When breeding, birds have many different interactions with the animals and plants around them, as well as with other aspects of the environment. To breed successfully, birds require a place to build a nest and materials to build it with, they need enough food for their young, and they need to protect their young from potential predators or other sources of danger.
Birds usually breed at the time when there is optimum chance for their chicks to survive. This usually coincides with the period when food for the young birds is most abundant. Clearly this varies from region to region, and from species to species, and is inextricably linked both to the feeding ecology of the bird, and the prevailing climatic conditions. Some species undertake extensive movements between regions (and some even move between continents) to experience suitable breeding conditions, while other species remain in the same area and wait for favourable conditions.
Some shorebirds (also known as waders) only breed when there is an abundant supply of insects for their young to eat. To do this they undertake regular migration between Australia and Siberia. After spending the Australian summer feeding on the coastal mudflats, they form great flocks and fly through eastern Asia to breeding grounds in the Siberian tundra, only stopping off at a few key sites to 'refuel' on invertebrates along the way. They time these movements so that they arrive just as the frozen wastes are thawing out, and there are clouds of insects swarming everywhere. They lay their eggs among the stunted vegetation, and the chicks that hatch are able to gorge on the insects.
The breeding season of some species of waterbirds, such as the Blue-billed Duck, is determined by the season, and they breed each spring, regardless of the conditions. During droughts, however, the size of their broods is usually much smaller than during wetter years, and the levels of survival are low. Some other species, such as the Freckled Duck and the Black-tailed Native-hen, are far more discerning (or more opportunistic), and usually breed after the wetlands have been flooded. This is a response to an increase in abundance of food, such as aquatic vegetation, invertebrates and fish, which is directly triggered by the inundation, and it ensures that there is plenty of food for the ducklings to eat.
Honeyeaters usually breed in spring, which coincides with the flowering of many native plants. Honeyeater chicks, however, do not eat nectar, so why do they hatch when the plants are flowering? The answer is twofold. As the weather warms up in spring, many insect larvae hatch and turn into flies, moths, wasps and the like, and it is these insects that are fed to the honeyeater chicks. Insects are often attracted to the flowers to feed, and many of them depend on taking nectar or pollen to maintain their population. Honeyeaters (and other insectivorous species) are able to catch unsuspecting insects attracted to the flowers as they feed. Another advantage to this timing is that nectar provides high levels of energy for the adult honeyeaters, and this allows them to chase flying insects extremely energetically, and increases their chances of a successful pursuit.
There are almost as many types of nests as there are types of birds. Some species barely build a nest at all. The nests of some resident shorebirds, such as the Pied Oystercatcher, simply consist of eggs laid in a shallow scrape in the sand, though some have a shell or two, or a strand of seaweed as decoration. Most parrots, cockatoos, owls and many ducks nest at the bottom of tree hollows, where the eggs are laid among a few pieces of rotten wood. Some other species of birds, such as the Australian Hobby, refuse to build a nest at all, and use old nests built by crows or other raptors. Most birds, however, build a nest to lay their eggs in.
Many birds, from gulls to eagles to honeyeaters, use a variation on a simple cup-shaped nest, made from intertwined sticks, grass or bark. A few species have taken these designs a little further. Most species of robins build a neat woven nest of grass, but tastefully decorate the exterior with moss or lichen. The Northern Fantail builds a simple cup-shaped nest, but with an amazingly long tail hanging beneath it. The Zebra Finch has added a roof to its cup-shaped nest to turn it into a sphere of grass with a roof to protect the eggs and young. The Spotted Pardalote also builds a spherical nest, and places it at the end of a long tunnel which is excavated into a cliff or bank of soil. The Yellow-rumped Thornbill builds a spherical nest, but with a fake nest on top to fool any passing cuckoos that may want to lay their eggs. The Yellow-bellied Sunbird builds an oval-shaped nest which hangs by a long cord made from bark, grass and fibres, all intertwined together, and there is often also a similar tail which hangs below the nest. The Mistletoebird also builds a nest which is made from spider webs and plant down, and looks like a small bag or purse hanging from a branch.
Probably the most unusual nests belong to the megapodes, such as the Orange-footed Scrubfowl. After building a nest which consists of a huge mound of soil and organic material such as leaf litter scraped from the floor of the forest, they lay their eggs in the huge pile of debris. As the organic material rots down, like a compost heap, it heats up, and it is this process which incubates the eggs. The adult birds usually stay close by so that they can scrape some material away if the nest becomes too hot, or scrape extra onto it if it cools down, and in this way the temperature of the eggs is regulated and maintained.
How Birds Protect Eggs
All species of birds lay eggs. Because each egg contains the embryo of a developing chick it is full of protein, and eggs are highly sought after by all sorts of predators which find them a nutritious food-source.
Most eggs that are laid in open cup-shaped nests have shells that are patterned with spots, blotches or streaks which act as camouflage to protect them from potential predators. Some are so well disguised that even when you are looking for them they are almost impossible to find.
The eggs of birds which nest in hollows or tunnels, or in nests with a roof, are well hidden from view, so they have no need for camouflage, and are often white. Other birds hide their nests by building them in dense shrubs where they are difficult to notice, or in prickly bushes which make it difficult for predators to move through the spines to get to the eggs, and other species build their nests in the outermost foliage of trees where most predators cannot reach.
Another way that birds protect their eggs or chicks from predation is by their behaviour. Some species, such as the Australian Magpie and the Masked Lapwing, aggressively swoop at intruders near the nest, which is a very effective deterrent. Other species, such as the Spotted Nightjar, are so well camouflaged that when an incubating bird is approached it remains on its nest, keeping its eggs hidden, and most predators will walk past, unaware that the nest or bird was there.
Some species nest in large colonies, where the presence of many birds means that there is an increased likelihood that any predator will be detected quickly. An offshoot of this is that some species of birds nest cooperatively, with a number of birds looking after a single nest.