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Over the past 30 years there has been extensive and increasingly sophisticated monitoring of bird populations after aerial 1080 operations. Scientists conclude on the present evidence that “the ecological costs of using toxins is much less than the damage if they are not used"⁷. In fact, when aerial operations are timed to decimate rat and stoat populations as well, then birds benefit, especially endangered species, as significantly more adult birds and nestlings survive.

This section summarises results from bird counts, bird banding, radio tracking and nesting studies carried out to assess the impacts of possum control on non-target species³³.

Many operational improvements have occurred since 240 native birds, mostly insectivorous species, were found dead after four aerial 1080 operations in 1976 and 1977. Those operations had all used poor quality baits – undyed, raspberry-lured, unscreened carrot baits, with a very high percentage of “chaff”. Some 40% by weight of the carrot was small fragments (chaff) of less than one gram, which were both extremely numerous and lethal to small birds. Screens now remove pieces of carrot weighing less than two grams. Raspberry lure is banned, carrot baits must be dyed green and the addition of cinnamon flavour deters birds while masking the 1080 odour to possums.

Since bait screening was introduced, there have been few dead birds found in ground searches after aerial operations. Between 1978 and 1993, extensive monitoring after 70 aerial 1080 operations, using screened carrots or cereal-based baits, found a total of 83 dead birds, of which 34 were native species. Blackbirds and tomtits were the species most commonly found dead. This was less than one detected native bird death per operation. Monitoring surveys indicate that few bird deaths occur from aerial 1080 operations as long as best practices are followed.

However, since not all birds that die from 1080 poisoning are found in searches, it is important to use other methods to assess 1080 effects. It is critical to determine whether the bird populations are more or less abundant after 1080 use, even though some individuals are killed. This is because the impacts on populations as a whole are more important for overall survival than the fate of a few individuals. One method for estimating population abundance is from 5-minute counts of birds seen or heard in areas before and after aerial operations. These results are compared with counts made at the same time in non-poisoned areas. This was done for 24 aerial 1080 operations, using carrot or cereal-based baits, between 1978 and 1993. The number of common bird species counted in poisoned areas did not change two to eight weeks after poisoning in relation to the numbers of birds counted in non-poisoned areas. This suggests there were no negative impacts on the populations from the 1080 operation.

Survival of kokako and fernbirds

One technique for more accurately tracking birds during 1080 operations is to band them with coloured leg-bands and check on their survival after the operation. During a number of aerial operations using 1080 cereal baits throughout New Zealand, 47 kokako were leg-banded. All 47 individuals of this endangered species survived. This result was particularly encouraging, as studies have shown that possums and ship rats are the major causes of poor nesting success for kokako. Seven fernbirds were also leg-banded, and all survived aerial operations using cereal baits.

Radio-tracking rare species

Another accurate way of examining the effects of aerial 1080 operations on rare or endangered species is by tagging them with radio transmitters. This has been done for about 230 birds, representing seven species, over the last decade. Birds are fitted with radio transmitters before the operation and then checked some weeks afterwards. The results, shown in Table 3, are very reassuring, indicating very low mortality from poisoned baits or secondary poisoning. Monitoring using radio-tracking is ongoing in various studies around the country.

The two dead birds (one weka and one morepork) both contained residues of 1080. Importantly, none of the endangered brown kiwi, great spotted kiwi, blue ducks or kaka died within a month of the operations. It is reasonable to conclude that aerial 1080 operations for possum control pose little risk to these species. On the contrary, bird populations benefit from the reduction of possums and other predators. For example, the kaka population in part of Pureora Forest Park increased by 33% within six months of aerial control in 2001. All of the 20 females with radio transmitters in the treated area survived, whereas in the nearby Waimanoa Forest, stoats killed at least five of nine kaka females during the 2001 nesting season.

In 2001, before an aerial 1080 operation in Tongariro Forest, 32 kiwis were radio-tagged. All were still alive six months later and 40% of their chicks survived. Compare this to the usual survival rate of kiwi chicks which was less than 5% before the 1080 operation. The aerial operation, therefore, appears to have significantly improved chick survival and left the kiwi adults unharmed as well.

Effects on insectivorous birds

Insectivorous birds are probably the species most at risk from aerial 1080 operations (see Susceptibility to 1080, Chapter 4.1). These small birds are also very vulnerable to predators such as rats and stoats. Their small body size means that carrot chaff coated with 1080 can provide a lethal dose, while their diets can include invertebrates that may have eaten some 1080 from baits. Robin and tomtit populations, two common insectivorous species, have been monitored after carrot and cereal bait aerial operations.

Robin and tomtit females usually raise two broods each year. Two aerial 1080 operations at Pureora Forest Park, in 1996 and 1997, reduced possum and rat populations to very low densities. The benefits to the robin population the following year are shown in Table 4. 

Robin populations increased by 28% after the 1996 operation, due to much better breeding success³⁴. The proportion of females also rose significantly (from 35% to 44%). This positive outcome was despite a failure to adhere to the protocols for preparing carrot baits during the 1996 operation. As a consequence, there was a high proportion of chaff produced that caused about 50% mortality in the robin population. In 1997, when the correct bait protocols were followed, robin mortality was only 9%. This was not significantly different from robin mortality in a non-treatment area. This underscores not only the importance of good operational procedures being followed, but also the significant benefits to small birds when aerial operations are timed to reduce predators just before the breeding season.

The success of possum and predator control in Pureora Forest Park has meant that, in the past two years, 150 North Island robins have been able to be transferred from the Park to boost numbers in four other reserves.

The results of monitoring tomtit populations suggest that sowing rates and type of bait are important influences on tomtit survival³⁵. Between 1994 and 1996, three aerial operations that used carrot baits, sown at 10–15 kg/ha, caused high losses of tomtits. One of these was the 1996 operation in Pureora Forest Park, referred to above, which had produced a lot of carrot chaff. By contrast, aerial operations in 1998 and 2001 that used larger 12 gram cereal baits, sown at the lower rate of 3–5 kg/ha, caused little or no mortality in the tomtit populations.

A recent study of five operations in the North Island tested the impacts of cereal and carrot baits on tomtits, using sowing rates of 3-5 kg/ha.³⁶ It confirmed that 12 gm cereal baits had little, if any, immediate impacts on tomtit populations at these sowing rates. After three aerial operations using carrot bait at 3-5 kg/ha, however, tomtit numbers initially declined by about 10-40%. The longer-term recovery rates of these populations were not monitored. The study authors stressed that these results may be specific to tomtits, since the 1997 carrot operation in Pureora Forest that had a substantial impact on tomtits had no apparent impact on North Island robins.

For this reason, in some operations, the use of cereal baits is preferred over carrot baits. Like robins, tomtit nesting success improved after a 1080 operation that poisoned most rats, possums and stoats³⁷.

Secondary poisoning effects

The small numbers of predatory and scavenging birds, such as harriers and moreporks, that have been found dead after 1080 operations have probably died from secondary poisoning after eating dead or sub-lethally poisoned possums, rodents, small birds or invertebrates. The low level of mortality observed is not regarded as a threat to the species involved. Secondary poisoning from eating invertebrates that have eaten some 1080, is theoretically possible for insectivorous birds. Monitoring results show that if such losses occur, they are quickly compensated for by the increased breeding success.

Table 3.
Survival of radio-tagged birds after aerial 1080 operations

Table 4. 
Breeding success of robins after 1080 control

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