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DIET OF RATTUS NORVEGICUS ON WHALE ISLAND,

BAY OF PLENTY,NEW ZEALAND.

by D.J . Bettesworth* and G . R . V . Anderson*

S U M M A R Y

The stomach contents of sixteen adult Rattus norvegicus trapped from several areas of Whale Island were analysed.

Stomachs of rats caught along the foreshore and boulder banks contained predominantly insect remains, with small quantities of intertidal invertebrates. 80% of volume of the stomach contents of rats trapped inland on the island was flesh and down of chicks of the grey faced petrel, Pterodroma macroptera.

No traces of rodents other than R. norvegicus were found on the island.

I N T R O D U C T I O N

Whale Island lies in the Bay of Plenty, off the east coast of the North Island of New Zealand. As part of the work undertaken on the island by the Auckland University Field Club scientific party in August 1970, a short programme of snap-trapping was planned to study the population of Rattus norvegicus, the only one of the three mammals present on the island likely to have a direct effect on breeding birds.

Although it is not known when the rats were first introduced to Whale Island, they now occur wherever there is sufficient cover, from the dune areas and the boulder banks along the shores, to the grassy slopes and bush of the summit. Rattus norvegicus is usually considered to be an opportunist in its eating habits, feeding on whatever suitable foods are available in the various habitats in which it occurs (Southern, 1964). In New Zealand, much damage to vulnerable populations of birds and other wildlife is attributable to predation by the three species of Rattus present; exulans, R. norvegicus and/?, rattus, (summarised in Wodzicki, 1950; and Best, 1968).

Whale Island is a breeding area for several thousand grey faced petrels, Pterodroma macroptera, which nest in burrows scattered over the island, but concentrated on the Summit and western slopes.

In early July a single egg of about 65x40 mm (Oliver 1955) is laid by each breeding pair. Incubation lasts about 54 days with the egg usually continuously guarded by one of the parents, although occasionally eggs may be left un­attended for short periods at change over of the incubating adults.

The main hatching period is towards the end of August. Newly-hatched chicks are guarded continuously for about two days and are subsequently visited every second or third night by a parent bringing food. (M.J . Imber, pers. comm.) It is at this time that the chicks are particularly vulnerable to terrestrial pre­dators, as they are too small to defend themselves adequately.

•Department of Zoology, University of Auckland.

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The remains of eggs and of half-eaten chicks dragged from burrows were not uncommonly seen in the areas of the island where burrows occurred, and in the absence of other terrestrial predators the damage was attributed to R. norvegicus. As the visit to the island coincided with the commencement of hatching of P. macroptera, a trapping programme was planned to examine the population structure and feeding of rats from areas of the island with concentrations of petrel burrows, in contrast to those areas with few or no breeding birds. A short trapping programme and search for signs of other rodents on the island was also made.

For a location map of Whale Island and accounts of the physical features and vegetation see Tane Vol. 17, 1971.

M E T H O D S

Trapping Programme Seventy light metal snap-traps and six heavier wooden based snap-traps were

on loan from Wildlife Section, Department of Internal Affairs, for use in the study. One live trap was also used. Traps were set out in four areas of the island in an attempt to obtain samples of rats from different vegetation types and varying densities of petrel burrows and breeding birds. A fifth trap-line was set in an attempt to trap R. exulans. Table 1 outlines the areas and the trapping programme in each area.

Each trap was attached to a light bamboo stake about one metre long, with a white-painted tip and a short lenght of'Fluorescent pink' trail marking tape tied to the top; the stakes served both to anchor the traps and to make their relocation easier. Traps were baited with a dough made of equal parts of peanut butter and flour, with a small quantity of water (Smith et al., 1969). Once set, the trap lines were visited each morning; any rats caught were collected for further study, baits taken or touched were noted and in open areas all traps were sprung and overturned to avoid catching small birds attracted to the bait. Each evening traps were rcbaited where necessary and reset.

Examination of Rats External Examination Collected rats were weighed on a 500g spring balance to the nearest gramme

and total and tail-length measured to the nearest millimetre. The rats were sexed and a careful search made for external parasites and any signs of scars or injuries before the stomach was removed. Once the body cavity was opened, the reproductive state was assessed and the rats were then discarded.

Analysis of Stomach contents The volume of removed stomachs was measured by displacement, the contents

removed and spread out in a petri dish for examination under low magnification. Food items were identified where possible and the volume of each of constituent item estimated as a percentage of the total volume.

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Table 1. Trapping areas and success rates.

Habitat Arrangement of

Traps Nights trapped

Trap nights

Baits taken

Rats Caught

Success Rates

1. Grass and sedge 75%

Manuka scrub 25%

grid 70 x 80m traps at 10m intervals

7 240 57 4 1:60.0

2. beach, boulders and sand dunes

Line of traps of 10m intervals.

6 62 27 7 1:8.9

3. bush Gully Pohutakawa dominant

Double Line of traps spaced at approx. 10m.

4 56 27 3 1:18.7

4. Summit of main hil l , high density petrel burrows

Circle of traps at grass/bush edge spacing about 10m.

3 84 43 2 1:42.0

5. Bush Gully , Pohutukawa dominant

Traps in low forks of large trees.

4 12 0 0 0

TOTALS 454 157 16 1:28.4

R E S U L T S

Trapping Programme

Table 1 denotes the areas trapped, the configuration of the trapline in each area and the capture rates attained. A total of 16 rats was caught in 454 trap-nights; an overall rate of 1 rat in 28.4 trap-nights, ranging from 1 in 8.9 trap-nights from the beach trap line, to 1 in 60 trap-nights from the grid of traps on Pa Hi l l (area 1). In comparison with the overall rate at which baits were taken (presumably byrats); 1 in 2.9trap-nights, the capture rate is low, attributable to the unsuitability of the lighter metal traps for a programme of this nature. Difficult to set critically, the traps also proved too light to kil l or hold the rats triggering them.

Table 2 gives the dimensions of all rats caught in the study. The sample is too small for any meaningful comparisons between the areas in terms of population densities. A l l rats were adult, all were in good condition with only one showing marked scarring presumably due to conflict.

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Table 2: Dimension of Rats caught

Sex Where Caught Total Length cm. Tail Length cm Weight gm.

M 1. Grid 43.0 18.5 360 M P r i d 38.0 17.0 285

F Gr id 38.4 17.1 250 F Gr id 38.4 17.4 300 M 2. Beach 30.4 12.4 175 M Beach 38.9 16.9 285 M Beach 40.2 17.2 300 M Beach 37.8 16.3 260 M Beach 37.9 16.9 285

F Beach 30.7 13.7 155 F Beach 34.4 15.8 205 M 3. Bush 35.0 15.2 235 F Bush 31.7 14.2 150 F Bush 37.5 17.3 255 1 4. Summit 39.3 18.3 270 1 Summit 37.0 16.5 225

The stomach contents of 15 of the 16 rats caught were analysed, one live-trapped animal had no food in its stomach when killed and is not included in the analysis. The animals caught on the beach (area 2, in Table 1) have been separated from the rats caught in the other three areas, which, although trapped from disparate vegetation types, seem to afford similar foods.

Table 3.

BEACH £. a r n

"0 c a 3

2 a

73 V

+ 09 —, 5'

r s- t $

AREA 2 i — M %

t Fur

^> a. s. Food Item

o' a.

ink)

a a.

Volume ml 0.4 0.3 - 1.0 3.1 13.1 1.2 - 1.2 - 0.1 2.9

% volume — 1.3 4.4 13.6 57.2 5.2 5.2 - 0.4 12.7

% Occurence - 33.3 33.3 33.3 83.3 16.6 33.3 - 16.6 33.3

INLAND AREA 1,3,4.

Volume mg 28.5 0.6 0.6 0.4 0.8 0.9 - 2.1 0.6 45.6 _ —

% volume

PL - 1.2 1.2 0.8 1.6 1.7 - 4.0 1.2 88.8 - -

+ 11.1 22.2 11.1 11.1 11.1 - 22.2 11.1 77.7 - -

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Rats from the two sampling areas show clear differences in diet, despite the small size of the samples. The rats from the beach had eaten a high percentage of arthropods from the littoral and supra-littoral, mainly fragments of the sand scarab, Pericoptis humaralis and Anisolabis littorea, an earwig commonly found under the debris marking high tide levels. The stomach of one of the beach rats contained a small fragment of skin from the abdomen of a skink, Leiolopisma sp. again found in the supra-littoral fringe. The limpet remains were identified from the radula as Cellana sp., in this area occuring quite high intertidally in the moist interstices of the boulder beach.

Seven of the nine rats caught in the inland areas had remains of petrel chicks in their stomachs, the fragments of down, flesh and blood accounting for 88.3% of food volume. Figure 1 shows the percentage volume excluding bait volume of food items. Plant material has been combined in one category and " U n i ­dentified animal" includes lizard remains.

-) 23 40 GO 80 100 0 20 4 0 60 SO 100

P E R C E N T A 6 C VOLUME

Small quantities of rat fur were present in three of the stomachs examined, probably attributable to grooming activity rather than to cannibalism.

DISCUSSION

The number of rats caught during the study was disappointing and was insufficient to give information of the population structure of the rats on the island. However, there a/e clear differences in diet between those rats caught on the beach and those caught in areas where petrels were breeding. Rats are usually considered to be omnivores (e.g. Southern, 1964), but during the breeding season of P. macroptera on Whale Island the rats in petrel breeding areas are carnivorous. The high percentage volume of petrel remains and the correspondingly low values for other items in those stomachs probably reflects the size of the food source represented by a single petrel chick; fewer alternative foods would be eaten in significant amounts by a rat satiated with readily available petrel chick. Plant material is thus a more important item in the diet of rats from the beach compared with those from inland areas, in contrast to its availability.

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Predation by rats on the petrel chicks does occur, but its extent and impor ­tance in the survival of newly-hatched chicks cannot be assessed from an examination of a small sample of the predator alone. However, further work by one of the authors (D.J.B.) on the rats on Whale Island, and the continuing work of M . J . Imber on the breeding biology of the grey faced petrel on the island should go some way towards providing an answer.

If predation on chicks is found to be seriously affecting the breeding success of birds on the island, some form of control may have to be undertaken. Whale Island is rugged and steep, with consequent difficulty of applying poisons and any attempt at eradication would have to allow for possible recolonisation by rats from isolated and inaccessible areas.

O T H E R R O D E N T S

With its history of early cultivation and later visiting by the Maoris of the Bay of Plenty, Whale Island would probably have supported Rattus exulans (the kiore), at some stage. During our stay on the island, despite a small trapping programme and search for faeces, no sign of rodents other than R. norvegicus was found. If Rattus exulans does occur it is very restricted and in very low numbers.

External Parasites Three species of ectoparasite were collected from rats trapped on Whale

Island and identified by R . L . C . Pilgrim. O f these, Pygiopsilla hoplia has been recorded before on R. norvegicus by Smit (1965), although he gives R. exulans as the type host. The apparent absence of Rattus exulans from the island makes this record interesting.

The chewing louse Eidmannella pellucida, identified from one rat, may have been due to contamination from a heavily infected pied shag and is not reliable record. Sucking lice, Polyplax spinulosa (Burmester 1839) were identified from the rats, their usual host.

A C K N O W L E D G M E N T S

We would like to thank M . J . Imber, Scientific Officer, Wildlife Service, Department of Internal Affairs for his suggestions and help while on the island and for information on the Grey Faced Petrel. Traps were supplied on loan by the Department of Internal Affairs. D. Towns identified the fragment of lizard and the ectoparasites were identified by Professor R . L . C . Pilgrim. The owners of the island, Mr C. Orchard and Mr & Mrs A . M . Duncan kindly gave permission for work on the island.

R E F E R E N C E S

Best, L.W. 1968 The ecology of Rattus rattus rattus (L.) in selected areas of the south Island, New Zealand. Unpublished M.Sc. thesis, University of Canterbury.

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Oliver, W.R.B. Smit, F . G . A . M . Smith, M . H . , Chew, R . M . , Gentry, J.B.

Southern, H . N .

Wodzicki, K . A .

1955 "New Zealand Birds", A . H . & A.W. Read, Wellington.

1965 Siphonaptera of New Zealand. Trans. R. Soc. N.Z. 7: 1-49.

1969 New techniques; for baiting snap traps. Acta Theriologica 14.

1964 'The Handbook of British Mammals' Blackwells Scientific Publications Oxford.

1950 'Introduced Mammals of New Zealand'. D.S.l.R. Bull 98: 1-255.