I am used to seeing lots of small brown birds back in the UK, so when I first arrived in Rwanda I was struck by how many different species of birds there were and how brightly coloured they were too. Examples that spring to mind include the various weavers, sunbirds, bee-eaters and kingfishers - to name but a few of my family’s favourites!
It was the bright vibrant colours and the subtle variations that can be seen across superficially similar species that played a large part in igniting my passion for birds. In this, I was in good company: early explorers of the tropics, including Charles Darwin and Alfred Russel Wallace, also commented on the amount of brightly coloured bird species compared to those in areas further north and south of the equator.
Until now, there has been no conclusive evidence that birds in the tropics are brighter, just an impression of a riot of colours and working theories. Wallace himself suggested there were just more birds in the tropics and that the colourful ones simply stood out more than their duller cousins. However, we can now be sure that there are more colourful bird species in the tropics than in temperate and polar regions.
In a recent study published in the academic journal, Nature, Ecology and Evolution, scientists examined the plumage of roughly 24,000 song birds and more than 140,000 photographs were taken, and there was a clear gradient of colourfulness between those birds found in the Tropics and those found further North and South. Whilst this study focused solely on songbirds I would expect the trend to be the same for all birds that call the tropics home given the bright and beautiful birds found here in Rwanda and throughout the tropics.
The exact reasons why tropical birds are so brightly coloured remains unknown but a number of theories were suggested in the study. One is that due to the density of each species there is a need for the birds to stand out in the ‘crowd’ so they can attract a mate – the huge number of each species found here in Rwanda is definitely something that stands out to me. The latest study also found that the brightest coloured birds were those found in dense, closed forest habitats as well those that eat fruit and floral nectar. When I read this part of the study I immediately thought of my trips to Nyungwe National Park in western Rwanda and the stunningly beautiful birds that call the park their home - most notably the beautiful variety of turacos, sunbirds and trogons. If it were not for their vibrant hues, visitors to the National Park would have an even tougher time locating them in the thick foliage. Luckily there are some excellent guides on hand to help...
The study suggested that another possible reason for the evolution of such colour diversity might be the need for brightly coloured visual communication in the dark environments found in tropical forests. A further possibility could be the stability of such environments found in the tropics which has enabled these species to evolve more than their northern and southern cousins. Again, Nyungwe National Park springs to mind as a prime example of such a stable environment. In other words, the resident bird population in places like Nyungwe can make the most of their protected habitats and invest more energy in their attire because these areas have experienced a relatively stable climate for millions of years, at least compared with temperate regions. The limited seasonal variation in terms of temperature found here in Rwanda and the rest of the tropics also mean that the resident bird populations don’t spend much of their energy migrating or trying to survive harsh weather conditions. Like other Rwandan residents, these birds can therefore dress to impress!
For some species, the male and female have the same or very similar colour plumage – the Malachite Kingfisher and Black-headed Gonolek are two such species found here in Rwanda where this is true. However, in a large number of species the male is more striking and brightly coloured than their female counterpart – the family of sunbirds found here in Rwanda are great examples of this. But why is this the case? Whilst there are unique theories for the colouration of each species, there are some broad concepts that help explain these stark contrasts.
One of the primary reasons is sexual selection - for these species, the male attracts a female by showing off his bright colours. The female prefers a colourful mate and uses the brightness of the male’s plumage as a way of determining the health and fitness of the potential mate, the brighter the better! For some species the male will compete for and then hold territory. They use their bright colours in conjunction with vocalisations to signal to other males of the same species that it is their territory and to stay away. Also, just as we use the bright colours to help us identify different species, so do the birds themselves – and in darker tropical environments, the brighter the better.
If we switch the question around and instead of looking at why the males are so colourful and ask why are the females coloured so differently then we reveal an additional reason. In almost all species it is the female that will do most, if not all of the nesting duties. So, the female’s plumage which might be seen by some as ‘dull’, ‘drab’ and ‘subdued’ cleverly helps to camouflage her and conceal the nest and the young chicks from predators.
For Wallace and Darwin, and for me it’s all these differences and intricacies that surround the 10,000+ species of the global bird population that make them so fascinating!
Bibliography
Cooney et. Al (2022) Latitudinal gradients in avian colourfulness, Nature, Ecology and Evolution, 6:22, p. 622-629 [Online]. Found at: https://www.nature.com/articles/s41559-022-01714-1