NOW we are in the swing of spring, many of our local birds have begun the process of nesting.

Obvious signs of this are birds carrying sticks and gathering grass and fine bark to build nests.

Perhaps less obvious is the nesting behaviour of hollow-nesting species because their breeding sites are confined to holes in trunks and branches of trees and stumps.

Many of these hollow-nesting birds do not actually construct a nest inside such hollows, rather they lay their eggs on a bed of rotting timber inside the hollow.

All of our parrots and cockatoos, owls, kookaburras and owlet-nightjars fit into this category.

However, birds like treecreepers, pardalotes, whiteface and Tree Martins do construct a grass nest within their chosen hollow.

Hollow trees, whether they be old or young, dead or alive, are incredibly important for a range of different bird species, not to mention arboreal mammals such as possums and gliders, microbats and many reptiles such as goannas, pythons, dragons, geckos and skinks.

Even small holes are valuable habitat, in fact most native species prefer a small entrance to their hollow cavity, at times having to squeeze through tight spaces in order to access their nesting site.

In most cases, hollows take many decades to form.

Large hollows required for larger species like kookaburras and owls are only available in trees well over 100 years old, often considerably older.

These mature veteran trees have survived a range of landscape and environmental processes such as storms, lightning, fires, insect-attack and fungal growth over countless years, and it is this process of weathering, scarring and limb fall that causes hollows to develop.

Younger trees have obviously experienced less hollow-forming conditions but it is important to allow these to grow to maturity to ensure that hollows will readily and continually develop and become available to the host of species that depend upon them.

Most importantly, it is vital to retain the large hollow-bearing trees that we have left, whether they be in the farming environment, in our native forests and along our watercourses and roadsides, as they are foundation habitat that cannot be replaced.