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Though they are charming and generous, providing me with excellent company and even more excellent food, there are still days when I can’t help wishing I could magically create a more equal male—female split on the island. Of course, I know that’s not possible. Just as I know that expectant parents cannot select the sex of their unborn baby – unless they have lots of time, even more money, and live in one of the few countries where embryo-screening for gender selection is legal.
 
Aride’s abundant wildlife certainly doesn’t have access to expensive embryo-screening clinics, so most people would assume the birds, lizards and turtles that breed here produce a random selection of male and female offspring, just like humans. Not so. When nesting hawksbill turtles – who provide me with much-needed female solidarity every few days – lay their eggs on our beach, the unborn turtles could turn out to be any sex. Only as the hatchling develops inside the egg is the gender determined: by the environmental conditions at the time. Higher temperatures in the nest result in more female hatchlings; lower temperatures produce a greater number of males. Though a turtle mother obviously can’t affect the world’s climate, the location she chooses for her nest, the type of sand present, whether it is shady or not and how deep she digs could all affect the percentage of males and females which emerge once the eggs come to hatch. While there may be some evolutionary benefit to these environmentally-influenced gender biases in turtle eggs, the link between the temperature of a beach-bound nest and the future life of a turtle in the deep blue ocean is not immediately clear. In one of Aride’s bird populations, however, the story is very different. 

Seychelles warblers (timerl dezil Sesel) are territorial birds who form long-lasting pair partnerships. Young birds can breed from one year of age, but many stay on in the territories in which they were born and act as ‘helpers’, lending their beaks to territory defence, predator mobbing, nest-building, egg incubation, nest-guarding and chick-feeding. Breeding pairs who are lucky enough to live in an area that has a large number of insect prey available (a ‘high-quality territory’) have been shown to benefit from the presence of these helpers, resulting in improved reproductive success. But helpers in a low-quality territory, where food is scarce, cause a decrease in the breeding pair’s reproductive output – because of competition for food and the territory itself. If you’re a female warbler in a low-quality territory there’s not much you can do. Fighting off the unwanted helpers takes time and energy; challenging for a more high-quality territory, even more so. But what if you could manage the number of helpers in a more subtle way?

Analysis of monitoring records over fourteen years on Cousin and Aride islands has shown that almost nine out of ten helpers are female, and that the handful of male helpers flee their birth territories much sooner than their female counterparts. So, if you could fly off to a gender-selection clinic, the answer is simple: give birth to females (who turn into beneficial helpers) in high-quality territories and produce males (who leave the territory) in low-quality ones. Further data analysis shows, amazingly, that the warblers do what we humans find impossible – they control the sex of their offspring depending on the type of territory in which they live. Almost three-quarters of the first-born young in low-quality territories are male; in high-quality territories over 85% of chicks are female. In medium-quality territories the gender split is almost half and half.

Sceptics might claim this is all just a massive coincidence – without an understanding of how gender can be determined in this way, it can be a little difficult to believe the statistics. But the translocation of twenty-nine warblers from Cousin to Aride two decades ago gives us access to valuable experimental data.

Experiments are important in science because they allow you to change something and observe what – if any – difference it makes to the norm. By comparing the results from a situation where a specific factor has been changed, you can identify which factors have a direct effect on the outcome, and which results are just caused by chance. In the case of warblers, the ideal experimental set-up would be to change the territory quality to see if it affects the gender of the breeding pair’s offspring.

Of course, it wouldn’t be ethically acceptable to do this to a stable population, but in this instance the birds were already being moved, as part of the translocation progamme to help build new populations. Look at the results of this fortuitous experiment and you’ll see that pairs who transferred from high-quality territories on Cousin to high-quality territories on Aride continued to produce female offspring, while pairs which were switched from low-quality territories to high-quality ones switched from producing males to mainly females.

Rather than blindly accepting their results, scientists are always questioning data and looking for alternative explanations. In this situation, perhaps male eggs, chicks or fledglings simply don’t survive as much in high-quality territories, meaning the biased numbers are not created by the birds themselves but by some external influence. However, the data implies that the gender imbalance is a result of biased egg production rather than the mortality of chicks. And since – unlike humans – the sex of a bird is dictated by the chromosome donated to the developing egg by the mother, it is the dominant female who is somehow manipulating the sex ratio of her children, and thus the population of her territory.

While none of this helps me manipulate the skewed gender balance of my own island population, it is important to know and understand the warblers’ behaviour. If the human managers of Aride decide to change the island habitat in any way, they need to know how the warbler managers – the dominant females – will respond. Conservation management is all about making the right decisions to look after habitats and manage wildlife populations effectively. But in this case, the wildlife itself is also making management decisions – albeit in less of a formal way – reminding us that we are just part of the management process, rather than the overall rulers of the conservation area. More personally, next time I feel like the odd woman out, I can take comfort from the fact that, despite the males’ puffed-up crest displays, it is the warbler females – as in many other Aride bird species – who ultimately rule the roost.

by Anna Faherty

The Island Conservation Society promotes the conservation and restoration of island ecosystems.