Species affected

To date, 36 species have been recorded with beak abnormalities through Big Garden Beak Watch. These range from the diminutive Long-tailed Tit and Coal Tit, to the colourful Ring-necked Parakeet and Goldfinch, to the unusual Skylark and Buzzard. Blue Tits head the list with over 250 records amassed - a particularly notable figure because back in July 2011, when the results were originally collated, Blackbirds (121 records) headed Blue Tits (117 records) on this list. Quite why there has been a sharp increase in records of Blue Tits with beak deformities is unclear and is of considerable interest.

Of course, some species are seen more frequently in gardens than others and, as a consequence, we might expect to observe more incidents of beak deformities in these birds. We can look to address this bias by using data from our year-round BTO Garden BirdWatch survey. Garden BirdWatch provides an average weekly count per garden for each species. By dividing the number of records of beak deformities for each species (as derived through Big Garden Beak Watch) by the average weekly count in gardens for that species (as derived through Garden BirdWatch) a ‘Beak Deformity Rate’ (BDR) can be calculated. BDRs are probably not that meaningful for very scarce garden visitors, so the selection of species for which this has been calculated is restricted in the table below.

Species

Number of records

Ave weekly count per garden

Beak Deformity Rate (BDR)

Blue Tit

261

2.74

95.26

Blackbird

154

2.60

59.23

Starling

64

3.56

17.98

Great Tit

60

1.76

34.09

Woodpigeon

37

2.21

16.74

Rook

35

0.30

116.67

House Sparrow

22

4.82

4.56

Jackdaw

21

1.15

18.26

Robin

17

1.22

13.93
Pheasant 10 0.26 38.46
Coal Tit 10 0.70 14.29
Collared Dove 10 1.64 6.10

Species with ten or more records of beak deformities as recorded through Big Garden Beak Watch.  ‘Average weekly count per garden’ is derived from BTO Garden BirdWatch from 2010–14. ‘Beak Deformity Rate (BDR)’ is calculated from ‘number of records’ divided by the ‘average weekly count per garden’.

Rooks have the highest Beak Deformity Rate
The overall picture changes considerably when BDR is considered rather than raw counts. Blue Tits and Blackbirds both still feature prominently, with high BDRs, but Rooks come out on top. These findings have exciting parallels with research that is ongoing in Alaska into beak deformities. Here, Black-capped Chickadees (similar to members of our tit family) and Northwestern Crows (which, like Rooks, are members of the crow family) have been
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