Thayer's Gull (thayeri)
(last update: January 22, 2013)
Thayer's Gull - Table 1.
From: Identification and Variation of Winter Adult Thayer’s Gulls with Comments on Taxonomy, by Steve N.G. Howell & Martin T. Elliott, IN: Alula 4/2001.
| Table 1. Correlation of patterns on P10 and P5 of 123 adult Thayer’s Gulls in central California (see Plates 2a, 2b). | ||||
| . | P10 score = 3 | P10 score = 4 | P10 score = 5 | Totals |
| P5 score 0 | 2 |
5 |
24 |
31 |
| P5 score 1 | 9 |
6 |
17 |
32 |
| P5 score 2 | 9 |
7 |
8 |
24 |
| P5 score 3 | 11 |
9 |
16 |
36 |
Totals |
31 |
27 |
65 |
123 |
Table 1 shows the results of our quantification of P10 and P5 patterns for 123 birds. Our data suggested that males may average less black than females but our sample was small, sex was only inferred, and age variation may negate any potential difference; thus, sexes are combined in Table 1. The positive correlation of low P10 scores with high P5 scores, and vice versa, is presumed to be due to age-related variation. In addition to a simple P10 / P5 quantification, complete wing-tip patterns were determined by MTE from video footage for 48 birds (Plate 3 illustrates the extremes and the commonest patterns of this sample). Of these, 48% (23 of 48) had the white tip to P10 and white mirror area on P9 contiguous with whitish on the inner webs (Plate 3, f, j-o), that is, the classic Thayer’s pattern, while 52% (25 of 48) had a complete blackish medial band on P10 (Plate 3, a-e, g-i). On P9, 96% (46 of 48; all except Plate 3, n-o) had blackish extending on to the inner web, forming a partial to complete medial band; 21% (10 of 48) had a complete blackish medial band (Plate 3, a-b, g). Thus, a dark medial band on P9 appears typical of Thayer’s Gull, contra figure 20 of Zimmer (1991) which, in our experience, shows an atypical Thayer’s pattern. Also of note (see below under Kumlien’s Gull, in Separation from North Atlantic Taxa) is that of 90 thayeri we observed, the black on the outer web of P9 was not bisected by the white mirror on 82% (Plate 3, a-g, l), whereas 18% had the black outer edge bisected by the white mirror (Plate 3, h-k).
Plate 2a. Wing-tip scoring criteria for P10.
Score 3: an isolated mirror separated from the pale basal inner web by a variably wide dark medial band and from the white tip by a narrow dark subterminal band.
Score 4: a variably complete white "tip" separated from pale basal inner web but contiguous with white tip.
Score 5: dark medial band incomplete; subterminal dark marks not considered in scoring.
© Martin T. Elliott.
Plate 2b. Wing-tip scoring criteria for P5 patterns.
Score 3: complete black subterminal band.
Score 2: incomplete subterminal bands.
Score 1: dark spots or marks.
Score 0 = unmarked. Note that score relates to extent (not tone) of dark marks.
© Martin T. Elliott.
Some illustrative birds:
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Thayer's Gull adult, November 22 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification:
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Thayer's Gull adult, November 28 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification:
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Thayer's Gull adult, December 05 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification:
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Thayer's Gull adult, December 07 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification:
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Thayer's Gull adult, November 30 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification:
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Thayer's Gull adult, December 02 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification: |
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Thayer's Gull adult, December 07 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification: |
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Thayer's Gull adult?, December 01 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification: |
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Thayer's Gull adult?, December 07 2008, Duluth/Superior, Wisconsin. Picture: Karl Bardon. Visit: Karl's Pbase site. Classification: Much black in wing-tip may suggest younger adult. |