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Whole Msum was influenced by the period (Fi^i = 38.5, p < 0.0001) and group (Fi^₄⁼ 5.4, p < 0.05) with Msum being 16.7% higher after the treatment than before and with clipped birds having a Msum 7.8% higher than control birds (no significant interaction, p = 1.0). However, when body mass was included in the model (F143 = 12.8, p<0.01) there was no difference between groups in Msum (p = 0.2). Mass-independent Msum was 14.7% higher after treatment than before (period: Fi^g= 29.9, p < 0.0001) (figurę 3.4A).

Haematocrit was not affected by group (p = 0.5) or the interaction period*group (p = 0.4) but was dependent on the period (F|_>76 ⁼ 38.2, p < 0.0001). Birds expressed an average haematocrit 8.5% higher after treatment than before (figurę 3.4B). Haematocrit was independent of body mass (p = 0.7).

Figurę 3.4 Changes in thermogenic capacity and haematocrit in Black-capped chickadees over the experimental period. Data are least sąuare means of (A) summit metabolic ratę (Msum) and (B) haematocrit before and after the treatment. Least sąuare means were extracted from an LME model controlling fór group, period and the interaction term group * period, with bird ID as random parameter. Body mass was also included in the model as a covariate for Msum.

3.5.2 Effect of pectoral muscle size and haematocrit on metabolism

When including haematocrit and muscle scores in LMEs, variations in mass-independent Msum were still affected by the period (Fi_i69= 4.3, p < 0.05,6.9% higher after manipulation) and remained independent from the group effect (p = 0.7). However, mass-independent Msum was positively related to the haematocrit level (F₂.69= 10.1, p < 0.0001; figurę 3.5) and was affected by muscle score (F2,69=4.1, p < 0.05; figurę 3.6). Therefore, for any experimental group, birds