Hippocampal volume is related to complexity of nesting habitat in Leach's storm-petrel, a nocturnal procellariiform seabird

The hippocampal and telencephalon volumes of the nocturnal Leach's storm-pe trel (Oceanodroma leucorhoa, n=15) were compared with published data for fo od-storing and non-storing Passerines. The hippocampus to telencephalon rat io of Leach's storm-petrels is intermediate between food-storing and non-st oring birds. Leach's storm-petrels taken from nesting burrows in wooded hab itat had a larger relative hippocampal volume than those taken from burrows in an open meadow. Relative olfactory volume did not differ between woods and open-nesting storm-petrels. The larger relative hippocampal volume of s torm-petrels may be associated with increased spatial demands of returning to their nests at night in the darker, more navigationally complex woods. I t is not known whether the larger hippocampus in storm-petrels from the woo ds is due to selection on different subpopulations or whether experience in a more complex environment results in greater hippocampal volume. Hippocam pal volume from the brain of one diurnal Procellariiforme, the northern ful mar (Fulmaris glacialis), fell within the range of non-storing species, whi ch supports the view that hippocampal enlargement in the storm-petrel is re lated to the spatial demand of returning to the nest at night.