The period of covering the eggs by the parents is continuing. I have noticed a slight change in behaviour in the last few days whereby fewer prey are being brought to the box as gifts for Diamond, so there is some movement. She may be hunting or finding prey in caches.
I can’t remove the eggs until they completely stop incubating. I would like to candle them to see whether they have been fertilised (which I doubt). It should be noted that most male peregrines do not attempt to breed until they are three (Olsen 1995). Gimbir is just two, so I think we were very lucky with Girri last year.
Questions are frequently raised in a situation like our current one, where the eggs are clearly non-viable, but the parents continue to cover them and keep them warm, sometimes called ‘over incubation’.
- Why do they do it?
Answer: There is a rise in the hormone prolactin at the onset of incubation and a decline with hatching. This can be rapid with precocial chicks (up and running about straight away), but more slowly in atricial young such as peregrines due to their need to be brooded until thermal independence. So there is considerable variability between species. Levels of prolactin are usually higher in females. The level in males usually depends on the amount of time he spends incubating (often around 30–40 %). Interestingly, the level of testosterone in males usually decreases concomitantly (Vleck 2002).
- Is there a ‘cost’ involved with this activity?
Answer: yes, although probably it is not that high. It is a sedentary activity, unlike hunting, but there some ‘costs’ involved in incubation. Maintaining a temperature gradient between the eggs and the environment requires a certain amount of energy, especially if the eggs are allowed to get cold and need re-warming. And of course it means they can’t spend time doing other things such as hunting or attracting suitors (Reid et al. 2002). In our case, the nest box provides shelter and a reduction in the temperature gap. Over incubation is also unlikely to interfere with the following breeding season which is many months away.
- How long will this behaviour continue?
Answer: There is very little information published on this behaviour. An early north American book by Skutch (1976) provides a table of species showing over incubation duration of about 50%, occasionally rising to 100 %. As peregrines’ normal incubation times is about 33 days, this could mean 16 to 33 days. This was exceeded by a pair of peregrines in New Hampshire where a pair of peregrines were observed in 1993 incubating a total period of 73 days i.e. 118% longer than normal. In Montreal two non-viable eggs were finally left after 90 days (157 % over !). That’s from Facebook, so I’ll double check that! So I can’t predict exactly when our pair will give up incubation as it is so variable. We have to let nature take its course.
REFERENCES
Journal of Raptor Research (1993). Peregrine Falcons incubate clutch of eggs for minimum of 73 days.” Journal of Raptor Research 27 (3, Article 11).
Olsen, P. (1995). Australian Birds of Prey. University of New South Wales. Sydney.
Reid, J.M., Monaghan, P. and Nager, R.G. (2002), Incubation and the costs of reproduction. In: Avian Incubation (Ed. D.C. Deeming), pp. 314–325. Oxford University Press, Oxford.
Skutch, A.F. (1976). Parent Birds and their Young. University of Texas Press. Austin.
Vleck, C.M. (2002), Hormonal control of incubation behaviour. In: Avian Incubation (Ed. D.C. Deeming), pp. 54-62. Oxford University Press, Oxford.