Developmental differences are correlated with relative brain size in birds: a comparative analysis

The possible relationships between relative brain size and developmental mode and between relative brain size and five measures of the length of the development period were tested across over 1400 species of birds. Using both conventional statistics and phylogenetically based comparative methods, significant differences in relative brain size were detected among modes of development. Across all species, there were significant relationships between relative brain size and each of the following developmental traits: incubation period, age of fledging, duration of postfledging parental care, and total period of parental care. In contrast, the age of first flight was not significantly correlated with relative brain size. The relationships between these five developmental traits and relative brain size varied among developmental modes and orders such that significant relationships were present within some modes and orders but not in others. Thus, developmental differences play a significant role in the evolution of brain-size differences, but the role depends upon the taxonomic level being investigated. This is likely due to the differential lengths of periods of neural and behavioural development in young birds. Our conclusions support the contention of previous studies that developmental differences have played a key role in avian brain evolution.