One of the joys of living near Shakespear Regional Park is that some of the increasing numbers of native birds begin to overflow into surrounding areas.
The fortunate residents of Shakespear and Everard Roads will have benefited most, but over the last month, and after eight years of hopeful watching, a bellbird/korimako has been a welcome visitor to our Matakatia garden.
This is the time of year when the annual bird survey is carried out at Shakespear. Members of the survey team follow fixed counting routes, which include bush areas and paddocks. The aim is to find out how the mix of species and the numbers of each have changed since 2011, when the pest-proof fence was completed, and the poison drop took place.
We don’t yet have an analysis of that data but a recently published study by Rachelle Binny, from Manaaki Whenua-Landcare Research, and her colleagues, which includes data from Shakespear and Tawharanui Regional Parks, gives clues about what to expect.
It will be no surprise that excluding mammalian predators from native forests benefits native plants and animals. But some of the effects may not be so obvious. Birds that benefit most, such as saddleback/tīeke, belong to families containing only endemic species, while birds from families where only some species are endemic, such as bellbirds and tomtit/miromiro, benefit less. It is believed that endemic species have evolved traits – such as cavity-nesting, reduced capacity for flight, and larger size – that make them more at risk from introduced predators.
Native (but non-endemic) species, such as silvereye/tauhou, tend to decline – as do non-native introduced species, such as chaffinch/pahirini. Decreases in non-natives begin to be noticed about seven years after predator removal and are possibly linked to competition from increasing numbers of endemics.
Skinks and geckos probably also benefit, but a lack of data makes it difficult to estimate by how much.
Removal of possums and other browsing mammals helps restore native forests. There’s an additional benefit when the native animals recover, as they help with pollination and seed dispersal.
Mouse numbers at Shakespear have grown rapidly since their predators were removed, and this has negative impacts, especially on the insects and other invertebrates that are part of their diet. Shortly after the pest removal at Tawharanui the mouse density there reached the highest levels recorded on the NZ mainland.
At Tawharanui, tūī and bellbird numbers have increased dramatically since 2004 while saddleback, whitehead/pōpokotea and robin/toutouwai have increased at a lesser rate. Fantail/pīwakawaka numbers didn’t change much but silvereye and grey warbler/riroriro declined. It is likely that the changes at Shakespear will be slightly different as they are dependent on many site-specific conditions.