Raising chicks: Why invest, when there is no test?

Macaque mother nursing her little one. Image by Freya Santana Cubas

Although house chores are usually shared among those living together, the concept of women being responsible for a tidy house and everybody’s full tummies is still present in our societies. The same applies to raising children, as women are often, once again, expected to invest more time with their kids than their male partners (as if carrying another creature for nine months inside of you and giving birth wasn’t enough).

My whole life I’ve been hearing people tell my mother how lucky she is for having a man that cooks for her. I feel like I was the lucky one, as being a good cook is definitely not a quality my mother possesses and I wouldn’t have come to enjoy food as much as I do without my dad’s culinary creativity (you’re great in many other ways, mum. Love you!).

One of the reasons why males might not invest as much in their offspring is because they may not be confident of paternity. Why invest energy to raise someone else’s child?

When my little sister was born and people saw her blue eyes for the first time they didn’t seem to understand how that was possible (both my parents and I have brown eyes) and they all kept asking the same question: “whose eyes are those?”, to which my father would always reply “the milkman’s” and make everyone laugh. My six year old self didn’t really understand the joke but I guess at some point (maybe after hearing it for the thousandth time and becoming older) I came to comprehend its meaning. As a woman you can always be certain you are your child’s mother, whereas men, unless there is genetic paternity testing or an unquestionable resemblance, are never 100% sure.

Different studies dealing with this topic have tried to identify how paternity is assessed in humans and correlations between paternal investment and various signals, e.g. face and odour similarities between fathers and children have been found.

South Island Pied Oystercatchers
South Island Pied Oystercatchers. Image by Maureen Pierre

In the animal world, paternal uncertainty is constantly present, even in monogamous species, such as the oystercatcher. This distrust can be reflected through males providing less effort in raising new-borns. Consequently, the possibility of not being the biological parent of your “offspring” raises questions about the disparity of the performance of males and females during the breeding period.

Do females invest more time and energy because they are certain the offspring in her care belong to her? Do males prefer to invest more time in other activities and leave the nursing to their partners? And, more importantly, is this effort and investment balanced between both parents?

Haematopus ostralegus finschi or South Island pied oystercatchers (SIPO) are an endemic bird species from New Zealand. Oystercatchers are normally found on coasts, but this species breeds inland in the South Island. After breeding, individuals usually move to estuaries and harbours in the North Island, where they mostly feed on molluscs (surprise!).

SIPO can be easily observed during breeding season, as they nest in open breeding territories. Also, they generally pair with the same mate for several years and display a high level of parental cooperation in favourable conditions for observation. In a study published in Notornis in 2002 and covering the topic of sex roles of the South Island pied oystercatcher, Jonathan Banks, supervised by Adrian Paterson, compared the behaviour of both sexes whilst breeding, including their energy expenditure and food intake.

The birds were monitored on farmland in the Mayfield area, Canterbury. After leg banding the individuals and determining their sex, observations began. The food intake observations consisted on recording the time each bird took to complete 60 probes and the number of successful probes. Successful probes were defined as a prey item being eaten or when the act of swallowing was observed. Each pair of oystercatchers was observed during the pre-egg period as well as after the first clutch was laid.

Regarding behavioural observations, these were recorded every two weeks. Observations began when the first oystercatchers returned to the breeding grounds and ended when the last birds left. They were given 10 minutes to adjust to the presence of the observer and their behaviour was categorised either as “territory defence” or “inactive”. While the word “inactive” is pretty self explanatory, “territory defence” referred to the act of running towards an intruder with raised wings, fanned tails, necks extended forwards and bills directed downwards. Object tossing may or may not be involved. I don’t know about you but I wouldn’t want to have an angry oystercatcher running towards me.

Energy expenditure was estimated for both males and females during the pre-egg phase, the post-egg phase and the whole of the breeding season. Each behaviour had an energetic cost, which was calculated from literature.

Following suggestions from a previous study, we could hypothesise that females would compensate for the high costs that come with egg production by investing less energy in the breeding phase. Despite this, Bank’s results show there were no substantial differences between female and male behaviour regarding food intake.

During the pre-egg and post-egg phases, females spent a greater proportion of the observations inactive while males invested their time in territory defence. This difference was only significant during the pre-egg phase and there was no other significant difference in the other behavioural categories (probing, preening, flying, walking). Even so, males still showed a tendency to spend less energy over the whole breeding season due to the high energy expenditure of egg production.

South Island Pied Oystercatchers.
South Island Pied Oystercatchers. Image by Tom Musson

This distribution of roles kind of rings a bell, despite male oystercatchers making a big effort after egg laying. The cost to females producing eggs and carrying them till laying is just so high that it would require males to conduct even more exhausting and intense activity to offset it.

Studying sex roles during breeding in a bird species allows us to think about how, as much as humans like to draw a line between our species and the rest of the natural world, we aren’t as dissimilar to other animals as we like to think. Animals often show behaviours or characteristics we would usually attribute to humans. If this applied to oystercatchers, it would just be a matter of time before we saw a bunch of females (or a Rockefeller?) walking down the coast fighting for gender equality.

So ­— whatever your role in your household or in your relationship is and whatever tedious and annoying chores your family makes you do, don’t complain too much. Help and collaborate and, if it makes you feel better, take a second to appreciate that you were not born as a male hyena (they have it rough!).

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The author Freya Santana Cubas is a postgraduate student in the Master of International Nature Conservation taught jointly at Lincoln University and University of Göttingen. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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