The research – a collaboration between researchers at Manaaki Whenua – Landcare Research and the Department of Conservation (DOC) – aimed to understand how rat diet may change over different years. Ship rats have a field day during beech “megamasts” (mass seeding events), when the flood of extra food in the forest leads to a population spike. These increased numbers of rats spell trouble for our native birds, but a crucial question remained – is it just the extra rats that are the problem, or are birds also a bigger part of the rat diet when the seed runs out?
“Answering this question is important to enable the best timing of predator control efforts for bird conservation,” explains lead researcher Dr Jo Carpenter. “If rats do end up eating birds more often when beech seed runs out and they begin to starve, that suggests pest control should be timed just before that happens. However, if they eat birds at a similar rate all the time, that suggests we need to always be keeping their numbers down.”
But, until now it’s been very difficult to tell exactly what rats eat, partly because they chew their food up so finely. The researchers got around this problem by using genetic techniques to illuminate the “ghosts of past meals” by screening the DNA of over 200 rat stomachs. These stomachs came from rats trapped at Lake Alabaster in Fiordland over three years following the megamast of 2019.
The results were sobering – the rats were eating 15 different native and introduced bird species (with a particular fondness for native silvereyes and introduced blackbirds), as well as 40 different plant species (mostly silver beech and broadleaf species, but also mosses and orchids). The proportion of rats consuming birds was surprisingly high – nearly one in five rats overall had bird DNA in their stomachs, with more birds eaten at lower altitudes where birds are more abundant. However, this proportion didn’t change over the three years – one in five rats ate birds all the time, suggesting that most rats did not switch to eating more birds when the beech seed bonanza finished.
“We were surprised by how frequently the ship rats were eating birds,” comments co-author Dr John Innes, “because most other studies have found birds are only an occasional part of ship rat diet. However, those studies have all had to pick through rat stomachs to find tiny fragments of eggshell or feathers, so it could be that bird consumption was under-estimated.”
So what does this mean for conservationists? “Our findings suggest that it’s the sheer number of rats in a mast year that are the problem for our native birds, rather than those rats also eating birds more frequently than usual,” explains co-author Dr James Griffiths, a science advisor at DOC. “This means it’s critical for rats to be kept at low densities where possible, while also timing that control to best protect birds when they are particularly vulnerable, such as during nesting.”
