Eggs
- Clutch control - The largest clutch size documented for a blue tit is an amazing 19 eggs. Beating this record, the grey partridge has gone one better with as many as 20 being recorded.
- Hard to swallow - A clutch of blue tit eggs can contain more calcium than the entire skeleton of the bird that laid them. That’s why egg-laying female tits often consume grit and snail shells to supplement their diet.
- Body building - The hard calcium-rich parts of a chick – beak, feet, scales and bones – come from the eggshell itself. The embryo absorbs this stored calcium as it develops, with the added bonus of making the shell thinner and subsequently easier to hatch from.
- The white stuff - Properly called albumen, egg white is an effective barrier against microbes that might cross over and cause disease in the developing embryo. It contains over a hundred antimicrobial proteins.
- Pink patch - For eggs to reach the perfect incubation temperature of 38°C, an incubating female sheds the feathers on her belly to form a brood patch that improves the transfer of body heat to the eggs.
- Rare as hens' teeth? - Nearly all birds have at least one ‘egg tooth’. Used to break through the eggshell, this small bump on the tip of their bill is reabsorbed soon after hatching. Woodpeckers have two; one on the lower part of the beak and one on the upper.
- Going to egg-streams - The mute swan is Britain’s largest flying bird and also lays the largest egg. In contrast the goldcrest lays the smallest egg; despite this her clutch of a dozen eggs can weigh over 1.5 times her own bodyweight.
- Equal opportunities - For blackbirds, dunnock, robins and tits, it’s the female that does most of the incubation work. Doves, sparrows and starlings are champions of equality, splitting the task between them.
- Sit on it - There are two strategies for incubation: wait until all the eggs are laid before starting to incubate or start when the first egg is laid. The former is the commonest, allowing all the chicks to fledge at the same time. The latter strategy is used by many birds of prey to cope with uncertain food supplies: when food is short, chicks that hatch first will survive even if the later ones don’t.
- Ground rules - Some eggs – such as those of terns and ringed plover – are deposited directly onto the ground. To fox predators they’re decorated with complex markings, making them hard to spot.
- Carried away - Parent birds will often carry away the empty shell when a chick hatches to avoid drawing attention to the nesting site. This explains the random appearance of mysterious eggshells on your lawn.
- Single fathers - A rare sight in the UK, male phalaropes (a group of shore bird species) have a dull colouration compared with females. Unusually, it’s the male that does all the incubating: the female partner isn’t involved at all in their chicks’ upbringing.
- Earlier birds - Since the early 1970s average egg laying dates for many garden birds have jumped forward by almost nine days – climate change is a big reason for this.
- All white inside - The eggs of most hole and cavity nesting birds are white. The adults don’t need to produce energetically expensive pigments since their eggs are hidden away from the gaze of predators.
- Mini egg - Eggs that are perfect miniature replicas of a species’ normally sized egg have variously been known as ‘runt’, ‘wind’, ‘witch’, ‘dwarf’ or ‘cock’ eggs. This phenomenon may be linked to poor nutritional resources being available to the female – or simple exhaustion.
- Breathing exercise - The seemingly impervious hard shell of a bird’s egg actually contains hundreds or thousands of tiny holes. These allow the exchange of gases between a developing chick and the outside world.
- Night-time is the right time - For most small perching birds, the process of shell formation occurs mainly at night.
- It's a cover up - Many birds such as ducks and geese will cover up their eggs when they leave the nest.
- All in a day's work - It takes about a day to build an egg. A journey down the oviduct starts with the yolk that’s fertilised, then the white and various membranes are added. The whole thing’s encased in calcium before decoration, colours and a final glaze are added.
- Eggshell paint - Two chemical pigments are responsible for the colour of eggs. Protoporphyrin makes reddish-brown colours while biliverdin produces blues and greens. Their relative amounts determine a particular species’ colour palette.
- Naked facts - The chicks of some birds are precocial, meaning they hatch already covered in downy feathers. Altricial chicks hatch naked; coming from eggs with smaller yolks, they have less food reserves and hatch earlier in their development.
