents a common occurrence. Other researchers have seen some mate fidelity across years (on studies of banded terns), but if one mate dies or disappears (or doesn’t show up as in this case) the other can quickly form a pair bond with a new mate.