The variation in size, colour and shape of bird eggs is part of what makes them so fascinating! This variety reflects the diversity of Australia’s birdlife.
The study of bird eggs is called “oology”, and is derived from the Ancient Greek word for egg, “ōión”. The hard-shelled structure that holds the yolk (yellow disc) and clear albumen (egg white) is what we all recognise from the humble chicken egg, for example. An egg shell is made from calcium that is deposited via mineral glands in the oviduct. As the egg moves through the oviduct, calcium is laid down and pigments are also deposited, which gives eggs their colours and markings. The surface of the egg is covered in small pores which allow heat and oxygen transfer to the tiny embryo that will eventually hatch into a fluffy chick.
The largest bird on the Australian mainland is the Southern Cassowary (Casuarius casuarius) and lays the largest eggs of all Australian bird species (138m long and 95mm wide). The clutch of four eggs are laid in a shallow scrape on the ground, and the eggs are a uniform pale green colour. At the opposite end of the scale is one of Australia’s smallest birds, the Weebil (Smicornis brevirostris), which usually lays two smooth creamy-buff coloured eggs that measure approximately 15mm long and 11mm wide.
The range of patterns and colours seen in Australian bird eggs are vast, from soft pastel pinks laid by species such as Honeyeaters, to cream coloured eggs marked heavily with dark brown and lavender scribbles, such as the eggs of the Regent Bowerbird (Sericulus chrysocephalus). The lustrous eggs of the Comb-crested Jacana (Irediparra gallinacea) are also marked with fine black lines, to assist with camouflage among the aquatic vegetation where it lays 3 to 4 eggs. The eggs laid by the Glossy Ibis (Plegadis falcinellus) and the White-faced Heron (Egretta novaehollandiae) are shades of pale blue-green.
Some birds use trickery to have their own eggs incubated by other birds and have these unsuspecting parents do all the hard work! The Horsfield’s Bronze-cuckoo (Chalcites basalis) is a brood parasite, meaning it will lay a single egg in the nest of a host species and leave it to the host parents to incubate the egg and raise the chick until it is ready to leave the nest.
The sizes and shapes of eggs are the perfect combination of structure and function: large enough to provide nutritional content and room for the growing chick, as well as being the right size and shape to be able to be properly incubated. The shape of an egg is usually larger at one end, and slightly narrower at the other. In the south-east of Australia, the Peregrine Falcon (Falco peregrinus) commonly nests on cliff ledges, and lays eggs that are pointed at one end, and well rounded at the opposite end (called “ovate”). This egg shape causes the egg to roll in a tight circle, which means that the egg is less likely to roll off the edge if it is accidentally knocked or pushed. Parrots and cockatoos, on the other hand, lay eggs that are white and mostly round which makes them more visible in dark tree hollows where they nest.
The collection of wild bird eggs was a very popular hobby during the 1800s in Great Britain and the United States of America, and avid amateur oologists that were also working in Australia at the time. A great example of this is on display in the museum’s Westpac Long Gallery, which has on show a variety of bird eggs uniquely arranged in a crafted wooden box. The box contains the eggs of Australian birds, including the Laughing Kookaburra (Dacelo novaeguineae), Australian Magpie (Gymnorhina tibicen), Galah (Eolophus roseicapilla), Rainbow Lorikeet (Trichoglossus moluccanus) and Superb Lyrebird (Menura novaehollandiae). Colour postcards of various birds accompany the eggs, which have been lovingly and skilfully arranged by this dedicated – but unknown – amateur scientist.
Alfred John North, one of the Australian Museum’s early ornithologists and egg collectors, was mostly responsible for caring for the museum’s egg collections and published the huge four-volume series Nests and Eggs of Birds Found Breeding in Australia and Tasmania (1901 – 1914). Today, the ornithology collection has over 22,000 clutches of eggs and can answer questions about bird biology, ecology, behavior and population dynamics.
Dr Leah R Tsang, Collection Manager & Scientific Officer, Ornithology, Australian Museum Research Institute
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