We get a lot of people asking us this question, particularly when they have spotted two robins together in their garden. If you have seen two robins together then it is likely that they are a male and a female as robins are fiercely territorial and females will only enter a male’s territory for breeding season.
But how can you tell which is which?
The honest answer is that you can’t. Although male robins are on average larger than female robins some females will be larger than some males which means you can’t use the comparative size between a pair as a reliable guide. A female may be heavier when she is carrying eggs, but will still be smaller on average if you were to measure her body parts, such as her wings and tail.
A lot of people think you can distinguish male robins from females by his brighter orange breast, or badge. And although recent spectrometry studies seem to confirm this, there is considerable overlap between the sexes, so it is virtually impossible to tell if you are only comparing a pair of robins.
There is a subtle difference between the shape of the peak on the head when looking from above. Females have more of a V-shape whereas the males’ peaks are squarer and more U-shaped, but again there is an overlap between the sexes so if you only have two robins to compare you won’t be able to tell.
A surprising number of people think that female robins don’t have red breasts – they do. And the white wing bars which many claim are only present in female robins are in fact present in both sexes.
Like goldfinches, for example, the overlap in characteristics between male and female robins means the sexes are only statistically distinguishable, rather than reliably distinguishable, like, say, chaffinches
One of the few ways you can tell is by observing their behaviour during breeding season. If you see two robins engaging in courtship feeding, then it will be the male who is feeding the female. If you see a robin building a nest or incubating eggs that will be a female. And if you’re lucky enough to see robins copulating then it will be pretty obvious which one is the male and which one is the female.
If you’ve spotted a robin in a garden or a park then that increases the chances that the robin is a male, as even in breeding season populations in gardens are usually more male-biased.
Even professional ringers and ornithologists with years of experience admit that determining the difference between male and female robins by eye alone is unreliable. The only way to be 100% sure is to do a DNA test.