behavioural adaptations of birds to environments where evaporation is high and water is in short...

BEHAVIOURAL ADAPTATIONS OF BIRDS TO ENVIRONMENTS WHERE EVAPORATION IS HIGH AND

WATER IS IN SHORT SUPPLY

S. J. J. F. DAVIES

CSIRO Division of Wildlife Research, Clayton Road. Helena Valley, Western Australia

(Receiwd 6 Jul): 1981)

Abstract-l. Behaviour that reduces the heat load or evaporation experienced by birds living in arid areas is reviewed. Many species have evolved hunting behaviour that enables them to remain inactive during the hottest parts of the day and thus greatly reduce the amount of metabolic heat that they need to dissipate. Flights to water are made at low ambient temperatures, either early in the morning or late in the evening. Fighting is rare in many species of desert birds, avoiding the excess generation of heat by this activity. Many arid zone birds maintain long-lasting pair bonds. avoiding the necessity for active. elaborate display before breeding and again reducing activity.

2. The observations on nomadism are discussed. No unifying principles that might control the behaviour of birds seeking widely separated areas of abundance of food have yet emerged.

3. Some species have evolved mechanisms, embodied in behavioural characteristics, that ensure that the eggs and chicks are sheltered from high temperatures and are provided with adequate moisture.

4. Birds have evolved many different kinds of behavioural adaptation to arid zones and representa- tives from many avian families live there, apparently successfully.

1. INTRODUCTION

The desert areas of the world are generally equated

with the arid and semi-arid zones of the geographers,

defined as areas where mean annual rainfall is less

than 250mm (Serventy, 1971). Water stress, induced

by lack of rainfall, absence of surface water or high

evaporation, affects many birds that live well outside

the arid and semi-arid zones, so that behavioural

adaptations to these stresses can be observed over a

substantial proportion of the world’s avifauna. Birds

have such a great ability to move quickly that in many cases their behavioural response to aridity is to leave the area in question and move to one with a more equable climate, but even for them such movement is not always possible, for example when they have nests or non-flying young, and behavioural adaptations then become apparent. Many species are, in any case, resident in areas which are arid for part or much of the year, and these species generally have evolved behavioural mechanisms which allow them to exploit favourable microclimates that exist even in the most arid places. Apart from these direct effects of aridity, birds also exhibit adaptations to its indirect effects, for example the sparse distribution of resources such as food, mates, breeding and roosting sites, and these adaptations are often exaggerations of traits apparent in species inhabiting equable climates. Usually they involve more than just one aspect of the species biology, for the bird must come to terms with its environment the whole time, so that any particular adaptation must be compatible with continuous survival.

Reviews that deal in part with behavioural adaptations of birds to aridity have been published by Keast (1959), Immelmann (1963), Serventy (1971), Dawson (1976), Maclean (1976) and Davies (1976). They have discussed at length the role of nomadic

behaviour in birds living in arid lands, but have given less attention to the day-to-day problems to which birds respond by changes in behaviour.

Behavioural and physiological processes are closely integrated in any vertebrate, so that it is difficult to divorce one discipline from the other in the discussion of any individual species, and it is simplest to say that this paper aims to complement the others in the series, dealing with reactions of the whole individual organism to its environment, rather than analysing the physiological processes in which one particular organ or organ system specializes. It looks at ways in which the behaviour of birds is adapted temporally, spatially and socially to deal with the controls imposed by water stresses flowing from the aridity of the environment. Deficiences or excesses of salt appear to be dealt with chiefly by birds’ physiology.

2. TEMPORAL ADAPTATIONS TO ARIDITY

2.1. Daily

Birds living in arid areas need to economise their water loss in every possible way, reacting to increased severity of water stress by according water-saving behaviour higher and higher priority in relation to other maintenance activities. The relationships have been investigated carefully in nightjars (Caprimulgidae) and Dawson & Fisher (1969) have reviewed some of the mechanisms the birds use in a study of Eurosto- podus, which feed throughout the night on flying insects, mainly moths. They therefore avoid activity during the day when excess heat production needs to be compensated for by evaporative water loss. During the day they remain immobile, and have such a low

metabolic rate [basal metabolism 0.83 cm3 02 (g/hr)-‘1 that no behaviour other than immobility is needed until the ambient temperature reaches

551

cllr714n I

558 S. J. J. F. DAVIES

4o’C. About that temperature gular flutter (Bartho- many related species. An individual was subjected to lomew er al., 1968) starts and the rate of evaporative a temperature of 56.5.C before it died (Dawson & water loss increases steeply. Eurostopodus has one of Fisher, 1969). The simple behaviours of nocturnal ac- the lowest basal metabolic rates amongst the nightjars tivity and diurnal immobility, supplemented by gular and is therefore more highly adapted to aridity than flutter to promote evaporative water loss which was

1lMt OF DAY

TIME OF DAY

Fig. 1. Daily patterns of drinking. N, total number of birds drinking: T, maximum daily temperature(s); SK Sunrise; M, midday; SS, sunset; enclosed in parentheses are the dawn-to-dusk watch number(s) from which all or most of the data were obtained. The pattern for the budgerigar was drawn using only lY,, of

the total number of individuals drinking (every 30 min period). (From Fisher et al.. 1972).

Behavioural adaptations of birds 559

70 r

A : PORT LINCOLN PARROT

60 - B : BOIJRKE PARROT

C : MULGA PARROT

50 -

40 -

30 -

20 -

16 22 26 30 34 38 42 46 50

MAXIMUM DAILY TEMPERATURE (‘C)

Fig. 2. Number of afternoon trips to drink of three species of platycercine parrots, all with drinking Pattern A. at various ambient temperatures. (From Fisher et (II.. 1972).

made up by nocturnal drinking, were therefore adequate to enable this species to occupy desert areas without preventing it from inhabiting less extreme environments.

Many species of diurnal birds confine their activity to cool periods, making long flights to drinking places early in the morning or late in the evening. Fisher et al. (1972) and Davies (1972) give information about Australian species and Prozesky (1969) and Wil- loughby & Cade (1967) information about African species. Each species showed a characteristic drinking pattern (Fig. 1). The small (40 g) Bourke parrot (Neop- sephotus bourki) drank before sunrise and after sunset in the observations of Fisher et al. (1972) which were made in the southern hot period (summer) between September and March. In Davies’ (1972) observations made in August, the species drank throughout the day, although they were most abundant before sunrise and after sunset and were the first species to arrive at the water. This pattern of drinking enables the species to avoid flying long distances during the daylight period, and hence to save using water in evaporative cooling to reduce a heat load caused by strenuous activity when ambient temperatures are high. Other species exhibit similar behaviour to a greater or lesser extent. Zebra finches (Peophila yuttata) in Australia (Davies, 1972) and Passer melanura in Africa both drank throughout the day. presumably because as small seed-eaters they need to take in water throughout hot days to maintain their water balance (Fisher er a/., 1972). In doing so they not only expose themselves to predation but to high ambient temperatures. emphasising how desperate is their need for water. Indeed, at very high shade temperatures (48’C) large scale mortality of these birds has been observed in Australia (McGilp, 1932). They died in the presence of water, and continued to die during the evening of a very hot day even though a storm had brought the ambient temperature down considerably.

The relationship between ambient temperature and the time at which birds visit water points is illustrated in Fig. 2 from Fisher et al. (1972) which shows an increasing number of parrots visiting a water point

with increasing ambient temperature. Thomas & Robin (1977) working with two species of sandgrouse Pterocles alchata and P. orientalis found that the birds visited water much later and in smaller numbers on a dull, foggy morning than on a clear one (Fig. 3). They analysed the drinking behaviour of the sandgrouse in relation to the length of flight and the ambient temperature to which the birds were therefore subjected

0 >

Fig. 3.(a) Air temperatures 5cm above the ground (0) in relation to the temporal distribution of drinking flight calls of alchara (b) and orient& (c) at their drinking point in the steppe study site, on two mornings chosen to illustrate the extremes of response elicited by the air temperature extremes experienced. The 107, onset (lo), modal (50) and 90”/, completion (90) times are shown for ulrhntu (a) and orientalis (b) at the moments when lW>,, 50% and 909, of the total number of calls (n) from each species had been heard on a given morning. Note how drinking flights on the cool misty morning (15 May: hatched) were markedly delayed compared to the warm clear morning (23 May:

plain). (From Thomas & Robin, 1977).


Fig. 4. The influence of air temperature near the ground (y) on the onset (x) or duration (x) of drinking flights in ulchutu (A) and orient&s (A). Linear least squares regression lines are plotted for combined data on both species. (a) The lo”, onset time occurred later when the air temperature then was lower (J = 36.5 - 1.78~; r = 0.557; @‘= 12; P < 0.05). (b) The duration of 80”,, of the calling activity tended to be longer when flights started in cool conditions 0‘ = 26.7 - 0.044.~: r = 0.462: @= 12; 0.1 > P > 0.05). (c) Increasing 800, activity duration resulted in 901,, activity completion occurring at higher

air temperatures (line: linear regression for points <35’C; .V = 20.1 + 0.063.x; r = 0.926; 4f= 10; P < 0.001; t-tests throughout). (From Thomas & Robin. 1977).

(Fig. 4). Their observations showed that the minimum period that a morning drinking flight lasted was 75 min. The birds always began their flight at least one hour after sunrise. On days when the temperature rose rapidly the flight started early, the number of birds involved was high, and the duration was short. Nevertheless the birds were still flying at much higher temperatures than those at which they became inactive at other times. On cool mornings, when the temperature increased slowly, fewer birds were involved in the flight, which started later and lasted longer, so that birds were still visiting the water point at midday. Thomas & Robin (1977) argue that such behavioural changes with changing rates of increase of ambient temperature shows that the birds anticipate the conditions of temperature stress that each day will bring. The response is to an interaction between ambient temperature and time of day, for the birds that do go to drink on a cool day do so later in the day and at a lower temperature than the birds that go on a hot day. Such anticipatory drinking behaviour is not observed in all species. The common bronzewing

(Phaps chalcoptern) of Australia characteristically drinks mainly in the evening (Fisher et al., 1972; Davies, 1972). Its close relative the crested pigeon (Ocyphaps lophotes) stays at water all day but visits most frequently in the morning, as does the African species Stropropelia senegalensis. The sandgrouse, therefore, appear to have evolved behavioural responses to temperature and time of day which help to shield them from heat stress during hot days.

Maclean (1976) describes an interesting behaviour in the Australian dotterel, Peltohyas australis, which may represent an adaptation to the need for increased water on hot days. These birds feed on succulent foliage during the day and take insects at night. Mac- lean lists the plants he saw Peltohyas eating and notes that the most favoured had a high water content (Table 1). He did not observe the birds drinking and considers that the succulent plants probably supply all their water requirements.

Other species exhibit less sophisticated methods of avoiding heat stress. Trost (1972) has described the behaviour of horned larks Ermophila alpestris perch-

Table I. Food plants eaten by Peltohyas, determined by direct field observations

Species Parts eaten

”

obsirv. I’(, water

of wet wt

Terrugonitr retrugonoidrs

Atriplrv halowrpu Ba.ssicl rmtriuxa Buhbnyia acropruru

Hrliprwum j7orihuntlum Wuhlenhrrgiu sp Bruchycome sp Gncphosis foliclra L.otus cruentus

Ctrlotis hispidula

lxpidium sp Ertrgrostis dielsii

Leaves Leaves Leaves Leaf. stem Fruits (?) Seeds Infloresc. Fruits ? Leaves ? seeds’? Seeds? Leaf tips Green inflores. seeds’?

17.24 87-89 17.24 X4-89 13.79 86 10.30 75 86

6.90 Dry 6.90 Dry 6.90 ‘,

6.90 D;y 3.40 ‘)

3.40 Dry 3.40 ‘, 3.40 ‘1


ing on the top of shrubs on hot days in the arid shrublands in which they live. The shrubs lift the birds above the ground surface where ambient temperature and radiation are highest, and expose them to better conditions for evaporative cooling than are available at ground level. Saunders (1977) has observed that white-tailed black cockatoos are inactive during the hot part of the day. when they sit on shady branches. Many other parrots in Australia behave in the same way, such as the budgerigar, Melopsittacus undulatus, whose diurnal cycle has been described by Wyndham (1980a). Both the cockatoo and the budgerigar have, under hot conditions, to give priority to heat avoidance at the expense of feeding behaviour, and in the case of the cockatoo this may sometimes contribute to the death of young in the nest.

Non-incubating sandgrouse seek shade during the heat of the day, although their incubating mates sit immobile in the sun (Thomas & Robin, 1977).

2.2. Semonal

Not a great deal of work has been done on seasonal adaptations to hot climates, that is to say on long- term behaviours that operate to reduce the heat load. There is no doubt that selection must operate against individuals that do not exhibit such adaptations. Plezczynska (1978) showed that in lark buntings, Calamospiza melanocorys, solar radiation is the major indicator of territory quality, which is reflected in the ability of males to acquire one or more mates. Nest- ling survival was significantly correlated with nest-site cover and experimental increase in cover resulted in high reproductive success. Fig. 5 shows the distribution of lark bunting territories in a field, superimposed on a contour map of solar radiation levels (depicted as light intensity). Increasing number indi-

cates increasing light intensity. The territories with good cover characteristics held bigamous males, the territories with fair cover monogamous males and territories with poor cover unmated males. Similar effects are likely to operate in many birds of the arid zone but have seldom been so well documented as in this study.

2.3 Aperiodic

The causes and controls of irruptive and nomadic behaviour in birds living where rainfall is erratic have not yet been identified in a single case. The aperiodi- city of the bird’s movements and the irregularity of the rainfall pattern differ only in degree from those observed in so-called regular, temperate climates. Much attention has been given to these phenomena in Australia. As Davies (1968) and Nix & Austin (1973) have pointed out, if standards are set low enough the rainfall pattern of central Australia is far more regular than is generally supposed. Furthermore Davies (1979) and Wyndham (1980b) have shown that the breeding and movement of birds in parts of the Aus- tralian arid zone is seasonally predictable, although in some years no movement occurs at all. Similar phenomena occur in temperate lands, but generally the variation in rainfall, although great, does not drop below the level at which survival is impossible, and the erratic movements of birds are over a small distance and usually involve only parts of the whole population.

That individuals of some species of birds do cover great distances in unpredictable directions is well established. Frith (1962) studied the movements of grey teal, Anas gibberfrons, in inland Australia in response to the steady drying out of the waters on which they had taken refuge in 1957-1959. Fig. 6

Fig. 5. Map of patchy alfalfa field showing territories belonging to males that were bigamous (no shading), monogamous (light shading) and bachelor (dark shading) superimposed over contoured 6th degree surface of the light variable. Increasing numbers indicate increasing light intensity. (From Plezc-

zynska, 1978).

S. J. J. F. DAVIES

SYDNEY _ /

Fig. 6. Place of recovery of grey teal banded in concentration areas in 1958 and 1959. Only one recovery is shown in each district. for the sake of clarity. (From Frith, 1962).

shows that dispersal from these refuges was random, complex and erratic. In the period November-March 1957-1958 birds were moving both from Dareton, NSW. to Buckiand Park, S.A., and in the reverse direction. As refuges began to dry up the ducks left them and appeared to go in ail directions, eventually concentrating on other areas where water remained. However the factors which prompted them to leave before the water bodies dried out completely and what enabled them to find other water bodies are undescrihed.

Davies rr a[. (1971) described the results of banding

emus in Western Australia during one of the periodic exodus movements in which the birds leave the inland and head for the coast. These movements tend to have a southwesterly direction in winter and a north- easterly one in summer (Davies 1968). The map (Fig. 7) which combines data from a 9 month period that included both summer and winter movements, show that the resulting dispersal resembles that of the grey teal, even though directional trends are apparent within each season. Again the factors initiating and controlling this movement are unknown.

Although it is clear that many Australian birds


i F---_ , 1 1 1 :

Koolyanobbing ‘\

Fig. 7. Map of part of Western Australia, showing the principal vermin-proof fences (Nos. 1, 2 and 3), and mileages along the fences, sites where emus were banded (a), and sites where bands were recovered (0). Individual banding and recovery sites are joined by a line except when movements took place along

the fences. (From Davies et al., 1971).

undertake nomadic movements, that similar movements occur in other continents, and that the result of these movements is to bring individuals away from areas where resources are scarce or absent, and into the areas where survival is possible, the mechanisms involved are unknown, and badly need thorough, experimental study.

3. SPATIAL ADAPTATIONS TO ARIDITY-

FINDING RESOURCES

All animals have the problem of finding resources on which to sustain life from day to day. In arid lands these resources may occur infrequently at one site so that the animal often has to move long distances once the resources at a site are exhausted before it finds another site at which the resources it needs are plenti- ful. Usually the direction of movement must differ every time the animal has to move. Davies (1978) after a lengthy study of emus has concluded that their

whole biology is adapted to the distribution of resources on which they depend. It is not clear how they find these resources, but it is clear that they depend for food on finding abundant supplies of seeds, fruits, flowers, insects and the growing shoots of shrubs and grass. Dry feed of any kind is rarely eaten. The principal adaptation to this diet lies in the bird’s ability to survive long periods of starvation. Adult emus weigh between 40 and 45 kg but can survive and travel at least until their body weight falls below 20 kg. Only when they are incubating are they forced to stay in one place and then only for 2 months. During incubation, undertaken solely by the male presumably in compensation for the resources required by the female to produce a clutch of 9-20 eggs, very little food is eaten. The male does not drink and rarely defaecates, so that during this enforced immobility he has no need of the resources normally required for survival. The breeding period is during the cool months, when rainfall is most likely to occur,


and soon after the chicks leave the nest they are able to travel considerable distances.

Emphasis has been given in this section to the erratic distribution of resources in the arid zone, but Wyndham (1980b) in a study of the food of the budgerigar, indicated that the food supply of this species may be more regular in one locality than is often supposed. The birds feed on a variety of seeds which are usually available within a short distance as a result of the redistribution of water after rainfall. Such redistribution occurs locally after small falls, and over large areas after substantial distant falls. Lindgren (1973) came to the same conclusion after a study of the Bourke parrot in Western Australia. It may be, therefore, that the problem of locating resources in arid areas is less great for many species than is often supposed, and this must certainly be true for resident species, Personal observations based on eight years banding of zebra finches at one water mill in arid Western Australia have shown individuals to be regularly retrapped at the same site, some surviving at least three years including a series of dry ones. Brooker et al. (1979) reported that banded individuals of five species remained for 20 or more months in or near a single isolated thicket (donga) on the Nullabor Plain, Western Australia. The species were Willie wagtail, Rhipidurct leucophrys, yellow-throated miner, Manorinujlacigula, singing honeyeater, Lichenostomus cirescens, black-faced woodswallow, Arturmus cinereus, and grey butcherbird, Cructicus torquutus. These birds, resident in an extremely arid district, must have found locally sufficient resources to survive and breed. Many other casual observations have shown that resident behavior in arid zones is success- ful in the sense that populations of resident species persist throughout long periods when conditions appear unfavourable. In this regard it is interesting that Newsome (1975) considers that both the euro, Macropus rohustus and the red kangaroo, Macropus rufus confine their activities to a small area during drought but are much less likely to exhibit resident behaviour during good seasons.

Detailed studies of the diet of desert birds are few, One exception is the work of Jones & Ward (1979) who have observed the red-billed queleas (Quelea quelea) in Nigeria and found that for breeding to be suc- cessful the males must start a colony with a good reserve of fat and be able to maintain that good con- dition locally. The females, on the other hand, must start with a good reserve of protein in the flight muscles (Jones & Ward 1976) which is metabolised during laying, and may be obtained at a distant locality before the colony forms. However the females must have access to a ready supply of carbohydrate at the colony so that they can accumulate fat reserves during yolk development. They bring with them enough protein for the formation of the first eggs of the clutch, but must collect protein locally for the later eggs and to restore the protein used during production of the first eggs. The search for protein (insects) is usually much more time consuming than the search for carbohydrate (seeds), so that seeds must be readily available nearby if the females are to have time to search for and find the necessary insects. In effect this means that the behaviour of queleas in arriving at a site where energy food is abundant and

breeding at once enables them to produce eggs from protein resources derived from a remote site and to take advantage of local abundances of carbohydrate food. A flush of grass, which provides the carbohydrate, is often followed by an abundance of insects (Nix, 1976). These insects would be available as food for the young, when they hatched. Such a food supply needs to be available locally in abundance so that the birds do not have to spend much time seeking it when feeding and brooding small chicks.

4. SOCIAL ADAPTATIONS TO ARIDITY

Several studies of the behaviour of arid land birds have reported a lack of intraspecific aggression. Thomas et ul. (1981) in a study of sandgrouse found little evidence of competition for food between captive individuals, and no evidence of a social hierarchy in the captive flock. They comment that the lack of aggressive behaviour could be related both to the wide dispersion of food in arid areas making it unpro- ductive to fight over single items, and to the disadvan- tage of an increased heat load incurred during fighting at very high temperatures. Aggressive behaviour has rarely been seen in the field. Wyndham (1980a) in his study of budgerigars found flocks to be made up by the aggregation of different individuals each day, and found no sign of a hierarchy in them. Fighting occurred over nest hollows but not in feeding or drinking flocks where close contact was frequent. The characteristic behaviours of woodswallows (Arturmus) which cluster in roosts at night and sometimes during the day as well, are examples of close contact behavior in arid zone species. Many Australian species shelter in caves and similar places when temperatures are high and the graphic account of Finlayson (1932) describing zebra finches, budgerigars, crimson chats (Epthianurua tricolor). magpie larks (Grullintr cymo- lenca), Willie wagtails (Rhipidurrr leucophrys) and bee- eaters (Merops ornutus) sheltering in and around rail- way carriages in a heat wave is an example of this behavior. Thousands of individuals were involved without any overt fighting.

Immelmann (1963) has emphasised the rapid attain- ment of sexual maturity in zebra finches (86 days to the first egg) as a preadaptation to desert life on the grounds that rapid reproduction when conditions were good would be an advantage in an erratic climate. Sexual development, both behavioural and physiological. certainly does seem to be rapid in desert birds. but there equally seem to be no good comparative studies of desert and mesic representa- tives of the same species.

lmmelmann (1963) has also emphasized the impor- tance of long pair bonds in ensuring that a partner is handy when good conditions occur. The argument is not completely convincing for Palaertic migrants seem to manage to breed rapidly and successfully after lengthy migrations which separate the sexes so that the males arrive well before the females. Equally in the quelea, the males establish the colonies in some species without the females and these birds are in- habitants of semi-arid areas. Saunders (1974) and Rowley (1974) both emphasized that an advantage of long pair bonds m the species that they studied lay in the ability to breed without elaborate display behav-


iour between the sexes as is characteristic for example, of many ducks. This could be related, however, to the need to save energy in the same way as lack of aggression in desert forms may be an energy-saving device.

Another aspect of sociai ~haviour that has received attention in deserts is the incidence of coop.. erative breeding in birds, that is of breeding groups comprising more than a simple pair. Rowley (1976) suggested that this was an adaptation to the irregular- ities of the Australian climate. Dow (1980) examined the distribution of cooperative breeding in Australian birds in relation to a number of environmental variables. but he found no general association with aridity.

5. THE PROBLEMS OF DEPENDENT CHICKS

For many desert species the breeding cycle, although it may start during equable weather, leaves young in the nest or in a dependent state after environmental conditions have changed so that they are subjected to heat stress. Even a species breeding in exposed sites in flowing tropical rivers has evolved mechanjsms that reduce the effects of dessication on eggs and chicks. Egyptian plovers (Pluciunus aegpp- tius) regularly carry water on their belly feathers to damp the eggs and chicks, and when they leave them for short periods, often cover them with sand (Howell 1979). Some arid-zone species build large and bulky nests which protect the nestlings from extreme environmental conditions. Bartholomew et al. (1976) studied nests of the sociaI weaver ~~~i~~f~jrus sociusf during a summer nesting period and found the temperature within the nest to be 5%X below ambient. This species builds large communal nests of grass containing nest chambers. sometimes more than 100, and in this way somewhat mitigates the heat stress. Many species of small, solitary-nesting passerine bird build their nests in the shade of vegetation (Maclean, 1976), often in a depression in the ground. In this way they reduce the heat load imposed on the young chicks. Other species, for example, the galah, Cocatuu ru.~ej~ffpj~~~, are remarkably good at retaining water (Skadhauge 1974) and this must be especially impor- tant for the chicks which have no access to water during the nestling period spent in a hollow tree.

Thomas & Robin (1977) summarized recent studies of the behaviour of sandgrouse which show that these birds have feathers on the breast specially adapted to carry water. The adults immerse themselves and return to the nest where the nestlings drink from the water held in the mat of these special feathers. Eggs may also be moistened in this way. The sandgrouse have species-specific behaviour to ensure that the water carrying capacity of the feathers is exploited to its full extent. No other group of birds are at present known to have similar abilities.

6. DISCiiSSION

The behaviour of birds living in arid environments differs from that of birds living in mesic environments in many, diverse ways that can be interpreted as adaptations to arid conditions. At an individual level these usually appear to reduce the likelihood of heat stress and involve such things as feeding at night,

making long flights during the cool of the day and remaining inactive in sheltered places when the ambient temperature is high. All these modifications are simple and may be thought of as direct responses by the individual to environmental cond~tjons, faced with the need to balance thermoregulatory requirements with the need for other maintenance behaviour, The longer the bird is inactive, the more food it will need when it becomes active again.

Some of the behaviours reviewed here, appear to be direct consequences of the controls exerted by an arid and often hot environment. These adaptations are of a long term nature and affect the survival and reproductive strategies of the whole species. For example a lack of aggression seems to be characteristic of many arid zone birds, not only those that form large flocks. Aggressive behaviour out of the breeding season is rare, and even in the breeding season it is usually confined to the immediate environs of the nest. The female lark buntings that actively select males in shady sites represent a modi~cation of behaviour that is an adaptation to ensure the success of the breeding cycle as a whole, although the responses that imple- ment it must be to perceivable differences between the breeding territories obvious at the time of mate selection. In this case the need for a shaded nest site apparently takes a higher priority than other terri- torial attributes. Sandgrouse seem to be exceptionally well adapted to life in arid areas, and their ability to carry water to their chicks, as well as the behaviour they use to maximise this ability, indicate the extent to which they have evolved to exploit arid areas.

Some of the most spectacular behaviour of arid zone birds. irruptive or nomadic behaviour, is much described but little understood. In order to compare the advantages of nomadic behaviour with the advantages of residency, much quantitative work needs to be done. Both behaviour patterns lead to the survival of some individuals but it is possible that each suits a different type of bird. Most. but not all, nomads are seed-eating species. Some that are not, are large, for example ratites and bustards, whereas many small resident species take insects. There are also a number of resident seed-eating species, including the zebra finch which is often wrongly placed in the nomadic category.

It seems that birds have a great ability to develop behavioural adaptations that assist them to maintain populations in arid areas. For each species the best balance between thermoregulation, feeding, breeding and drinking is different, depending upon, amongst other things, diet, size and season. A great variety of behavioural adaptations result and it is impressive how many avian families contain species living in arid areas. The behaviour of birds is astonishingly flexible.

~c~~u~1~~~g~~~nfs-r am grateful to Dr G. J. Caughley, Dr D. Thomas and Dr C. H. Tyndaie-Biscoe for comments on an earlier draft of this paper. The Zoological Society OF London, the Cooper Ornithologist Society (Condor), the C.S.I.R.O. Editorial and Publications Service, the pub- lishers of Science (the American Society for the Advance- ment of Science), and the Royal Australasian Ornitholo- gists Union for permission to reproduce material originally published in their journals.


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behavioural adaptations of birds to environments where evaporation is high and water is in short...

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