Moving polewards in winter: a recent change in the migratory strategy of a pelagic seabird?

Table 1 Differences between species and times of the year in stable isotope ratios.

Source	Type III Sum of Squares	df	Mean Square	F	P	η ²
Dependent: δ¹³C
species	17.9	1	17.9	9.7	0.002	0.044
time	16.6	1	16.6	9.0	0.003	0.041
species × time	1226.7	1	1226.7	666.1	<0.001	0.760
Error	386.7	210	1.8
Total	1698.9	213
Dependent: δ¹⁵N
δ¹³C	138.2	1	138.2	131.9	<0.001	0.387
species	303.1	1	303.1	289.4	<0.001	0.581
time	18.3	1	18.3	17.5	<0.001	0.077
species × time	3.5	1	3. 5	3.3	0.070	0.016
Error	218.9	209	1.0
Total	1540.3	213

Carbon and nitrogen stable isotope ratios were measured in feathers of Thin-billed prions and Wilson's storm-petrels (winter vs. summer), tested using GLM. Location (δ¹³C) was included in the model for nitrogen to account for differences in baseline levels with latitude (e.g. see Fig. 2B).

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