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Plover females are sensitive and sensible when it comes to chick care

posted Mar 16, 2021, 1:45 PM by Clemens Kuepper

Snowy Plover females have a very peculiar behaviour: they frequently desert their broods leaving back their young chicks. Their male partner then has to deal with the offspring alone, an example from nature for a modern single parent family. After abandoning their broods, Snowy Plover females quickly re-mate and start a new breeding attempt with a new partner. This can happen in the same breeding area or the females can travel up to 1000 km and try their luck elsewhere, which has also ramifications for the genetics of the species.

This unusual emancipation enables the plover female to have a lot more chicks than when staying with the brood and caring for the young. Deserting the brood though is a delicate balancing act as the abandoned chicks receive less parental care and as a consequence may not survive as well as if the female would have stayed around. Fortunately, plover chicks are nidifugous meaning that they can already run and search for their own food within a few hours after hatching from the egg. The parents are mainly required to brood the chicks when it’s cold, warn them from predators and protect their offspring from rival plover families. Yet, a few years back we observed at our site that in families, in which the female deserted, the chicks did not survive so well as in families where the mother stayed around to care. Could this strange behaviour perhaps explain why Snowy Plovers are threatened and their numbers are going down in many places? Are the females doing something that may have been good in the past but is now rather hurting their own reproductive success.

Kriszti Kupán decided to look at the dynamics of female desertion in more detail. She re-analysed the previously published data, where we had established that single parent families have reduced chick survival. She used our well curated data on Snowy Plover brood care and chick survival for the years 2006 until 2012, published as part of the CeutaOPEN data base. To make this happen, Kriszti showed incredible dedication and persistence: she started the analysis in 2015 during her maternity leave and from late 2017 she continued this part-time whilst raising her (full disclosure here: ‘her’=’our’) own three kids. You can imagine that we are really happy that the study got finally published in the journal Behavioral Ecology this week and it also received some nice media attention.

So what do we describe in this paper? Kriszti, first found that the length of female care in Snowy Plovers is, as in other plovers, related to seasonality and brood size. Early in the breeding season almost all females desert their broods shortly after hatching. Later in the season, they stay longer and a few stay then even until the chicks are independent. This makes sense: early in the season, chick survival is very high and females have lots of remating opportunities because there is still enough time to finish a new breeding attempt successfully. Also, in our population there are almost always lonely males available as for every adult female there are about two adult males. She also found that females would stay longer when their broods were larger whereas when there was one chick only, they quickly left the brood. Snowy Plover clutches contain most of the time three eggs but we had previously experimentally reduced and enlarged broods, which came in handy here. Also, in nature occasionally chick adoption occurs. She also found that the present brood size, the brood size on a given day was a very good predictor whether a female would continue to care or not. Previously, researchers had mainly looked at the initial brood size. Yet, if the initial brood size would be so important for females when deciding how long to care, they would have little flexibility and ride out their decision to care all the way through the end.

This was clearly not the case. Instead, Kriszti found that when the brood size would drop because a chick died, the female often deserted shortly after. This resulted in a very strong temporal relationship between chick death and female desertion. Right before or after a chick died, the female was much more likely to desert than when all chicks would survive. To us this shows that Snowy Plover females are incredibly sensitive to the condition of the chicks and they flexibly adjust their parental care accordingly. But there is more to it: there was a strong seasonal pattern for this association. Early in the season, when conditions were good enough for one parent to raise the brood successfully alone, females always deserted. We assume that most of these females re-mated quickly, for a bunch  of them we actually were able to confirm this as they stayed at our site. Conversely, late in the season when there was not enough time to have another successful breeding attempt, the female would stay with the family. In the middle of the season, the females committed themselves first to care but when things with their brood seemed to go South and chicks started to die, they abandoned the sinking ship and looked for new mating opportunities. Because there are many more males than females in our population, the males don’t have the re-mating option. A deserting male would struggle to find a new mate and so they are better off caring for the chicks alone no matter what.

It seems after all that the Snowy Plover females are doing the very sensible thing: they leave the brood when they are not needed because the chicks survive anyway or the chicks die despite the female caring for them. Hence the deserting females are certainly not to blame for the decline of Snowy Plover populations.