American Oystercatcher clutch. Beth Howard, Mass Audubon

American Oystercatchers are fiercely territorial, and a pair of nesting oystercatchers generally prefers some space between them and their nesting neighbors. However, there is an unusual oystercatcher oasis in Boston Harbor where nine pairs nest within a maximum distance of about 275 meters! This leads to all sorts of fun and unusual behavioral observations regarding these generally-territorial birds.

For example, Mass Audubon monitors have noticed several instances of creche-like behavior, which is when chicks from multiple broods are collectively tended by an adult or adults. This is not at all typical of American Oystercatchers. However, there are two pairs on this island who nest less than 50’ from each other each year, and for several years, they have left both broods with one adult while the other three adults go off to forage. The adult that stays with the chicks switches chick-tending duties with another adult periodically. This collaborative chick-rearing is not what we typically expect from this highly territorial and aggressively defensive species!

On this island oasis, we have also seen collaborative nesting, where a “throuple” (three adults) came together one year to lay eggs and share incubation duties. Additionally, we have seen a case of adoption (by choice, not necessity!). In this situation, one pair of oystercatchers initially had three chicks and another pair had two. The pairs’ nests were within 100’ of each other, and when the chicks hatched, they often hung out together near the territorial boundary. The adults would not allow the other adults into their territories, but the chicks were allowed to freely come and go between territories. At one point, one chick from the pair that had two decided not to return, and instead joined the trio and were fed and defended by the adoptive parents as a fourth member of the brood until they all fledged. The solo chick also fledged, and continued to spend time with the other chicks, but stayed with the original set of parents.

Some Things We Learn from Banding

At the southern end of the Boston Harbor Islands is a small harbor with four small islands. As the established population of American Oystercatchers in Boston Harbor is successful, their chicks and other young oystercatchers often return to the same general area to try to establish a territory of their own. One year, a young banded bird showed up with a mate and established a new nesting territory on one of the islands. Because of the band, we were able to learn that this was a bird who was three years old and had not previously nested elsewhere.

American Oystercatchers. Beth Howard, Mass Audubon

We were able to see how young birds move in and search for territories by following this bird’s nest attempts over eight years. Year after year, we saw the pair try out different sections of the same island, then different types of substrate, and eventually, three different islands before settling on what appears to be their final choice of nesting location. We can see them learning and refining their process with every attempt. They are choosing areas with more forgiving substrate, and are nesting higher above the wrack line to avoid tidal overwash. Avian predators are hard to avoid, but the pair is learning to choose locations with a good vantage point for spotting predators approaching, while also using natural features to conceal their location as well as they can.

We see a lot of oystercatchers moving through the area, and without a banded bird being part of this pair, we might not have understood that it was the same pair switching up their locations from year to year, and trying new things. Because of the bands, we are able to observe and understand a little bit more about how young birds arrive on scene, establish their own territories, learn from their prior choices, and make new decisions on where to nest each year. We can see their confidence grow as they return, claim territory, make a nest, and lay eggs earlier and earlier each year. This year, they chose a good spot and nested at about the same time as the older, more experienced pairs did. Perhaps this will be the year they fledge chicks from what appears to be their preferred territory!