ATOLL RESEARCH BULLETIN NO. 182

THE MURINE RATTUS, EXULANS, AND NORVEGICUS AS AVIAN PREDATORS

by F. I. Norman

Issued by THE SMITHSONIAN INSTITUTION Washington, D.C., U.S.A.

January 15,1975 Contents Page Introduction 1 References to Predation by 3 Apterygiformes 3 Procellariiformes 3 Pelicaniformes 4 Ciconiiformes 4 Ans eriformes Gallif ormes Charadriiformes Columbiformes Psittaciformes Passeriformes lliscussion References THE MURINE RODENTS RATTUS RATTUS, EXULANS, AND NORVEGICUS AS AVIAN PREDATORS

by F. I. Norman-1 /

INTRODUCTION

Few have adventitiously accompanied man around the world more than members of the and of these, the most ubiquitous must surely be the --Rattus group. Three species, R. rattus Linn., R. norvegicus Berkenhout, and R. exulans T~eale),have been h~storically associated with-man, and their present distribution reflects such associations. Thus although -R. exulans is generally distributed through the Pacific region, -R. rattus and norvegicus are almost worldwide in distribution, having originated presumably in Asia Minor, as did exulans (walker 1964.). All have invaded, with man's assistance, habitats which previously did not include them, and they have developed varying degrees of commensalism with man. Neither --rattus nor norvegicus has entered antarctic ecosystems, but Law and Burstall (1956) recorded their presence on subantarctic Macquarie Island. Kenyon (1961) and Schiller (1956), however, have shown that norvegicus has become established in the arctic where localised populations are dependent on refuse as a food source during the winter.

More frequently, reports bave been made of the over-running of islands by these alien rats. Tristan da Cunha is one such example (Elliott 1957; Holdgate 1960), and Dampier found rattus to be common on Ascension Island in 1701 though presently -norvegicus is more widespread there (nuffey 1964). Rattus spp., and -Mus musculus Linn., are present on some of the Galapagos islands (Absal. 1965). In the Pacific, an influx of ratitus occurred during the Second World War (~ohnson1945; Marshall 1955), and these may have di-splaced --exulans, a prior introduction (Morrison 1954). The rats, once introduced into areas of abun- dant food and no extreme of climate, may cease commensalism and exist independently of man. This may occur more readily in disturbed habitats where --rattus generally predominates, perhaps because of i-ts wider range of food acceptance (e.g., Fall, Medina, and Jackson 1971 ) . The diet of these species, and of the Rattus group as a

I'Arthur Rylah Institute for Environmental Research, 123 Brown St., Heidelberg. Victoria. 3084. (Manuscript received April 1972--Ed.) whole, tends to be of an herbivorous nature. However rattus, -norvegicus and exulans have generally devel-oped an omnivorous diet which has ultimately led to active predation and even cannibalism (e.g., Bailey and Sorensen 1962; Murphy 1936; Stei.niger 19148; Troughton 1962).

Of especial concern to conservationists have been the persistent reports that the introduced rats were avian predators It is the purpose of this paper to present and briefly review such statements.

On islands two factors may act to initiate predation of birds by rats. Firstly, the alien rats tend to be scavengers, and secondly, there may be a requirement for water other than that supplied by metabolism of the normal dietary in-take.' Cott (1952) suggested tha-L sea bird eggs must be util.ised as a water source, and Habs and Heinz (1951) considered that the brown obtai.ned water requirements from eggs during birds' breeding seasons and from dew or rain at other times. Norman and Baudinette (1969) kept rattus on a diet containing only ten per cent water and found that they could not maintain even reduced body weight without suppl.ementary water being provided. On many islands obli.gatory water requirements may easily be obtained from the more vascularised plant tissues or storage organs. But in extreme conditions it seems that rats must become predators, particularly of el:-gs, to survive and that this source could be extensively exploj-ted. Thus Fisher and Baldwin (1946) observed that, "many of the Pacific Islands offer no large quantities of vertebrate food other than bj-rds and eggs, (and) the pressure from increases in numbers of rats would be exerted upon bird popula-Lions."

Listed. below are summaries of references to predation of birds and their eggs, thi.s listing includes and adds to those given by Cott (195%). Though extensive, the references indicate that few systematic observations on the food preferences of rats have been conducted on islands where bird and rat populations co-exist, and more particularly on those islands where decreases in bird populations have been recorded. Usually statements regarding the diet of rats include generalisations relating to the content of avian material taken. Thus Steiniger (1948) stated that -norvegicus, which fed extensively on plant seeds on Norderoog (in the North Sea), had almost exterminated the bird population there. Bailey and Sorensen (1962) considered that norvegicus fed on seeds, fruit and roots, shellfish, dead seals, sheep, and birds but Austin (1948), who provided one of the few quantitative reports on regular predation, noted that mussels were eaten by norvegicus. For the Cape Cod area, Austin (19148) gave results of 19 years observations on Sterna -hirundo Linn., S. dougalli Montagu and S. paradisea Pontoppidan. He stated that the rat norvegicus was tEe greatest deterrent to successful nesting. The daily kill during the breeding season was between three and 20 birds, and caches of 25-150 eggs were found. Killing, in Austin's s-tudy colony, followed the breeding pattern, thus adults were taken as they becan nesting, eggs were later preda-Led as were chicks as they hatched. Howcver Austin provided no evidence of direct observation, and nor did Habs and Heinz (1 951 ) , who considered that norvegicus destroyed 60 percent of the S. hirundo eggs laid on ScharhBrn off Cuxhaven, Gcrmany, amounTing to some 6000 eggs within a month. More recently Kepler (1967), supporting his report photographically, has detailed losses of Diomedea immutabilis Rothschild on Kure where exulans caused considerable predation of incubating adults. Howcver Korman (1970), and Fall et al. (1971), found that rattus did not cause any prcdation even though specimens inhabiting ground nesting colonies of sea birds were collected, nor did the latter , authors observe predation of incubating adults by rattus or -exulans .

REFERENCES TO PREDATION BY RATS

APTERYGIFORMES

Heaphy (in Moncrieff 1938) considered that rats were the most likely cause of the decline in numbers of Apteryx spp. in .

PROCELLARIIFORMES

Alsatt (1945) believed that rats aided the decline of -Pterodroma hypoleuca on Laysan. Bailey and Sorensen (1962) felt -that rats were "probably responsible for the practical elimination of Campbell Island (New Zealand) as a breeding place of small petrels and shearwaters. " Bell (in Merton 1970) recorded enormous losses of young Pterodroma neglecta in the Kermadec group as being caused by -R. exulans. Campbell and Mattingley (1907) state that rats were causing destruction of Pelagodroma marina in Victoria, Australia Davidson (in Merton 1970) notes heavv CEE uredation of -. , < -- & -Pterodroma neglecta by norvegicus on Raoul Island in the Kermadec group as being caused by R. exulans. Fisher (1952) mentioned that rats were predators of -Fulrnarus glacialis. Harris (1970) considered that rattus were almost certainly responsible for the poor breeding success of Pterodroma ~haeopygiain the Galapagos. Holdgate and Tiace (2961) thought that rats were associated with declines in petrel popuiations on Tristan da Cunha and South Georgia. Johnson (1945) considered that rattus lives on the Midway Islands largely at the expense of and particularly Pterodroma hypoleuca; the young of the albatross (?~iornedeasp.) were also taken. Kepler (1967) recorded exulans feeding on incubating -Diomedea immutabilis on Kure, and noted the absence of chicks of --Pterodroma h oleuca. L-6-t L-6-t that rattus were responsible for the destruction of eggs of breeding Puffinus pacificus and P. -griseus on Lion Island, Australia. Larson (1967) stated that --rattus killed nestling Pterodroma gia on Maui, . MertonPmted that Pterodroma neglecta had decreased on Kermadec Island following the introduction of rats. Munro (1945) thought that ~terodromahypoleuca and Bulweria bulwerii would decline unless rats were destroyed on Midway. Murphy (1936) lists perhaps the most extensive series of observations on rats predating seabirds and their eggs. At Tristan da Cunha, he noted that Pelagodroma --marina was "one of the species which would most quickly have been wiped out by .... rats," and he felt that rat predation was sufficient to account for the vast reduction of Pachyptila desolata there. Similarly the decrease of -P. forsteri on the same island was attributed to rats. On South Georgia, Murphy recorded great quantities of Pelecanoides georgicus bones in the rat burrows. The related P. urinatrix is restricted to islands without rats in the Tristzn group. He also noted that rats were enemies of -Loomelania melania at the nesting grounds. Murphy and Mowbray (1951) found no trace of Pterodroma cahow in localities where rats were present in Bermuda. Olstad (1930) recorded norvegicus living in burrows of Procellaria aequinoctialis and Cott suggested that they took "here as elsewhere .... toll of eggs and young as opportunity offers . " Stead (1936) found Pterodroma ~ycroftieggs eaten by rats (exulans) and stated that alien rats had almost exterminated the shearwater, Puffinus tenuirostris, on Stewart Island (~ew zealand). Thorensen (1967) suspected that exulans was preying on eggs of Pelecanoides urinatrix on Stanley Island, New Zealand. Tickell (1962) considered that prions, Pachyptila desolata, and other burrowing petrels had suffered heavily from rat predation on the Falkland Islands.

PELICANIFCRMES

Gross (1912) caught a rat (MUS alexandrinus = 5. rattus) sucking an egg of Phaethon americanus (= lepturus) in Bermuda. Murphy (1936)-considered that Phaethon aethereus was ~reatlvreduced on St. Helena. aartiv bv alwne hunters and - , & . "still more because of the ravages of rats and feral cats," and -P. lepturus on Bermuda had as its principal enemy _R. exulans.

CICCNIIFORMES Layard (1850) recorded eggs of Ardea sacra as being sucked by rats in the Duke of York Islands (notseen, quoted from Cott 1952). Berger (1970) considered that rats were destroying nests of Anas wyvilliana in Hawaii and thought that they were predators of A. lavsanensis on Laysan. eve ridge and~aniel(1965) thought that ducks decreased on Mokoia Island, New Zealand, as a result of depradations by -norvegicus. Coward (1914) eggs of Anas ~latyrhynchosas being sucked by E- (: norveg&anot seen, quoted from cott 19rz)T~ - ~uilkr(1966) mentioned that norvegicus was a predator of Cygnus atratus eggs in Tasmania. Guile-) considered that introduced rats took eggs of Cereopsis novaehollandiae on breeding islands in Tasmania.

Steininer- (1948), found remains of wild ducks in rat burrows in Germany.

GALLIFORMES Alsatt (1945) thought that --rattus had affected Porzanula ~almerion Laysan. Bailey (1956) also mentioned the disappearance of Porzanug following the overrunning of Midway islands by rats. Bent (1932) recorded egg losses in Phasianus colchicus, -Colinus virginianus and Lophortyx gambelii nests in America. Berger (1970) considered rats were a serious predator of --~allinula chioripbs on Hawaii and thought that rats (and other predators) played a role in the destruction of Pennula sandwichensis. Coward (1914) recorded eggs of Fulica atra as being predated by norvegicus in England.

Cott (1952), ~, felt that there was little doubt that eggs of Gallinula chloropus were taken by rats in Britain. De Groot (1927) stated that -norvegicus took eggs of Rallus ohsoletus in California. Elliott (1957) considered that rattus was the second most important predator of Porphyriornis nesiotis in the Tristan da Cunha group, man being more important. Fisher and Ba1dwi.n (1946) thought that the "main factor in the extermination of the rail (~orzanulapalmeri) was the overrunning of both islets by rats," at Midway. Holdgate and Wace (1961)thought that Porphyri~nis populations had almost certainly decreased following the introduction of rats to Tristan. Johnston (1945) states that a decrease of Porzanula occurred following the introduction of rats onto Midway Atoll. Middleton (1935) mentioned that porvegicus would take eggs of Perdix erdix in Great Britain. NobmOhorded rats taking eggs of Porzana pusilla and 2. bailloni in Andalucia. Austin (1~.948) in .,e5vin~: - details of losses. consi.dered that -norvegicus was the main factor in losses of Sterna dougalli, -S. --hirundo, and S. paradi.sea at Cape Cod, U.S.A. Belcher (i

Layard (1880) recorded that rats sto1.e eggs of Dacula pinon and -D. pistrinaria on the Duke of York Islands.

PSI'JTACIFORMXS

Heaphy (in Moncrieff 1938) considered that rats had caused a reduction of Stringops habrophila in New Zealand.

PASSERIFORMES

Alsatt (1945) attrj~butedthe disappearance of Psittirostra --cantans on Laysan to rats. Bailey (1956) also considered that rats caused the decline of Psittirostra in the Midway area. Beveridge and Daniel (1965) mentioned depradations of rare birds caused by rats and considered that populations of -Philesturnus carunculatus and Xeriicus longipes were threa-tened by them on Great South Cape Island, New Zealand. Blackburn (1968) thought that exulans apparently preyed upon eggs and young of certain species and that this "... predat:ion seems to account for the disappearance of the Pied Tit (Petroica macrocephalus toitoi)," from Cuvier Island, "and of the Yellow-breasted Tit (~etroicam. macrocephalus) from Inner Chetwock Island in Cook Strait,T in-New Zealand. Bull (1946) found that norvegicus was the major predator of eggs and young of Turdus merula and -T. philoma eggs in New Zealand. IIabs and Heinz (1 951 ) found several dead and half-eaten -Sturnus vulgaris on SctiarhBrn and attributed them to rats. ~endcrson(1965) stated tha-t since the "recent rat inf'estati-on, the robin (~e.troicaaustralis), bush wren (~enicus -longipes), and fernbird Gtata) have, of course, gone from ...I1 Solomon Island, New Zealand. Johnson (1945) considered that --Psittirostra cantans disappeared following the introduction of rats onto Midway Islands. Kenyon (1 961 ) thought that norvegicus had externlined Melospra melodia and Troglodytes-troglodytes- in Amchitka, Alaska. Merton (1965) stated that Phi.lesturnus carunculatus has been all but ex-terminated following the arrival of raktus on South Cape Island, New Zealand. Stead (1936) recorded exulans eating deserted eggs and youngs of Prosthemadera novaeseelandiae on Taranga Island, New Zealand. Steini.ger (1948) found rat burrows containing remains of S-tiurnus vulgaris, Saxicola torquata, and Regulus regulus in Germanv., - Stoner (1937) recorded -norvegicus as a predator of Riparia ri aria in America.

Wayne-?-z 18 ~,9) -nave some evidence of oreda%ion bv rats of eggs of--- Turdus merula and Erithacus r~beculain ~riiain.

UISCUSSION

Many authors have attributed the disappearance of bird populations, particularly those on islands, to the effects of introduced predators. Almost without exception rats have been proposed as the most important predator, and such suggestions have been generally accepted. Indeed, Lack (1966) considered that the threat of actual or even potential predation by rats (or other mammals) caused shearwaters and petrels to nest on islands without them, and suggested that occurred where rats and cats have been introduced. More recently Fleming (1 969), commenting on Xeplerls (1 967) observations on ICure, has postulated that -exulans could have been responsible for the reducti-on of some 27 species (seven genera) of moas (~inornithiformes)and perhaps carinates such as Aptornis, Notornis and Cnemiornis. Such considerations are not supported by a closer examination of the references presented above.

Attention has been paid to the more specific examples of rat predation on birds and their eggs in the references listed above, more generalised statements are legion. However, of the 75 references swarised, 39 (52 per cent) refer to rats in general and only 36 relate to identified species, 17 to no=- --gicus, 10 to rattus, and nine to exulans. More importantly, only two authors record direct observa%ions of active predation, these being Gross (1912), who caught a --rattus sucking on egg of Phaethon americanus and Kepler (1967), who photographed exulans feeding on nesting giomedea immutabig, and none gave evidence of having examined food habits for any area mentioned. Pertinent too is the distribution of predation records in relation to ordinal groups. Seabi-rds (~rocellariiformesand Charadriiformes) predominate and account for some 35 references, but mortality in colonies is frequently extensive, caused by a variety of factors including desertion, food shortages, etc., and subsequent carcases may have been eaten by the alien scavengers. Indeed most references to predation of seabirds (and other species) are circumstantial.

Much of the alien rats' reputations as predators are based on suppositions and these have not been confirmed by food studies conducted on islands where birds and rats co-exist. Beveridge and Daniel (1965) showed that norvegicus fed mainly on plant material on Mokoia Island, New Zealand, and Best (1969) considered that birds were not important in the diet of -- gicus sampled in forest areas of New Zealand. Fall et al. Tm) found that on Eniwetok Atoll in the Marshall Islands, the diets of both --exulans and rattus were predominantly vegetarian though rattus did take insect material, and the investigators were unable to show evidence of predation of birds even though samples (and observations) were made in and near colonies of ground nesting terns. Norman (1970) showed that plant material made up most of the diet and considered that rattus, inhabiting an island in Tasmania where extensive numbers of Puffinus tenuirostris bred, did not actively predate, although unattended eggs were apparently taken. Confrontation experiments between rattus and the shearwater, though perhaps artificial, suggested that the rats avoided any contact, either with unattended young, or with incubating adults.

Thus it appears that the rats' role as an avian predator has been overestimated, and apart from a very few statements supported by direct observation, there are few data to implicate rats as causative agents in localised population declines. Other introduced mammals, such as cats, could well have played a more substantial part in bird population reduction and the role of man himself has generally been ignored. Certainly deserted eggs or young eaten by rats will not effect population size, and in most areas now colonised by the aliens a balance has probably been established between the rats and the local avifauna. For islands where the colonisation dates can be established, it would be profitable to determine such inter- actions which exist so as to examine ecological changes which may have taken place during a known period. Certainly the basic facts should be established before widespread control campaigns are undertaken.

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