Sons and plovers: young birds are mummy’s boys
Male red plovers raise daughters, and females look after sons, mystifying researchers. Tanya Loos reports.
Australia’s smallest shorebird, the red-capped plover, is a sexually dimorphic species, with a brightly coloured male and a dull coloured female. When brooding the eggs, the parent plovers divvy up their time by having the less conspicuous female on the nest by day and brightly coloured male on it by night.
When the nestlings hatch, the parental care is then determined by the age of the nestlings. When the brood is young, the female does the bulk of the parenting. As the young approach independence, the male then takes over.
But researchers at Deakin University in Australia have uncovered a new twist to the story of plover parenting – as the brood ages, the fathers spend more time with the brood if the nestlings are female, and if the nestling sex ratio is skewed to males it is the females who provide more care.
Daniel Lees and his team carried out radio-tracking on an intensively studied population of red-capped plovers (Charadrius ruficapillus) in Cheetham Wetlands, in the Australian state of Victoria. Parental care of nestlings was measured by attaching transmitters to 42 chicks in 42 broods (comprising 36 nests containing two chicks and six containing just one). Observations of direct parenting were made, and the age of the chicks was either known or assigned using a model based on the length of a lower leg bone.
After hatching, parental care progressively shifted from a female-dominated state to a male-dominated state, but then became determined by the sex ratio of the nestlings. This is the first recorded case where both male and female parents increase their provision of care for young of the opposite sex. The paper states these results “defy unambiguous explanation”.
One possible explanation for the sex-specific trend could be related to predator avoidance. The young chicks are cared for by the duller female, thus avoiding the attentions of predators such as Little Ravens (Corvus mellori). As they age, and are better able to hide and run from predators, the bright male takes over. But this does not explain why adult male red-capped plovers are preferentially caring for female-biased broods.
Only one other study describes behaviour of adults preferentially caring for young of the opposite sex, a 1985 paper on the European robin (Erithacus rubecula). Lees acknowledges it, but states that it comprised a very small sample – just seven. However, the study’s author, DG Harper from Cambridge University in the UK, suggested that young of the opposite sex may represent a better investment, as young of the same sex may eventually compete for mates in the same population.
A mother–son pairing has been observed in hooded plovers (Thinornis cucullatus) Could the feeding of opposite sex nestlings be related towards some sort of fundamental attraction to the opposite sex?
Lees suggests that these explanations are “perplexing, somewhat speculative and require additional studies for confirmation”.
The red-capped plovers are certainly keeping the researchers on their toes, with a note published in Researchgate confirming these curious little birds are the first shorebirds to practice reversed mounting when copulating – with the female bird on top. This practice is fairly common in grebes and cormorants but very rare elsewhere in the bird world.
The study is published in the journal Behavioural Ecology.