Spring has arrived, the American robin is foraging among the leaves, perhaps in lawns, hopping about, peering sideways, watching for any slight movement of the soil, then pouncing, pulling out a worm.
Robins are seen all winter although they are mostly migratory. If there are enough berries available to survive the winter, they will stay and patiently wait until their favourite food – earthworms – is available. Earthworms thrive in our lawns, are essential to our gardens and provide food for the birds, but they are not native to North America. They were introduced from Europe as early as the 1700s as unexpected passengers in the horticultural trade between the two continents.
In the thrush family, our robin is not related to the smaller European robin but was named American robin by early settlers because of their similar red breasts. Common throughout North America, they can be found everywhere, whether in urban areas or rural forests.
Robins pair up every spring. The male courts the female by serenading her with song at dawn and proudly puffing up his white-striped throat. They are quite indiscriminate in where to build a nest. Outside cities, they’ll build in any protected area, typically in a tree or large shrub. At city homes or cottages, nests can be found on upper window ledges, above doors, right outside a kitchen window. The female takes charge of nest building, which can take up to a week; the male helps by getting supplies such as twigs and grasses. The first of three or four eggs per brood will be laid about three days later. Robins typically have two broods per season, the male looking after fledglings while the female is busy with a new clutch.
Robins’ eggs are a lovely greenish-blue colour. Perhaps you’ve found eggshells on the ground? They are usually tossed far from the nest, so predators are not alerted to the location. If you find shells that are cleanly split, it’s from a hatched chick. If the shell is crushed and unevenly broken, it’s probably a victim of a predator – many animals welcome a meal of bird’s egg.
How do eggs get their colours? Egg formation from yolk to laying is a 24-hour process. (Bird reproduction varies from species to species and can be quite astounding. Interested readers can look this up!) Seasonal hormonal changes trigger egg production. The yolk is released from the oviduct where it is fertilized during mating. Then albumin, the egg white, forms around the yolk, which takes about four hours. It then travels to the uterus where glands in the lining expel calcium carbonate to form the shell, which takes another 15 hours or so. Then finally colouration, another five hours. A single egg can weigh up to 12 per cent of a bird’s weight – only one egg is produced at a time so the female can still fly without being weighed down by eggs!
Birds are the only backboned animal that lays coloured and patterned eggs. The colours and patterns depend on the species and, more importantly, where they nest. Species such as shorebirds that lay their eggs on the ground produce speckled or streaked eggs that blend into their surroundings, so well camouflaged that they can barely be seen among rocks, pebbles and leaves. Those that lay all-white eggs, like owls and woodpeckers, don’t need to colour their eggs because their nests are safe in a tree cavity. Songbirds that lay coloured eggs, such as the robin, tend to have open nests. The colour plays a protective role: darker surfaces heat up faster and protect more against harmful UV rays; lighter shells keep them cooler but protect less against the sun. Brown pigment patterning has also been shown to strengthen the eggshell.
Despite the wide variety of patterns and colours, they come from only two pigments produced from the lining of the uterus after shell formation – blue (biliverdin) and reddish-brown (protoporphyrin). The resulting colours are dependent on the proportions used. Precisely how a bird produces colours that seem almost spray painted on the white calcium carbonate shell is still one of the mysteries of bird biology.
Fun facts: The ostrich produces the largest egg, weighing up to 1.35 kg and measuring 15 cm in length. The smallest egg comes from a hummingbird, weighing 0.56 grams and about the size of a coffee bean.
It’s a great time to watch birds in our gardens. Say goodbye to some you’ve been watching over the winter as they head north and welcome the new birds arriving for the summer.
Jeanette Rive is a Glebe bird enthusiast and regular Glebe Report contributor.
Dedicated to Glebe resident Mary Marsh, committed nature and animal lover, friend to many.