Both traits showed significant FA, but also DA (). DA should not compromise the FA values used here, if owing to a systematic bias (Van Dongen 2006
). Signed FA values also showed high leptokurtosis (). A closer look revealed that leptokurtosis appeared to be due to a single individual in each case showing clearly outlying FA values. We thus removed these individuals from further analysis. This resulted in markedly reduced kurtosis of the signed FA (−0.49 and 0.48 in wing and tail feathers, respectively), providing no evidence for antisymmetry (Palmer & Strobeck 2003
). High values of hypothetical repeatability, r
, suggest substantial between-individual variation of developmental instability in the traits studied ().
Table 1. Decomposing the variation of wing and tail feather measurements of treecreeper nestlings (n = 165) into fluctuating asymmetry (FA), directional asymmetry (DA) and measurement error (ME). (K denotes kurtosis of the distribution of signed FA and r the hypothetical (more ...)
The analysis of unsigned FA values showed a sex-related association between corticosterone and FA (). In males, a high corticosterone level was associated with high FA (β
(95% CIs) = 0.046 (0.011, 0.080), F1,96.5
= 6.95, p
= 0.01), whereas in females FA was unrelated to corticosterone level (β
(95% CIs) = −0.017 (−0.046, 0.013), F1,62.7
= 1.32, p
= 0.26; ). FA also differed between traits, FA of wing feathers being 0.06 units higher than that of tail feathers (). Using corticosterone values not corrected for autolysis did not change these results. There was still a significant interaction between corticosterone level and sex (F1,159
= 6.44, p
= 0.012), as males showed a positive association between FA and corticosterone (β
(95% CIs) = 0.004 (0.001, 0.007), F1,96.7
= 6.91, p
= 0.01), while females showed none (β
(95% CIs) = −0.002 (−0.004, 0.001), F1,63
= 2.13, p
= 0.15). These results also remained unaltered when the potential size dependency of FA using log transformation was taken into account (Palmer & Strobeck 2003
), or when the two individuals with outlying FA values were included in the analysis (results not shown).
Association of corticosterone level, trait, sex and brood number and their selected interactions on the unsigned FA of treecreeper nestlings (n = 163). CIs, confidence intervals.
Association between autolysis-corrected corticosterone level and average individual FA in male (filled circles and solid line) and female (open circles) treecreeper nestlings.