Garrick Tremain, of Wakatipu, asks :-
In winter in the Whakatipu Basin we feed the waxeyes and are normally inundated with them. This winter, not a one. Where are they?
Derek Onley, an ornithologist at Waitati, responded.
Waxeyes (silvereyes, white-eyes; they are all the same species Zosterops lateralis) start nesting in September and continue through into February. The young can be still be in the nest in March and a pair of birds can raise up to three broods, of three or more young, in that time. Well before Christmas flocks of young birds can be seen moving around a district searching for food – fruit, nectar, insects – and sugar water if you still put that out over summer. By autumn numbers can build up to four or more times the spring population.
In the Dunedin area, in a couple of winters in the 1990s, several Ornithological Society members trapped and banded over 2,000 silvereyes in their gardens. Overall, less than 1 in 10, were ever caught again and many birds appeared to stay around for little more than a week. One garden nearer the centre of town seemed to be little more than a drive-by, fast-food, outlet with less than 1 per cent of banded birds ever seen again. As these figures suggest, the return rate the following winter was low; varying from almost none in the most suburban of areas to 10 per cent in rural areas with more bush.
The corollary to this ability to produce lots of young is that life is short. If it were not so we would rapidly be inundated with silvereyes – 100 this year could become well over 1.5 million by 2018. Although the oldest banded silvereye in New Zealand lived for over 11 years, the average for adult birds must be nearer 2-3 years and if you are a juvenile you are likely to have considerably less than an even chance of surviving your first winter; 60 per cent or more of young birds are likely to die in their first year. In contrast, the royal albatrosses of Taiaroa Head which live a long time, up to 60 years, do not start to breed until their tenth year or so and then do so only once every other year.
The silvereye lifestyle goes a long way to explaining the occasional dearth of birds at garden feeders overwinter, as there is considerable scope for large fluctuations in the total population, something common to many small birds in the more temperate and seasonal regions of the world. Variations in weather, invertebrate abundance, fruiting and flowering of food plants (good and bad flax flowering years is a good example) can all have large effects on the numbers of young produced, their subsequent survival and the numbers of birds that arrive in your garden overwinter.
So was it a poor year in the Whakatipu Basin? Was it a cold wet spring? Was food in short supply? Was it a poor breeding season? Well, maybe. Unfortunately life is not quite as simple as that and it is just as likely to have been a good one. Food may be abundant out there. It may have been an excellent year for fruit, native and exotic berries, wild and cultivated apples, all of which form a large part of a silvereye’s autumn diet. The Basin may be swarming with silvereyes but life is good and there is just no need to flock into the fast food joint in your garden.
That we cannot easily answer a question like this about one of our commonest of birds does not reflect well on the level of information, particularly longer term monitoring data, available in New Zealand. Unlike the UK and the USA where monitoring programmes for common land birds have been in place for well over half a century it is only in the last few years that a very basic scheme, Landcare’s Garden Bird Survey, has been set up. Please sign up for it through www.landcare.co.nz and we might be able to answer such questions as this with a little more confidence in years to come.