Long-term decline and geographical variation in the numbers of moulting Common Eiders Somateria mollissima in Shetland

Figures & data

Table 1. Counts of individual Eiders during Shetland-wide moult flock population surveys in 1977–1993.

General survey area	Eider count (individuals)
	1977	1980	1981	1982	1983	1984
Shetland (excl. FI & FO)	13800	8794	8081	9595	8880	10113
Fair Isle	400		1018	690	680	565
Foula	300				300
TOTAL	14500				9860
General survey area	Eider count (individuals)
	1988	1989	1990	1991	1992	1993
Shetland (excl. FI & FO)	7684	8272	6783	5840	6051	5959
Fair Isle	1000	795	551	294	280	345
Foula		375	400	336	356	240
TOTAL		9442	7734	6470	6687	6544

FI = Fair Isle; FO = Foula. (1977: Jones & Kinnear 1979; Shetland 1980–84: Heubeck 1987; Shetland 1988–92, Fair Isle and Foula 1981–92: Heubeck 1993a; 1993: Heubeck 1993b).

Figure 1. Map of Shetland showing the location of the 30 survey areas used for moulting Eider population surveys in 1996–2019 (see online Table S1 for additional details). Coastline and open water habitats were surveyed within the boundaries of each area (polygons or circles). Survey areas categorised as exposed coast are underlined (e.g. 1), those categorised as sheltered coast are not underlined (e.g. 2). Triangles indicate where counts significantly increased (pointing up) or decreased (pointing down) across years (1996–2019, P < 0.05; survey area 9 showed no significant trend). Fair Isle is situated 38 km southwest of the southernmost tip of Mainland Shetland. * indicates survey areas with no missing counts from 1996 to 2019.

Table 2. Counts and estimated counts of Eiders during Shetland-wide population surveys at the time of moult (mid-July to mid-September), 1996–2019.

Survey area	Eider count (individuals)										z	P
	1996	1997	2001	2002	2005	2006	2009	2012	2015	2019
1	62	48	41	54	41	42	11	20	18	14	−7.89	<0.001
2*	171	115	685	341	654	978	1366	635	1181	721	30.44	<0.001
3*	60	190	193	663	573	203	389	305	727	585	21.75	<0.001
4*	50	5	27	4	9	299	667	578	24	76	18.39	<0.001
5	369	354	297	283	233	212	232	102	101	41	−13.54	<0.001
6*	198	232	283	307	231	165	252	239	106	186	−5.39	<0.001
7*	164	120	65	111	24	36	5	0	0	0	−19.32	<0.001
8*	35	159	223	319	271	115	182	115	55	25	−9.59	<0.001
9	53	54	54	54	55	101	36	22	15	106	0.22	0.823
10*	25	10	38	110	72	90	241	489	379	116	24.37	<0.001
11*	122	80	57	63	34	9	29	19	7	23	−12.85	<0.001
12*	236	28	71	140	170	100	80	7	5	25	−14.80	<0.001
13*	222	377	319	100	36	106	30	50	46	29	−24.32	<0.001
14	8	0	0	2	1	3	0	0	0	0	−2.35	<0.019
15-	47	175	104	29	44	8	18	11	0	0	−12.72	<0.001
16	90	32	12	39	36	30	24	48	17	18	−5.64	<0.001
17	90	96	123	130	150	188	151	167	235	252	4.81	<0.001
18*	29	12	0	0	0	0	0	18	0	0	−5.15	<0.001
19*	70	68	4	22	11	0	4	72	160	123	11.43	<0.001
20	5	15	2	7	7	0	3	12	8	19	2.02	<0.044
21	129	145	207	223	190	109	666	499	240	480	9.85	<0.001
22*	368	429	246	455	282	372	110	322	60	2	−22.69	<0.001
23	56	62	4	36	2	22	0	5	6	0	−10.46	<0.001
24	4	0	13	19	37	43	61	15	35	213	14.21	<0.001
25	28	206	169	145	76	129	0	35	3	0	−13.79	<0.001
26	370	375	399	405	422	377	493	259	884	308	2.76	<0.006
27*	2201	1451	1317	1039	1117	812	297	242	60	61	−63.40	<0.001
28*	207	211	195	312	361	255	31	66	16	6	−20.48	<0.001
29*	966	1322	1035	831	209	355	122	97	91	101	−51.08	<0.001
30*	293	477	239	376	211	126	282	178	120	109	−17.34	<0.001
Empirical total*	5417	5286	4997	5193	4265	4021	4087	3432	3037	2188	−48.29	<0.001
Estimated overall total	6727	6849	6423	6619	5560	5285	5782	4627	4599	3639	−41.23	<0.001

Survey areas 1–30 correspond to the areas in Figure 1 and Table S1. Survey areas categorised as exposed coast are underlined (e.g. 1), those categorised as sheltered coast are not underlined (e.g. 2). Bold italic denotes estimated counts (e.g. 41), when empirical counts were missing (1996–2019, see methods). Empirical total = sum of all counts for given year from the 17 survey areas with no missing counts (^∗ indicates no missing counts, 1996–2019). Estimated overall total = sum of all empirical counts and estimated counts for given year, from all 30 survey areas. z = test statistic from generalised linear model of counts on year (estimated counts entirely excluded, except for the model for Estimated overall total).

Figure 2. Eider counts from Shetland-wide population surveys at the time of moult (mid-July to mid-September), 1977–2019. Dotted lines = linear regressions of counts on year. Triangles = total counts from surveys in 1977–1993. Squares = estimated overall total counts from surveys in 1996–2019 (β = −136.2 individuals per year, z = −41.2, df = 9, P < 0.001). Circles = empirical total counts from surveys in 1996–2019, from the 17 survey areas with no missing counts (see methods; β = −138.2 individuals per year, z = −48.3, df = 9, P < 0.001). These data series are indices of population change from large-scale focused sampling of the Shetland coast.

Figure 3. Eider counts at the time of moult (mid-July to mid-September) in areas categorised as exposed coast and sheltered coast, 1996–2019. Dotted lines = linear regressions of counts on year. Squares = estimated overall total counts (Exposed: β = −221.1 individuals per year, z = −92.7, df = 9, P < 0.001; Sheltered: β = 84.9 individuals per year, z = 35.9, df = 9, P < 0.001). Circles = empirical total counts, only including survey areas with no missing counts across years (Exposed: β = −204.3 individuals per year, z = −88.7, df = 9, P < 0.001; Sheltered: β = 66.2 individuals per year, z = 36.9, df = 9, P < 0.001).

Figure 4. Proportion of Eiders that were adult male (%) among the sample of birds identified as adult male or female/juvenile during Shetland-wide moulting Eider population surveys in 1996–2019. Dotted line = linear regression of proportion of adult males on year (β = −1.24 per cent per year, t = −4.62, df = 9, P = 0.002). In each survey year, ≥ 86% of all Eider counted were categorised as either adult male or female/juvenile.

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