Spotlight: Stopovers as “pre- and post-natal support” for migrants

If you’re lucky, when you are expecting a baby, you have access to pre-and post-natal care. In humans, this refers to physician or midwifery support before and after birth, but also to having the space and time to eat and rest well. Migratory shorebirds also need safe places to eat and rest pre- and post-breeding. In this issue of Wader Study, two articles describe stopover sites that provide this space en route to and away from breeding grounds. These articles also show what studying birds as they pass through these sites can tell us about other parts of the annual cycle, including breeding.

Let’s begin with the post-breeding period. In both humans and birds, this is the time that seems to get the least attention. Many new parents find themselves on their own with their new baby in the days after birth, and similarly not much is known about the stopover sites birds use post-breeding. I was lucky to have had a child in the Netherlands where they have a wonderful thing called kramzorg¹. For a week after I gave birth, a kraamverzorgster came to my home and helped with everything from teaching me to breastfeed to washing the dishes and making peanut butter sandwiches (which was all I wanted). It was revolutionary for me to discover this kind of care (and the need for it) around a topic I thought I knew well – having a baby. In much the same way Lyons and colleagues write about a well-studied species, but shine light on a period of the annual cycle that hasn’t been well studied – post-breeding stopover. They report on a mark-recapture/resight approach to study migration and stopover ecology in the Red Knot, Calidris canutus rufa, at Mingan Archipelago National Park Reserve in eastern Canada².

The researchers’ first task was to collect the mark-resight data. They conducted surveys on four limestone islands (Niapiskau, Quarry, Grand-Île, and Nue de Mingan) at the western end of the Mingan Archipelago Reserve between 11 July and 3 September 2008. Between 9:00 in the morning and 3:00 in the afternoon they looked for knots, counted their numbers, and noted the proportion with uniquely engraved leg flags (marked birds). They detected five-hundred thirty-five (535) adults with leg flags. Most of these birds had received their flags as part of prior work at other locations along the flyway in Canada, the United States, Brazil, Argentina or Chile. Blood samples had been taken when the birds were captured and marked. These samples were used in molecular techniques to determine the sex of the birds. Then the 535 observations were converted into encounter histories, one for each bird, and analyzed mathematically using a Jolly-Seber (JS) model.

The study had four goals: (1) use mathematical models and mark-resight data to describe when the birds arrive, how long they stay, and when they depart; (2) use data on when males, females and young arrive to infer how well the past breeding season went; (3) estimate how many birds use the sight, taking into account that birds pass through and that not all birds will be counted; (4) formalize the method for using quantitative mark-resight data to understand migration and stopover ecology in migratory birds.

Using their field observations and the mathematical model, the authors estimated approximately 9,500 birds (8,355–10,710) used the stopover site at the Mingan Archipelago in 2008. The birds stayed on average about 11 days with a minimum-length-of-stay (MINLOS) about half that. The 535 marked birds seen during the field season represented approximately 64% of the total number of marked birds, or how many would have been seen if every marked bird could have been seen. Mathematically, the model that best explained the data with the fewest variables (the most parsimonious model) assumed that this resighting probability stayed constant.

On the other hand, the probability of birds arriving, and how long they stayed, was not constant. Adult birds arrived in Mingan in two distinct waves. The first group arrived in mid-July and were mainly females who had likely nested successfully, and males and females whose nests failed. The second group arrived from August 8^th to 11^th and were mainly juvenile birds and males that nested successfully (and thus stayed longer to care for young).

Though seemingly a simple description of timing, the authors point out that these results are important because the timing and type of birds arriving in Mingan tell us something about the success of the prior breeding season. In 2008, a large fraction of the stopover population arrived late in the season and most were juveniles and males. This suggests successful breeding. Being able to infer something about the breeding season from 1700 km away, makes the modelling described in this article a powerful tool, especially for species whose breeding sites are not easily accessible and whose nests are not easy to find.

Sticking with Red Knots, but switching to subspecies C. c. roselaari, Buchanan and colleagues report on between-year variation in stopover timing at Grays Harbor, Washington, USA³. Here the focus is on spring migration and the pre-breeding period. In knots, more is known about pre- than post-breeding stopover sites (the famous Delaware Bay for C. c. rufa comes to mind); however, less is known about stopover sites for C. c. roselaari, whose flyway spans the Pacific side of North America⁴. Therefore, the authors took the opportunity to synthesize data in two previous surveys of Grays Harbor, conducted in 2009 and 2010, with data collected more recently in 2016.

Grays Harbor is a large estuary on the Pacific coast that is dominated by extensive flats at the confluence of seven rivers. Knots use areas in these mudflats that are too far from shore to be visible, particularly in the second half of the migration period, so researchers have to get creative to see the flocks. In this case they used airboats. From these boats, they estimated knot numbers by scanning the flocks and counting in increments of 20, 50 or 100 birds (depending on the size of the flock). The researchers found that knots passed through the area later in 2016 than in 2009 and 2010.

Though the study was not designed to determine the cause of this variation in timing, the authors suspect that the later passage in 2016 was influenced by conditions in Mexico because knots appear to fly directly from wintering areas in Mexico to coastal Washington during spring⁴. The authors mention possible causes including variation in numbers of predators, such as Peregrine Falcons Falco peregrinus, or differences in the timing of food resources in Mexico.

Whether human or bird, having a safe and resource-rich place to rest and eat is important for reproduction in both the pre- and post-natal periods. Both of these papers remind us of the importance of stopover sites used by migratory shorebirds en route to and from breeding areas, and both papers remind us that these sites can provide a treasure trove of information, even on a well-studied species.

1. For those unfamiliar with the Dutch and Belgian kraamzorg system see: https://en.wikipedia.org/wiki/Kraamzorg and also https://www.babble.com/parenting/the-incredible-post-birth-service-all-dutch-women-receive/
2. Lyons, J. E._, A. J. Baker, P. M. González, Y. Aubry, C. Buidin & Y. Rochepault. 2017. Migration ecology and stopover population size of Red Knots (Calidris canutus rufa) at Mingan Archipelago after exiting the breeding grounds. Wader Study 124(3): 197-205.
3. Buchanan, J. B., L. J. Salzer & V. Loverti. 2017. Between-year variation in the timing of peak passage of spring migrant Red Knots at Grays Harbor, Washington, USA Wader Study 124(3): 238-240.
4. Carmona, R., N. Arce, V. Ayala, A. Hernández-Alvarez, J.B. Buchanan, L.J. Salzer, P.S. Tomkovich, J.A. Johnson, R.E. Gill, Jr., B. McCaffery & J. Lyons. 2013. Red Knot (Calidris canutus roselaari) migration connectivity, abundance and nonbreeding distribution along the Pacific coast of the Americas. Wader Study Group Bulletin 120: 168-180.

PDF is available here.