‘Green incubation': avian offspring benefit from aromatic nest herbs through improved parental incubation behaviour

Abstract

Development of avian embryos requires thermal energy, usually from parents. Parents may, however, trade off catering for embryonic requirements against their own need to forage through intermittent incubation. This dynamically adjusted behaviour can be affected by properties of the nest. Here, we experimentally show a novel mechanism by which parents, through incorporation of aromatic herbs into nests, effectively modify their incubation behaviour to the benefit of their offspring. Our study species, the European starling, includes in its nest aromatic herbs which promote offspring fitness. We provided wild starlings with artificial nests including or excluding the typically selected fresh herbs and found strong support for our prediction of facilitated incubation. Herb effects were not explained by thermal changes of the nests , but by modified parental behaviours. Egg temperatures and nest attendance were higher in herb than herbless nests, egg temperatures dropped less frequently below critical thresholds and parents started their active day earlier. These effects were dynamic over time and particularly strong during early incubation. Incubation period was shorter in herb nests, and nestlings were heavier one week after hatching. Aromatic herbs hence influenced incubation in beneficial ways for offspring, possibly through pharmacological effects on incubating parents.

Plasticity in diurnal activity and temporal phenotype during parental care in European starlings, Sturnus vulgaris

“…Incubating females were more active later in the day suggesting that females benefit by self-feeding more, and being more active, just prior to fasting associated with night-time incubation which can involve increased energy demand (de Heij, van der Graaf, Hafner, & Tinbergen, 2007). An alternative explanation is that lower overall levels of early morning activity are associated with constraints of incubation with females delaying leaving the nest until ambient temperature has increased so that eggs cool less, decreasing subsequent rewarming effort (H. Gwinner, Capilla-Lasheras, Cooper, & Helm, 2018). However, this is not consistent with our finding that incubating females first left their nests earlier in the morning than chick-rearing females (see below).…”

Climate as an Evolutionary Driver of Nest Morphology in Birds: A Review

“…The spotless starling uses unpigmented feathers and aromatic plants to enhance antimicrobial protection and nestling survival (Ruiz-Castellano et al, 2018). The European starling adds aromatic herbs to their nests that improve nestling condition through stimulating higher parent attendance and longer incubation bouts (Gwinner et al, 2018).…”

Development of nestling blue tits (Cyanistes caeruleus) is affected by experimental addition of aromatic plants

“…Studies focusing on nestling weight found no influence of aromatic plants on nests of European starlings, 6,9,15 blue tits, 4,12,16 spotless starlings, 11 and tree swallows. 13,14 However, an increase in nestlings’ weight was observed in European starlings, 8,17,18 but no influence of aromatic plants on nestlings’ tarsus length was observed in another study, on the same species, 15 nor in spotless starlings. 11…”