BTO Garden Bird of the Month – March: Dunnock

The dunnock may look like quite a dull bird, but it has a fascinating sex life. 

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Dunnock.
Jill Pakenham/BTO

The dunnock may look like quite a dull bird, but it has a fascinating sex life. 

Known as the ‘hedge sparrow’, the dunnock is commonly mistaken for a female house sparrow.

However, although it favours hedges, it is most definitely not a sparrow. Instead, it is part of the accentor family that favour insects and small seeds.

Complex social system

This unassuming bird may seem quite boring to the casual observer, but it actually has a fascinating social system, and it is this time of year that it really starts to get interesting.

Although many dunnocks have monogamous pairings (one female and one male), others have more complex arrangements.

Unusually for a bird species, both female and male dunnocks have their own breeding territories.

The territories of males are usually larger and overlap with female territories, allowing them to mate with two or more females (known as polygyny).

Females will also often mate with more than one male (known as polyandry).

There are even cases of polygynandry, which can involve two or three males mating with three or four females.

If that wasn’t complicated enough, some male territories may be shared by two males, one alpha and one beta. The two work together to defend the territory against intruders.

The alpha spends a lot of time guarding his females from the beta.

If the alpha’s females get a chance to mate with the beta it’s a bonus for them as more food will be provided by the males, increasing their chances of a successfully raised brood.

Cuckoo in the nest

Unfortunately for dunnocks, they are one of the main hosts for brood parasitism by cuckoos.

A female cuckoo will remove one of the dunnock’s eggs and then lay one of her own.

In other hosts, the cuckoo egg matches that of the host. However, this is not the case with dunnocks and the cuckoo egg looks very different from the bright blue egg of the dunnock.

You’d think that the dunnock would notice a strange egg and get rid of it, but they are very rarely rejected. This suggests that dunnocks are a relatively recent host that has not evolved a response to this act of parasitism.

This doesn’t seem to be affecting dunnocks as they can rear two or three broods in a season, which is good news for the declining cuckoo. 

© Shane Hinkley/BTO

A cuckoo-free dunnock nest. © Shane Hinkley/BTO

Dunnocks in the garden

Dunnocks are originally woodland birds, but their association with human habitats has grown and they are now a common garden bird.

In the spring they are seen in over 80 per cent of BTO Garden BirdWatch gardens.

The European songbirds mainly feed on insects, but during the winter and early spring will visit bird tables and forage for small seeds. They will also eat finely grated cheese and breadcrumbs.

Little brown bird

The dunnock is described by its name which derives from the Old English word for ‘little brown’. This is because, from afar, they do look drab.

However, if you take a closer look, you’ll see that dunnocks are actually quite attractive.

They have a blue-grey head and breast, light and dark brown streaky back, brown-streaked flanks and pink legs. Their bill is thin and black, perfect for eating insects. 

If you’re not sure whether you’ve seen a sparrow or a dunnock, listen out – dunnocks have a high-pitched ‘tseep’ call, or a sharp, ringing song, lasting about two seconds. It is similar to that of a wren, but is shorter and more forceful.

The British Trust of Ornithology (BTO) works in partnership with over 40,000 volunteer birdwatchers to chart the fortunes of UK birds.

Among the surveys that we coordinate is our popular Garden BirdWatch, the largest year-round survey of garden birds in the world.

Each month we highlight a bird for you to look out for in your garden.

For more information about Garden BirdWatch or to speak to the Garden Ecology Team please email gbw@bto.org

 

Read previous BTO Garden Bird of the Month blogs.

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