Depending on the nest box entrance size (large, medium, small etc), nest box occupants are at risk of predation or competition by a number of different native and introduced species.
Many will have read by now the discovery by researchers, of Sugar Glider predation on Orange-Bellied parrot females when they are sitting inside tree hollows or nest boxes on nestlings and eggs in Tasmania, where Sugar Gliders have been introduced.
On my property, I have recorded footage of Brushtail Possums dragging a famale sitting Wood Duck off her nest at night, attempting to kill her before she escaped into the darkness with her life. I have also observed Ravens and Currawongs enjoying a snack of Wood Duck eggs, that Brushtail Possums will happily raid any nestbox they can fit into, and that the resident Sugar Gliders really enjoy eating Crimson and Eastern Rosella eggs.
I would consider that Brushtail Possums and Sugar Gliders pose the greatest risk to nesting birds, they are also tree hollow users themselves. Other common nest raiders include Ravens and Currawongs, while other native predatory birds would no doubt enjoy a snack of an egg, nestling or possum should the opportunity arise. Introduced nest box raiders include black rats and cats. Unwanted nest box box competitors include the introduced Indian Myna, Starling, and European Honeybee.
Brushtail Possums will eat eggs, chicks and adult birds (if they can), and have also been found to occupy up to 38 different tree hollows throughtout a year. That’s quite a lot of real-estate to be hogging, in a landscape where tree hollows are often a scarce resource! On our property, I have found that birds will avoid nesting in a nest box where a Brushtail used to sleep, regardless of the bedding being refreshed, nest box relocated and possum tree gards installed to prevent access by Brushtail Possums. I have noticed that our smaller possum residents, Ringtails and Sugar Gliders, will not use nest boxes that Brushtail Possums can fit into.
To prevent access by Brushtail Possums to nest boxes (or tree hollows), I’ve found that installing 1 m wide polycarbonate plastic around the tree trunk below and above (if tree canopy is touching another tree canopy)) a nest box is very effective. To exlude Brushtails, a guard at the base of the tree for a high nest box is sufficient, so long as no shrubs or adjacent trees are touching any part of the tree above the tree guard. Just be sure to cut the length of the polycarbonate plastic with sufficient room to let the guard out as the tree trunk expands, every other year. This strategy would also be affective at preventing access by cats and black rats, I would think.
I am concerned about the likelihood of the 1 m wide x 2 shiny plastic possum guards on the trees scaring more fussy nesting birds away. I have now painted some plastic guards in matt grey just in case, but this question does not yet have an answer. White-throated Treecreepers, Wood Ducks, Crimson and Eastern Rosellas and Kookaburras apparently don’t mind.
Sugar Gliders also enjoy snacking on the eggs and chicks, and adults, of generally smaller hollow-nesting bird species.They are often very common in locations where they naturally occur, and have also been introduced to Tasmania where they are impacting on the endangered Orange-Bellied Parrot’s nesting success and adult female population. Sugar Gliders will use around 5 different tree hollows throughout a year, and I have noticed that this is quite seasonal.
To prevent access by Sugar Gliders to nest boxes (or tree hollows), I have found that installing 1 m wide polycarbonate plastic tree guards right above, and right below a nest box is necessary, regardless whether the tree’s canopy is or isn’t touching o the canopy of a nearby tree.
Also important is selecting nest box installation trees that are several meters away from tall shrubs and other trees, such that no branches small or large are even close to the nest box or tree guard region. Unlike Brushtail Possums, Sugar Gliders can fly and are very light climbers, so strategies need to reduce the ability of a glider to land on the tree trunk in between the tree guards where the nest box is installed.
Sugar Gliders love eating Rosella eggs and chicks, I have discovered. And will attack sitting female Rosellas at night to get to their eggs and chicks.
I have observed Ravens and Currawongs raiding Wood Duck nest boxes on our property, and there are number of other predatory native birds that no doubt would enjoy an egg or nestling snack if the opportunity arose.
While it is difficult to prevent access to nest boxes by birds, one can learn from bird nesting behaviour and copy with nest boxes. Birds vulnerable to predation choose nesting spots where they can come and go without being easily seen, generally away from busy locations, with nests obscured such as by vegetation, a tangle of fallen branches etc. Nests are made to be invisible, for all but the most expert of nest spotters. Careful selection of nesting location, nest design and discrete behaviour by parents going to and from a nest to feed young will reduce the chances of the nest being discovered by a predator.
In applying this knowledge to nest boxes, I have painted most nest boxes to match the colours and patterns of the trunks of our native woodland trees. I try to install the nest boxes where a bird can look for danger before entering a nest box (and so giving the nest location away) from nearby cover of leafy branches. Unfortunately because of Sugar Glider and Brushtail Possum predation risk I cannot installing the nest boxes in amidst leafy branches. However for locations without this issue, installing nest boxes in amongst dense vegetation would be a great protective strategy. I have also spread out the larger lunch-sized snack boxes (Wood Duck nest boxes) across the property, from originally being all grouped together. This has reduced the risk of duck action being spotted (single duck pairs rather than several), and if one nest box is discovered, the raiders don’t then just go from one to the other, as the other boxes are discretely located elsewhere.
I don’t have experiences with Indian Mynas and Starlings as tree hollow competitors, however will add a section on this in the future.