Of Eggs and Colours

We’re now over halfway through the incubation period and all appears to be going well, even if there’s not a great deal of action.  I’ve spent a few hours down at St George’s on some early mornings and have seen an awful lot of the non-incubating bird sitting about on one of its several favoured perches near to the church.  No photos to show for it I’m afraid!

On further reading of Tim Birkhead’s comprehensive and very readable study of birds’ eggs, it’s been fascinating to learn about why – and how – eggs are coloured.  The mechanics of ‘how’ are not fully understood, but there are some good hypotheses as to ‘why’.  First, a quick comparison of eggs from some of the nests with webcams not too far from Sheffield, which I’ve noticed from the twitter feed.

March 29

These, of course, are the eggs of the Sheffield pair, a deep russet-brown with some darker flecks.

Lincoln eggs

Next we have the unusually large clutch of the Lincoln cathedral pair, markedly paler in their background colour and with reddish blotches.

Wakfield eggs

Then it’s the Wakefield pair, whose ringed male is tantalising us with thoughts that he may well have come from the birds ringed in Sheffield in May 2014.  Perhaps he followed the copy of the St George’s nest platform that Jim Lonsdale took up to Wakefield!  These eggs are obviously darker than Lincoln’s but paler than Sheffield’s.

Notts eggs

And finally Nottingham’s pair, also with four eggs.  These are the closest in colour to the Sheffield clutch.

So there’s quite some species variation between clutches, and individual females are known typically to lay the same number of eggs from year to year, and the same colour.  Which brings us to the first hypothesis as to why eggs are coloured and/ or patterned: the ability for a bird to recognise its own eggs.  Given that Peregrines do not nest communally and are extremely unlikely ever to find confusion as to which egg is theirs among many in close proximity, this seems a non-starter.

The second idea is equally unlikely: as a means of avoiding brood parasitism.  Tim Birkhead describes as an ‘arms race’ the struggle for some species to keep the design of their eggs complex enough to allow them to identify eggs laid in the nest by parasitic species, such as cuckoos or cuckoo finches.  This is not a problem faced by Peregrines, so that’s two strikes.

Osprey-Peregrine 15 June 2013

The third notion revolves around camouflage and conspicuousness.  Over the last few years, it’s been evident that the Peregrines attack Crows, gulls and other raptors when they overfly the nest site (as above, taken when an Osprey passed St George’s previously), so there may be some merit in considering the role that colour could play in camouflage, with darker eggs less likely to be noticed by passing potential predators.  Not entirely convincing, I agree.   One fascinating experiment to explore the benefits of white eggs in relation to darker eggs involved painting Ostrich eggs (which are white) dark brown, with the result that the darkened eggs were 3.6 degrees C warmer than the white eggs.  In very hot regions (so not Sheffield!) this can be a problem, as it can raise the temperature of the egg above the limit for survival of the embryo, but in temperate regions it may be beneficial to produce darker eggs to maintain a higher temperature when they are not being incubated.  This might be especially relevant in the case of Peregrines, with the first laid egg(s) routinely left unincubated for extended periods in the early stages.  Perhaps.

However, that still wouldn’t explain the variation between the clutches shown above, all of which are in the same climate zone.  I’ll leave that one to the scientists…  So many unanswered questions.  As you’ll have gathered, Tim Birkhead’s book is one I’d heartily recommend: not only does it provide answers to many questions about birds’ eggs and their success, it also makes you think about things you’ve never thought about, even if once he raises them you wonder how you can have never considered them before.

So, 12-14 days to go before the St George’s clutch are likely to hatch: fingers crossed!



  1. I am following a pair in Phoenix Arizona, very hot there, just got back from holiday and the eggs appear dark brown. Four eggs, one damaged, should hatch end of April. Am 75yrs old and so excited.

  2. I am following peregrines in Phoenix Arizona, just come back from holiday there. There are four eggs, one damaged, should hatch end of April. Eggs dark brown mottled, new Mum, experienced Dad. Am so excited,

Leave a Reply to Sonia hennigan Cancel reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s