Brain size, head size and behaviour of a passerine bird

Abstract

A recent increase in comparative studies of the ecological and evolutionary consequences of brain size in birds and primates in particular have suggested that cognitive abilities constitute a central link. Surprisingly, there are hardly any intraspecific studies investigating how individuals differing in brain size behave, how such individuals are distributed and how brain size is related to life history and fitness components. Brain mass of the barn swallow Hirundo rustica was strongly predicted by external head volume, explaining 99.5% of the variance, allowing for repeatable estimates of head volume as a reflection of brain size. Repeatability of head volume within and between years was high, suggesting that measurement errors were small. In a 2 years study of 501 individual adult barn swallows, I showed that head volume differed between sexes and age classes, with yearlings having smaller and more variable heads than older individuals, and females having smaller and more variable heads than males. Large head volume was not a consequence of large body size, which was a poor predictor of head volume. Birds with large heads arrived early from spring migration, independent of sex and age, indicating that migratory performance may have an important cognitive component. Head volume significantly predicted capture date and recapture probability, suggesting that head volume is related to learning ability, although morphological traits such as wing length, aspect ratio and wing area were unimportant predictors. Intensity of defence of offspring increased with head volume in females, but not in males. Barn swallows with large heads aggregated in large colonies, suggesting that individuals with large heads were more common in socially complex environments. These results suggest that brain size is currently under natural and sexual selection, and that micro‐evolutionary processes affecting brain size can be studied under field conditions.

Behavioral plasticity is not significantly associated with head volume in a wild Chestnut Thrush (Turdus rubrocanus) population

“…Neither sex difference in head volume nor variance in head volume was found in Chestnut Thrush. Our result is not in line with a study of the Barn Swallow (Møller 2010), in which males have larger head volume than females, while females have greater variance than males (Møller 2010). Møller (2010) Fig.…”

“…Brain size measured using head volume has been validated across species (Iwaniuk and Nelson 2002;Møller et al 2011). At intraspecific level, the strong positive correlation between head volume and brain mass had been found in Short-tailed Shearwaters (Puffinus tenuirostris) (Iwaniuk and Nelson 2003), Budgerigars (Melopsittacus undulatus) (Iwaniuk and Nelson 2003), and Barn Swallows (Hirundo rustica) (Møller 2010). However, in female Great-tailed Grackles (Quiscalus mexicanus), linear measurements of skull were a poor predictor of head volume from computerized tomography scans (Logan and Palmstrom 2015).…”

“…So we use the change of activity in the presence and absence of the novel object to quantify the behavioral plasticity. Brain size is tightly correlated with head volume, so we use head volume as a proxy for brain size (Møller 2010;Öst and Jaatinen 2015). For reasons discussed above, we predicted that behavioral plasticity is positively correlated with head volume.…”

“…Although intraspecific variation in brain size is smaller than interspecific variation (Gonda et al 2013), there also exists an association between brain size and behavioral flexibility at the within-population level, including migratory behavior (Møller 2010;Fuchs et al 2015), food hoarding (Roth and Pravosudov 2009), antipredator behavior (Öst and Jaatinen 2015) and breeding strategies (Jaatinen and Öst 2016). Thus behavioral plasticity should be linked with brain size at the intra-population level (Mery and Burns 2010).…”

“…All birds trapped were weighed (to the nearest 0.1 g), measured for tarsus length (to the nearest 0.1 mm), wing length (to the nearest 0.1 mm), and head size (width, breadth, and height to the nearest 0.01 mm), and banded. We estimate head volume as the product of head length (minus beak length), head width, and head height to the nearest 0.01 ml according to Møller (2010). We captured 71 adult birds.…”