The Great Green Macaw is a magnificent bird. It evolved to live in the equally magnificent lowland tropical rainforests of the Caribbean. Imagine flying above the canopy of the forest, just tree tops as far as the eye can see, with the occasional green giant standing above the rest like an island in a sea of green. These trees are called emergent trees; so named because they emerge from the top of canopy. In these trees, which were hundreds of years old, there would be cavities big enough for Great Green Macaws to nest.
Unfortunately, we have lost a lot of that forest and now have a patchwork of cattle pasture, pineapple plantations and primary and secondary forests. One of the questions I wanted to investigate during my research was how this quite dramatic change in the landscape impacts the Great Green Macaw’s breeding. There are a number of different ways it could have affected them. For example, the cues they use to select a suitable cavity might no longer lead them to choose a good nest site. People experience this when we are also selecting a house to live in. Before cars became a central feature of our lives, living next to a busy road was advantageous as it meant getting to work was easier. However, if you live near a busy road now, you are more exposed to harmful pollutants, noise and dangerous driving. These days, you might not even need to get to work as working from home is increasingly more common. So, the cues from the past are no longer suggestive of a good place to live. We are fortunate because humans can consciously change this kind of choice, but for a lot of animals, it is innate and so cannot be changed.
So, I set out to find out a few things. First, what characteristics do Great Green Macaws use to select their nest cavity? Second, is reproductive success associated with any of the characteristics they use to select nests? Finally, how many cavities are available for a recovering population?
To do this, we measured a lot of cavities, both used and unused, to see if there were any differences in the characteristics. If there were, then we would know that Great Green Macaws are selecting for these characteristics. We also monitored the reproductive success, or the number of fledglings produced, from each active nest. With this data, we could look at whether reproductive success is correlated with certain characteristics.
What did we find? Well, Great Green Macaws in our study population select for large entrances. This makes sense, they need big entrances. After all, they are big birds. But primarily, they are attempting to reduce the risk of predation, one of the primary causes of reproductive loss, by nesting in deep cavities and isolated trees. To support this, we found that cavity depth is associated with reproductive success, so the deeper the cavity, the more likely it is to produce more chicks.
Finally, we determined that high-quality cavities, the deep ones, are potentially in short supply. We think this because we found a lot of nests that were in sub-optimal cavities, suggesting that there are not enough high-quality ones to go around. A good example of this are the use of cavities that flood or are perennially predated.
The apparent lack of high-quality cavities might not be a surprise when you consider that the large hardwood trees that are able to accommodate cavities large enough for Great Green Macaws are often those that are missing from forests. These hardwood trees are the most valuable in the logging industry, so they are often the first to disappear. This should emphasise how important the remaining large trees are. Not only this, but the small trees of today will grow large enough to develop these cavities in the future, if only they are given the chance. We need to protect all the trees so we can ensure that the Great Green Macaw population recovers and that there will be sufficient nest sites.
If this blog sparked your curiosity and you want to deep-dive into the science, check out my study here.
Tom Lewis, PhD
Board Member
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