Action: Artificially incubate eggs or warm nests
- A replicated, controlled trial in the UK found that great tits Parus major were less likely to interrupt their laying sequence if their nest box was heated, although there was no effect on egg or clutch size.
- A small study in New Zealand found that no kakapo Strigopus habroptilus eggs or chicks died from chilling following the use of nest warmers. Before this a nest had been lost to chilling.
If incubating parents spend a long time away from the nest then the eggs may cool and potentially develop abnormally or die before hatching. For very intensively-managed species with very low populations, it may therefore make sense to warm eggs gently whilst parents are away from the nest.
Supporting evidence from individual studies
A replicated, controlled trial in 1991 in woodland in Oxfordshire, England (Yom-Tov & Wright 1993), found that blue tits Parus caeruleus nesting in heated nest boxes did not have significantly heavier eggs or larger clutches than those in unheated boxes. However, birds were less likely to interrupt their laying sequence in heated boxes (33% of 16 heated nests had interruptions vs. 67% of 14 unheated nests). Heat was provided by a small ‘night light’ candle, 8 cm below the bottom of the box, which raised the temperature in the box by an average of 6oc, saving roosting blue tits approximately 0.77 kcal/night, comparable to 35% of the energetic cost of producing an egg.
A small study on Codfish Island, South Island, New Zealand (Jansen 2005) found that no kakapo Strigopus habroptilus eggs or chicks died from chilling between 1997 and 2005, following the use of specially designed nest heat pads to keep eggs and chicks warm while the female is off the nest. Before pads were used, a nest containing three eggs lost failed, apparently due to chilling of the eggs and chicks as the female spent large periods of time away from the nest.
- Yom-Tov Y. & Wright J. (1993) Effect of heating nest boxes on egg laying in the blue tit (Parus caeruleus). The Auk, 110, 95-99
- Jansen W.P. (2005) Artificial incubation of kakapo Strigops habroptilus eggs and brooding of chicks while in the nest, Codfish Island, New Zealand. Conservation Evidence, 2, 6-7