Aromatic plants in nests of blue tits: positive effects on nestlings

For altricial birds, detailed studies of parental care include egg incubation, brooding and chick feeding, all of them being crucial to offspring survival. Few studies have explored nest building and maintenance from this perspective, although the nest is the first environment experienced by hatchlings. On Corsica, blue tits, Cyanistes caeruleus, incorporate fresh fragments of several species of aromatic plants in the nest cup throughout the nesting period, and replenish the nest with fresh fragments of the same plant species quickly after experimental removal. In this study, we experimentally tested whether aromatic plants used by blue tits affect the growth and condition of nestlings. Because we expected the effects of plants to depend on environmental conditions, we tested this hypothesis both in enlarged and in control broods. Aromatic plants positively affected chick mass gain in enlarged broods. Chicks in nests containing aromatic plants also had higher haematocrit levels. In addition, in 1 of the 2 study years, feathers developed faster in aromatic-treated nests than in nests where aromatic plants had been replaced by a neutral nest material (moss). However, no significant effect of aromatic plants on chick body size at fledging was found. We propose several hypotheses on the proximal mechanisms whereby aromatic plants affect chick growth and condition.