The problem with Canada geese
For much of the year, the Canada geese problems are not obvious around Pauatahanui.
Because the birds’ preferred food – short pasture grasses and herbaceous wetland plants – is readily available, the geese feed in small groups over a wide area and their impact on the environment is not great.
But at other times, geese will descend in groups of up to 30 on the tidal basins that are the feeding spots for native wading birds, stripping the fringing salt-marsh plants and fouling the water.
Other native birds, including spoonbills, herons and kingfishers, are unable to feed at prime locations when Canada geese are roosting in large numbers (up to 40 birds) on the banks at the mouth of the Pauatahanui stream and from Horokiri Stream to Ration Creek.
The problem gets worse at the start of the breeding seasons, when pairs are seeking safe nesting sites.
A good nesting site needs quick, easy access to a reliable waterway and a plentiful food source nearby for parent birds.
Such sites are essential until the goslings fledge, and then throughout the summer until the moult is over and the adult birds can fly again.
Suitable breeding and moulting sites are often limited, so several pairs of geese will nest around the same site.
The geese will defend their nest sites aggressively, to the extent the gander will hold an intruding duck under water until it drowns.
An average nest will contain between six and nine eggs.
So, as happened in the Pauatahanui Reserve when the geese first nested there, we had seven adult birds (one a non-breeder) and 26 goslings roosting along about 15 metres of grassed track, feeding there and in the adjacent pond.
Canada geese are large birds that eat and poo a lot, so it doesn’t take many in such a small area for a month or more to do damage.
Public access tracks are fouled to the point where they become unusable. But more important is the intrusion into and fouling of habitat used by native species, such as herons, shovelers, and especially pied stilts.
For the past few years, a tidal basin in the reserve has been the preferred nesting site for up to six Canada geese breeding pairs.
However, the tidal basin is also the nesting site and main feeding ground for the native pied stilt.
By using the shell islands, where the stilts normally nest, as lookout posts, and by fouling and muddying the stilt feeding ground, the Canada geese endanger an important southern North Island pied stilt breeding colony.
And by feeding ravenously on the fringe of the tidal area, the geese are seriously damaging the herbaceous salt-marsh plants.
The concentration of these geese into small niche areas for extended periods causes the problem and creates the need to control their numbers.
We had hoped that population control of geese in the reserve could be achieved by finding the nests and preventing the eggs from hatching, but this has not been effective.
In 2013 there were no goslings hatched in the reserve, but late in the breeding season, three pairs of geese that had raised clutches farther afield brought their goslings into the reserve, so our efforts at controlling numbers were largely ineffective.
In North America, where Canada geese are increasingly entering the urban environment, experience indicates that interfering with breeding by addling eggs is not enough to offset the effect of the 20 to 30-year life expectancy of the geese.
From being rare visitors of one or two birds in the 1980s, the local flock is now more than 70 birds and growing rapidly.
Canada geese are increasing throughout New Zealand and are causing so much damage to cropland and the habitat of native birds and plants, that the species is now on the pest bird list.
It is obvious that to protect the habitat of native birds and plant species, control of Canada geese must be tackled on a broader basis than just addling eggs.
Menace: A Canada goose with goslings.