Types of deformities

The types of beak deformities experienced by birds in British and Irish gardens are quite wide ranging. For ease of comparison, we have classified each Big Garden Beak Watch record according to the principal beak abnormality exhibited. For the four species for which we have the most number of records – Blue Tit, Blackbird, Starling and Great Tit – we have been able to compare the frequency of the principal beak deformities experienced.

When considering sightings of all species, overgrown upper mandibles have been logged most often, followed by both mandibles over grown and then by crossed mandibles. However, pooling all Big Garden Beak Watch records together in this way means that fascinating between-species differences in the frequency of different beak abnormality types are obscured.

Deformity type

All species

Blue Tit

Blackbird

Starling

Great Tit

Both mandibles overgrown

202

92

10

36

26

Crossed mandibles

146

55

24

2

10

Fractured beak

67

3

45

0

0

Gap between closed mandibles

32

1

19

1

1

Lateral curvature

31

2

14

2

0

Lower mandible overgrown

93

26

19

9

2

Other type

26

2

5

0

2

Upper mandible overgrown

261

118

31

18

22
Principal peak deformity types of birds recorded through Big Garden Beak Watch. Please note that some birds are considered to have more than one deformity (e.g. crossed with gap).
Overgrown upper mandibles have been most frequently spotted in Blue Tits, while Blackbirds have tended to fracture their beaks.

In general, the deformities experienced by Blue Tits relate to their beaks being overgrown. Over half of beak deformities in Starlings involve both mandibles becoming elongated, and this deformity is also that which is most frequently observed in Great Tits. There are no records of fractured beaks in Blue Tits, Starlings or Great Tits. In contrast, the most regularly observed beak deformity of Blackbirds is where individuals have seemingly fractured their beaks. Why is this happening? Are Blackbirds colliding with hard surfaces, like windows? Are their beaks being weakened in some way? Currently, we just don’t know. 

Reasons for these apparent between-species differences are unclear and require attention. You can help by telling us about any birds that you have ever seen in your garden with a deformed beak.

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