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Spotlight: Shorebirds bending gender roles

A couple has just had children. Almost immediately after, Mom’s life returns to normal. Dad cares for the children. Does this grab your attention? In human society, issues of who does what in parenting are alive and well.

What if the parents aren’t human?

Shorebirds have diverse parenting systems that range from both parents cooperating to one or the other sex taking full responsibility. Thus, for many species in the shorebird world, males caring for the chicks has long been the norm. For example, in most pair-forming sandpipers (socially monogamous Scolopacidae), both parents help when the chicks are very young and then one parent – usually the female – leaves. In Red Knots Calidris canutus, Purple Sandpipers C. maritima and Great Knots C. tenuirostris the female leaves as soon as the eggs hatch.

However, a few Red Knot females may be bending the gender roles in a species that generally takes these roles to the extreme. In this issue of Wader Study, Tomkovich and colleagues describe rare cases of female knots attending broods (staying with the kids) for the first time1.

Whenever rare or surprising things are found, scientists question a few things: (1) Are the methods strong enough to ensure the rare find is genuine? (2) If so, why haven’t we seen this before? (3) Why is this rare thing happening?

The first question then, is how did Tomkovich and colleagues find these parenting-females in a species where males and females look alike and nests are notoriously hard to find because the birds breed over vast swaths of tundra?

First, the researchers stuck to well-studied sites. They studied breeding C. c. rogersi knots near Meinypilgyno Village, Chukotka in Far-Eastern Russia and C. c. roselaari knots near Nome, Seward Peninsula in Alaska.

Knot on nest at the study site near Nome, Seward Peninsula, Alaska, USA (photo:James Johnson).

The Chukotka site is 10 km2 of coastal dry-plain tundra and the Alaska site comprises 5 smaller locations over 18 km2 of montane dwarf shrub tundra.

Knot on nest at the study site near Meinypilgyno Village, Chukotka, Russia (photo: Pavel Tomkovich).

To find birds, nests and young, the researchers conducted daily surveys during pre-nesting, occasionally during incubation, and every one to three days during brood-rearing. Two to five observers walked in parallel about 100 m apart to cover as large an area as possible.

This systematic searching helped the researchers find the birds (and their broods), but how did they know which bird was which and – more importantly for this study – whether the one taking care of the brood was male or female?

First, they had to mark the birds. This is one reason to use a well-studied site, where birds are already marked as a matter of course for other studies. The knots were individually marked with a numbered metal ring. In most cases, the birds also had an engraved leg flag or a unique combination of a flag and colour bands so that researchers could recognize them at a distance.

To tell who was male and female, the researchers working in Alaska took blood samples when banding the birds and determined the sex using molecular techniques. The researchers working in Chukotka also used molecular sexing techniques in some cases, but they also noting mating behavior, measured cloaca sizes, and even watched birds lay eggs (if it lays an egg it’s a female).

The hard work paid off and from 2009 to 2017 the researchers found a total of 62 broods in Chukotka, and 127 broods in Alaska. All broods were attended by a single parent. In four cases, the bird attending the brood was female: one female in Chukotka and two females in Alaska (one who attended broods in two successive years). Importantly, for all cases, molecular sexing from blood samples was used to confirm the sex of the bird.

This brings us to the next question: Why haven’t we seen this before? This is the first report of such behavior in Red Knots. Are gender roles changing for these birds? In humans, parenting comes in many varieties: single parents, co-parents living apart, co-parents living together, same-sex parents, gender fluid parents. The variety does seem like a modern thing, though people have likely been coming up with interesting and successful ways to raise kids for a long time. Now though, we are more interested in studying it. As a result, we might be seeing, acknowledging and accepting more variety. Might this be true for birds too?

In the paper, the authors list several earlier studies that report no female parenting in Red Knots. However, they note that the incidence of females caring for broods in their own study was very low (only ~2%) and that such rare females might always have been there, undetected. Why? Because males and females look the same, because observing techniques like nest cameras are new, and because molecular sexing is new. Perhaps more and better study is allowing us to see more variety. To really know whether knot behavior is changing, the researchers will have to keep watching. Then they’ll be able to see if this low level of female care stays the same or if it is increasing over time.

Finally, we ask why this rare behavior is happening. This study can’t tell us the answer, but the authors discuss several possibilities. The most obvious is that the male deserted the brood or died, forcing the female to stay and raise the kids. This may have been the case for the Chukotka female. The authors report that from 2012–2015, she always paired with the same male and he stayed with the kids. But the year she was found with the chicks, he was not seen at all. Rare cases of females attending broods have also been found in Purple Sandpipers and a removal study on this species has shown that if the male is experimentally removed just before hatching, most female Purple Sandpipers assumed brood care2. So perhaps the females took care of the kids because they had to.

Another possibility is that broods were split between the parents shortly after hatching and the male was attending the other chicks elsewhere. But the authors note that this seems unlikely. In the case of the female in Chukotka, she was taking care of a single chick and there were no other broods of the same age found in the area. In Alaska, in one case a female was attending four chicks, not a reduced brood.

Whatever their reason for parenting, these rare single moms parented much the same as the common single dads. They were “good” parents: leading chicks to foraging areas, brooding chicks, and displaying strong anti-predatory responses if researchers (or other threats) came too near. Similar findings, that females can parent just as well as males, have also been found in Purple Sandpipers2. In knots, parenting didn’t seem to harm female survival either. All three females attending broods in Alaska and Chukotka were observed the following breeding season.

This leads to the question of why female knots don’t parent more often. The authors themselves wonder “why female chick attendance is rare in Red Knots, and other socially monogamous sandpiper species, while they apparently are capable of successfully raising chicks”. Many wonder the same about humans, but the other way around. Single mothers far outnumber single fathers, even in places single fathers are on the rise3. Yet, dads are quite successful at raising children (some male mammals can even lactate4).

Of course, there is also the larger question of why such a wide variety of parenting strategies exists at all5. We still don’t know – for shorebirds or humans – so we might as well enjoy the variety.

  1. Tomkovich, P.S., J. A. Johnson, E. Y. Loktionov & L. H. DeCicco. 2018. Brood attendance by female Red Knots. Wader Study 125(1): xx-xx.
  2. Pierce, E.P., L.W. Oring, E. Røska & J.T. Lijfeld. 2010. Why don’t purple sandpipers perform brood care? A removal experiment. Behavioral Ecology 21: 275–283.
  3. Webb, A. 2017. Single parents worldwide: Statistics and trends. Blog post in Spaced Out Scientist 18 Jul 2017 at https://spacedoutscientist.com/2017/07/18/single-parents-worldwide-statistics-and-trends/ with links to original OCED and census data.
  4. Swaminathanm N. 2007. Strange but True: Males Can Lactate. Posted in Scientific American 6 Sep 2007 at https://www.scientificamerican.com/article/strange-but-true-males-can-lactate/.
  5. Székely, T., G. H. Thomas, I. C. Cuthill. 2006. Sexual conflict, ecology, and breeding systems in shorebirds. BioScience 56: 801-808.

PDF is available for download here: http://www.waderstudygroup.org/article/10744