What to know about sexual dimorphism
By Emma
March 2025
Introduction
When I first began birding, I found it a difficult task to remember all the names of the birds I was seeing. It quickly became confusing, especially when two completely different looking birds share the same name. I would think, Surely the bird couldn’t have changed that dramatically in appearance within a few weeks? But no - this can all be explained by sexual dimorphism! Of course, this isn’t exactly a revelation, and the story is a bit of a fabrication. Sexual dimorphism is widely known, with many children encountering it early on – whether through watching The Lion King or seeing a male and female peacock at their local zoo. Yet, it remains an incredible phenomenon and is fascinating to observe in nature. Here are some facts and identification tips which will help you in your birding adventures.
What is sexual dimorphism?
Sexual dimorphism occurs when females and males of a species have significantly different physical appearances, which can include differences in colouring, shape and size. It is widespread within the animal kingdom and is extremely common in birds, this blog will predominately focus on sexual dimorphism within birds especially in respect to visitors to Scotland. Sexual dimorphism has evolved due to factors like natural selection, where males and females can often follow different evolutionary pathways, as well as sexual selection, where individuals of one sex develop traits that increase their success in securing mates.
A common trait of sexual dimorphism in birds is a disparity in size between the sexes, more than not, males are larger than females. This is likely due to sexual selection, as males of the same species compete for female mates. Often, a larger male is perceived by female as a stronger candidate for reproduction. In addition, for being larger, male birds also tend to have more colourful plumage and elaborate features, such as a larger beak. These traits collectively help attract females by making the male more visually striking. However, these conspicuous features can also make males more vulnerable to predation. This increased risk serves as an indicator of strong fitness, signalling to the female that the male has good genes and would be a suitable father for her offspring.
Sexual dimorphism in birds directed at males
Interestingly, sparrows show variation in sexual dimorphism, with some species exhibiting it while other do not. House sparrows, for example, display sexual dimorphism. Males are marginally larger than females and have distinct features, including a black bib, grey head (during summer), white cheeks (during winter) and a streaky brown back, back of the neck and wings. In comparison, females are much harder to distinguish, as they are predominately brown with a brown-grey underside, streaky brown back and no signature black bib, allowing them to blend more easily into foliage. On the other hand, in tree sparrows, both males and females share a similar black bib, brown head, white cheeks, a streaky brown back and signature black cheek spots. This variation highlights the considerable differences seen in sexual dimorphism across bird species.
A common trait among female birds is their relatively drab, brown, dull appearance. This is understandable, as it helps them to blend into the ground and surrounding foliage, making them less visible to predators. When bird watching, I have noticed this trend, particularly in ducks, especially since they are frequently seen at a distance, such as out at sea. Often, I struggle to identify female ducks until I spot a male nearby, highlighting the similarity in the appearance of many female ducks. When observing three common duck species in St Andrews - Mallards, Common Eiders and Teals – the males of each species look considerably different. Male mallards have a brown breast, a distinct dark green head, a yellow beak, a black rear and a blue wing patch. Breeding Common Eider males are black and white with a pale green patch on their neck. Lastly, male Teals are grey, with a speckled breast, a chestnut-coloured head and a distinct green patch around their eye. In contrast, the females of all these species share a predominantly brown appearance. However, each has subtle distinguishing features, female Mallards have a blue wing patch, female Common Eiders have slightly paler tip of their beak, and female Teals have a green wing patch. This pattern highlights a noticeable trend in sexual dimorphism, where males exhibit more vibrant colours while females have more cryptic plumage for camouflage.
Sexual dimorphism in birds directed at females
The general trends of female birds being smaller and less elaborate in appearance is not always the case. In some bird species, females are larger or more colourful than males. One example is the Phalaropus family, which includes the red-necked Phalarope, a rare visitor to Scotland. In this species, females are the more colourful sex during the summer. Males and females appear relatively similar, with a grey body and head, a white belly and during the breeding season, an orange/red neck, a darker grey body and a distinctive white chin. However, females display brighter colours than males, making them more noticeable, such as a marginally larger orange/red patch on the neck. This reversal likely occurs because, in this species, females compete for mates rather than the other way around. Unlike most birds, the females do not participate in raising their young. Instead, they focus solely on finding another mate to continue the cycle.
Another example of reverse sexual dimorphism is seen with black-tailed godwits, where females are larger and heavier than males and have longer beaks. The also display a brighter orange plumage during summer, making them more distinct against the males.
Other taxa
Although this discussion has focused on birds, they are by no means the only taxon that exhibits sexual dimorphism. Humans also display sexual dimorphism, with distinct physical differences males and females. An especially fascinating example is found in spiders, such as the orb-weaving spider Zygiella x-notata, where females are significantly larger than males, often reaching up to ten times their size. In some species, extreme size dimorphism is linked to sexual cannibalism, where females consume males after mating. This behaviour can result in low male fitness, potentially selecting for smaller males over evolutionary time as smaller males are more agile for sneaky mating. However, selection pressure can also favour larger males, as their size may help them avoid being eaten.
Sexual dimorphism is also observed in plants, such as Sagittaria latifolia where male flowers are not only larger than female flowers but also more numerous.
Conclusion
This odd, peculiar trait is widespread across the world and various taxa, making its study an exciting gateway to further discoveries in wildlife. By appreciating the unique appearances and behaviours of the species around us, we can inspire people to connect with nature and take action to protect it. Species that exhibit sexual dimorphism are just as vulnerable to the effects of climate change as any other, with house sparrows experiencing a staggering 71% decline in the last 50 years. Now is the time to explore, understand and protect the natural world while we still have the chance.