Foraging ecology of blue ducks Hymenolaimus malacorhynchos on a New Zealand river: Implications for conservation

Biological Conservation 74 (1995) 187 194 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0006-3207/95/$09.50+.00

0006-3207(95)00029-1

ELSEVIER

F O R A G I N G E C O L O G Y OF BLUE D U C K S Hymenolaimus malacorhynchos O N A N E W Z E A L A N D RIVER: IMPLICATIONS FOR CONSERVATION C l a r e J. V e l t m a n , a K e v i n J. Collier, b* I a n M. H e n d e r s o n a & Lisa N e w t o n ~ "Department of Ecology, Massey University, Private Bag 11-222, Palmerston North, New Zealand bScience and Research Division, Department of Conservation, Box 10-420, Wellington, New Zealand

(Received 6 October 1994; revised version received 20 January 1995; accepted 20 January 1995)

foraging birds preferred mayfly and net-spinning caddisfly larvae to all other benthic macro-invertebrates during the breeding season (Ormerod, 1985). This observation showed that riverine birds can, like fish (e.g. Scrimgeour, 1986), discriminate amongst prey and suggested that resource selection patterns should be investigated as part of designing a conservation strategy for blue ducks. Blue duck populations in New Zealand's North Island are small and fragmented, and recolonization of formerly occupied rivers does not appear to be happening (Cunningham, 1991). Effective conservation cannot begin until 'agents of decline' are found and reversed (Caughley, 1994). If present-day populations of vulnerable species persist because they occupy habitat unfavourable to the agent of decline, a systematic comparison of present and past ranges may help identify any such agent (Caughley, 1994). This logic was employed by Ormerod et al. (1985), who related dipper populations to abiotic and biotic features of their habitats and found that one physical feature (gradient) and one biotic feature (abundance of caddisfly and stonefly prey) were associated with high densities of breeding dippers. Since caddisfly larvae were the preferred prey for the provisioning of nestlings, it might be hypothesized that when dipper populations decline, it is because something is depressing the availability of preferred prey. Unlike dippers, which also take aerial prey and fish (Ormerod, 1985), blue ducks are restricted to aquatic invertebrate prey although they consume fruit under exceptional circumstances (Harding, 1990). Adult pairs defend all-purpose territories of approximately 1 km in New Zealand mountain rivers and streams throughout the year (Eldridge, 1986; Williams, 1991). In an analysis similar to that by Ormerod et al. (1985), Collier et al. (1993) found that sections of rivers supporting blue duck populations had significantly greater proportions of stonefly nymphs and lower proportions of dipteran larvae than sections without blue ducks. This observation led us to hypothesize that, like dippers, blue ducks rank prey in some way and that the agent of decline operates by reducing the densities of preferred prey. Translocations of blue ducks to establish

Abstract We investigated whether blue ducks Hymenolaimus malacorhynchos preferentially capture prey that have themselves become rare or that need to be present for successful re-establishment. Working at the Manganuiateao River in New Zealand, we measured the densities and relative abundances of benthic invertebrates, numbers of prey fragments in faeces of adult ducks, and foraging behaviour of adult ducks. Invertebrate densities on stones ranged from 3 741 m : to 14,417 m 2. Stone and boulder communities were dominated by cased caddisfly larvae or Chironomidae larvae in most months. Patterns of apparent selectivity varied, but Trichoptera larvae in the family Hydrobiosidae and in the genus Aoteapsyche (Hydropsychidae) ranked highly, and cased caddis larvae consistently ranked low, in the diet. Discriminant function analysis indicated that apparent prey preferences were partly related to whether foraging blue ducks were gleaning from the tops or undersides of rocks in the river. Canonical correlation analysis showed that ingestion of stonefly and mayfly larvae was associated with diving behaviour, but it was not possible to predict the ingestion o f other prey from foraging tactics. No single prey category was so highly valued by the blue ducks we studied that it might limit population establishment at new sites. Keywords: Blue ducks, benthic macroinvertebrates, conservation.

INTRODUCTION Blue ducks Hymenolaimus malacorhynchos Gmelin consume aquatic invertebrate larvae that they glean from the surfaces of and the crevices between submerged rocks in fast-flowing mountain rivers, principally in forested catchments (Kear & Burton, 1971; Collier & Lyon 199t; Collier 1991). This habit is shared by rather few bird species: five other ducks (Veltman et al., 1991) and a passerine (Ormerod, 1985). In the only well-studied case, that of the dipper Cinclus cinclus, *Present address: Ecosystems Division, NIWA, PO Box 11-115, Hamilton, New Zealand 187

188

C . J . Veltman et al.

new populations would clearly be hazardous if preferred prey were absent from release sites. The area of foraging substrate required to produce a sufficient biomass of invertebrate prey to meet the daily energetic requirements of an adult blue duck is very small compared with average territory sizes (Veltman et al., 1991). Blue ducks may defend such large areas of river as an insurance against prey depletion during floods, or if spatiotemporal variations in prey availability favoured the evolution of obligate specialists (Glasser, 1984). Conservation efforts for a bird specializing on certain prey would necessarily take a different direction from those for a generalist predator. We investigated the foraging ecology of adult blue ducks by sampling prey potentially available in the benthos and comparing them with prey consumed by six ducks at bimonthly intervals over 11 months on the Manganuiateao River in central North Island, New Zealand. This river supports the largest remaining North Island population of blue ducks (Williams, 1991), and is representative of central North Island rivers with catchments on Mt Ruapehu. We also collected data on foraging behaviour of the ducks to determine if the capture of any prey was associated with particular foraging tactics.

METHODS Sampling sites and months Data were collected between January and November 1989 from the Manganuiateao River which originates on the western slopes of Mt Ruapehu, central North Island. The river flows in a south-westerly direction for 80 km through alpine tussock, beech Nothofagus spp. forest and pasture before entering the Whanganui River. The river and its 600 km 2 catchment are described in greater detail by Cudby and Strickland (1986). Individually colour-banded adult blue ducks which occupied two readily accessible territories were chosen for this study. These territories were separated by three intervening territories, a distance of approximately 3 km. The composition of both territorial pairs changed during the course of our study and, as a result, our faecal and behavioural samples were collected from two female and four male blue ducks at the two study sites. We visited both territories to collect faecal and behavioural samples at approximately the same time that invertebrate samples were taken (16-19 January, 14-18 March, 2-3 April, 9-11 May, 11-22 July, 2-9 September, 24-26 October and 3-5 November). Floods in July disrupted our sampling programme, so data for consecutive weeks in July are presented separately. Invertebrate samples During each bimonthly visit, aquatic invertebrates were collected from the shallow rapid most favoured by the ducks during foraging in each territory. These remained the preferred foraging areas for ducks in each

territory throughout the study. Each of the two sites was sampled on two occasions; during early morning and in late afternoon when blue duck foraging activity in the non-breeding season is greatest (Veltman & Williams, 1990). In July, floods disrupted the sampling programme and samples were collected from each site only in the afternoon. Invertebrates were collected in three ways at each site on each sampling occasion to provide information about the communities inhabiting different-sized substrates and microhabitats. First, five stones (surface area 0.04-0.13 m 2) were randomly selected and all the invertebrates on the upper surface and sides were cleaned into a 0-5 mm mesh net using a brush tapered at one end to resemble a duck's bill. These 'upper stone surface' samples represented the invertebrate fauna likely to be most accessible to blue ducks. Each stone was then carefully manoeuvred into another 0.5 mm mesh net and carried to the river bank where the lower surface and sides were scrubbed into a bucket. This procedure produced the 'lower stone surface' samples which would have been relatively inaccessible compared with upper stone surface fauna to blue ducks foraging during daytime. Surface areas were estimated in January and March by selecting stones with dimensions closely similar to 10 reference stones of known surface area (estimated by wrapping in aluminium foil of a known weight per unit area), and in the other months by the method of Graham et al. (1988). Invertebrates were also collected from large boulders by brushing accessible sides into a 0-5 mm mesh net held in the current downstream of the boulder. Accessible surfaces of several boulders were sampled in this way for 1 min on each occasion (early morning and late afternoon), to give two 'boulder' samples per site per month. All invertebrate samples were stored in 4% formalin. Preserved samples were passed through 1 mm and 0.5 mm mesh nested sieves in the laboratory, and were sorted on white trays and at 10× magnification respectively.

Faecal samples During observation of focal animals, we recorded the location of droppings as they were deposited on exposed boulders. Once the ducks had moved away, we collected droppings separately into zip-top plastic bags and froze them. Thawed droppings were suspended in water and disrupted ultrasonically. The suspension was then subsampled using a Folsom plankton-splitter so that an unbiased fraction of approximately 200 fragments was extracted for microscopic analysis. Potential prey all had sclerotised body parts resistant to digestion which therefore could be identified by comparison with reference slides after passage through blue duck guts. Terminal segments of beetle larvae (Elmidae), mouth-hooks of flies (Muscidae) and all mandibles and clypera (other insect larvae) were removed from the subsample for identification and counting. When numbers were based on mandibles and clypera, we counted two mandibles and one clypeus as

Foraging ecology and conservation of blue ducks Table 1. Prey occurring in faecal samples, grouped into the eight categories used during analyses Category name Cased caddisflies

Aoteapysche Hydrobiosidae

Mayflies

Stoneflies

Aphrophila Chironomidae Other

Order

Taxa

Beraeopteraroria Moselyi Pycnocentria spp. Confluens hamiltoni Ti_llyard Pycnocentrodes spp. Olinga feredayi McLachlan Helicopsyche albescens Tillyard Trichoptera Aoteapysche colonica McLachlan Trichoptera Neurochorema spp. Itydrobiosis spp. Costachorema spp. Psilochorema spp. Ephemeroptera Deleatidium spp. Austroclima spp. Coloburiscus humeralis Walker Nesameletus sp. Zephlebia spp. Plecoptera Zelandoperla spp. Zelandobius spp. Acroperla spp. Diptera Aphrophila neozelandica Edwards Diptera Chironomus spp. Other genera Muscidae Ephydridae Elmidae Archichauliodes diversus Walker Miscellaneous species Trichoptera

equal to one individual ingested. Using this procedure, the number of prey in eight taxonomic categories (Table 1) could be calculated. The varying levels of taxonomic resolution reflected our ability to identify invertebrates from small fragments.

Behaviour samples A focal animal procedure was used to sample foraging behaviour during daylight hours. Once foraging ducks were sighted, observers remained in concealment amongst riverbank vegetation at distances of 10-100 m from them for as long as possible or until the birds ceased foraging. Each minute, observers watched one duck for 10 s and noted whether the bird performed at least one occurrence of diving, up-ending, headdipping, grazing (standing on a rock and collecting exposed and submerged prey), and surface-pecking foraging actions during this time. We also scored whether the bird was foraging in a rapid or a pool.

189

' m o n t h ' (six bi-monthly collections); 'site' (two nonadjacent shallow rapids); 'sample type' (faecal samples or all-stone-surface benthic samples) and 'prey category' (as in Table 1). Prey selectivity was indicated by a significant interaction between 'sample type' and 'prey category'. Parameter estimates from this interaction were measures of how much the proportions of each prey category differed between the use (faecal) and availability (stone surface) samples, and can thus be interpreted as resource selection coefficients. We tested for changes in selectivity between sites and amongst months by considering all those higher order interactions involving 'sample type' with 'prey category'. Parameter estimates of these interactions indicated how resource selection coefficients changed between months and sites. This approach was similar to the use of log-linear selection models (Heisey, 1985), except that availabilities were estimated by the model, and we did not have to assume Poisson error distributions for the number of prey estimated from fragments in faeces. Although tests of significance of the selection coefficients can be performed, their interpretation would be influenced by the initial choice of potential prey items (Johnson, 1980). Instead, we used these coefficients to rank the prey categories from most to least preferred and performed a Tukey multiple range test (Zar, 1974) to detect significant differences in relative selectivity. RESULTS

Changes in prey abundance between seasons Mean densities of invertebrates on stones (all surfaces, sites and sampling occasions combined) ranged from 3741 m 2 in January to 14,417 m 2 in M a y (Fig. I). Densities did not differ significantly between times of day or sites, but significant differences were evident between months (analysis of variance on log-transformed data, p < 0.001). Data collected in July were omitted from the A N O V A because samples were collected only in the 18000 16000 14000 E o~

g

12000

////

10000

///I /111 //// ////

8000

////

//// ////

6000 E 4000

//// 2000 0 Jan

Mar

May

Jul

Jul

Sep

Oct

Month

Statistical determination of prey selectivity Numbers of each type of prey in faecal and benthic samples were converted to proportions which were transformed to arcsine values (Zar, 1974). A factorial analysis of variance was performed using the factors:

Fig. 1. Density (x _+ 1 SE) of aquatic invertebrate larvae colonizing stones in the Manganuiateao River on seven sample dates. Samples were collected from two sites on consecutive days at bi-monthly intervals (n = 20, both sites combined) except for July when floods meant that samples were collected in consecutive weeks (n - 5 for each site).

C.J. Veltman et al.

190 10o ] (a)

9oI

8o

i

;

•

~

40-

~o-

,o: so :

i ~

i ~

~o-

;

;

~

i~

i

•

~o= Jan

I Mar

i May

i Jul

i Jul

8o9°_L"

~

~ ~ ~

0 ,

- (b)

10o

i Sep

7o7 / / /

60:

7, / /

50-

20

f

-

¢ = Nov

0 Jan

Mar

May

Jul

EL

Jul

Sep

Nov

Jul

Sep

Nov

Month

Month

10

15 "l (C)

(d)

9 13 12

8

7 6 5 4

O l m

}!2n

v-

Jan

Mar

May

Jul

Jul

Sep

Jan

Nov

Mar

May

Jul Month

Month

10- (e)

30

(f)

25

~

20

N

15

"6 P.O_

g_

o.

~ Jan

Mar

,.-r-~ ~ - ] - ~ _r-l__ May

Jul

Jul

Sep

5

o

Nov

Jan

Mar

Month

May

Jul

Jul

Sep

Nov

Month

lo- (g) 987o

6 5

"6 E

4

o~ 2

'

_YL

o Jan

Mar

r-~ May

Jul

Jul

Sep

Nov

Month

Fig. 2. Relative abundance of eight invertebrate taxa on boulders (11), all stone surfaces (D), and upper stone surfaces ([~). Note that vertical axes vary between prey categories. (a) Chironomidae: (b) cased caddisfties; (c) mayflies; (d) Aphrophila; (e) Hydrobiosidae; (f) stoneflies; (g) Aoteapsyche.

Forag&g ecology and conservation of blue ducks

191

Table 2. Number and percent (in parentheses) of faecal samples containing fragments of prey in each category

Cased caddisflies

Aoteapysche Hydrobiosidae Mayflies Stoneflies

Aphrophila Chironomidae Others

January

March

May

July

September

11 (100) 8 (72) 10 (90) 10 (90) 1 (9) 8 (72) 11 (100) 7 (64)

8 (72) 6 (54) 10 (90) 8 (72) 6 (54) ? (63) 11 (100) 7 (64)

12 (80) 6 (40) 15(100) 4 (26) 3 (20) 6 (40) 14 (93) 5 (33)

4 (40) 7 (70) 5 (50) 10(100) 6 (60) 6 (60) 5 (50) 1 (10)

8 (100) 2 (25) 1 (12) 6 (75) 7 (87) 5 (62) 8 (100) 1 (12)

November 8 (100) 7 (87) 8(100) 8(100) 4 (50) 6 (75) 5 (62) 1 (12)

Table 3. Interactions between factors confirming prey selectivity (ANOVA on arcsine transformed proportions of prey categories). 'Sample type' refers to benthic or faecal samples, p < 0.0005 for all tests

Effect

Sum of squares

d.f.

F

67.72 21-87

1192 7

54.98

34.71

35

17.46

1.82

7

4.58

7-91

35

3-98

Error Sample type × prey category (overall prey selectivity) Month x sample type × prey category (selectivity differs by month) Site x sample type × prey category (selectivity differs between sites) Month × site × sample type × prey category (selectivity differs between sites in some months)

afternoon and sites were not sampled after the same antecedent flow conditions. Invertebrate densities were significantly higher in M a y and September than in other months ( S t u d e n t - N e w m a n - K e u l s test, p < 0-05). Densities in March were significantly higher than in January or October. Chironomidae larvae were the most abundant animals in samples of all surfaces of stones, in samples from upper surfaces of stones, and in boulder samples (Fig. 2(a)). Cased caddisfly larvae were also numerous, although they tended not to be on upper stone surfaces (Fig. 2(b)). M a y f y larvae were relatively abundant on stones in October (Fig. 2(c)), but none of the other taxa comprised more than 10% of the total invertebrate fauna in any month. Note, however, that large changes in absolute abundance of these animals happened during the study (Figs 2(d)-(g)). Diet of blue ducks

& Taylor, 1990) in the diet of blue ducks. When all months and sites were considered together, cased caddisfly had significantly lower selectivity compared with all other prey categories which were positively selected to varying degrees (Table 4). Significant higher-order interactions in the A N O V A confirmed that selectivity differed between sites and amongst months (Table 3). For example, Chironomidae ranked highest in selectivity in March and September but lowest in N o v e m b e r (both sites combined, Table 5). In five of the six bi-monthly samples, cased caddisfly larvae ranked lowest in selectivity. In July the mayfly Nesameletus became an important component of the diet. Larvae of these species prefer water of low (<0-5 m.s ~) velocity (Jowett et al., 1991). Under the Table 4. Resource selection coefficients for blue duck foraging obtained from the parameter estimates of the sample type x prey category term of the ANOVA in Table 2

A total of 63 faecal samples was collected, from which approximately 7500 fragments comprising 3835 individual prey were identified. Blue ducks consumed prey in all categories. Chironomidae and cased caddisfly larvae occurred in the majority of samples; Hydrobiosidae and Aphrophila neozelandica (Diptera: Tipulidae) larvae were also frequently ingested, and diet varied between months (Table 2).

Positive coefficients indicate selection, negative coefficients indicate 'avoidance'. Similar superscripts link prey categories for which the resource selection coefficients were not significantly different (p < 0.05).

Evidence for selective predation by blue ducks

Stoneflies Chironomidae Others Cased caddisflies

Using A N O V A , we found a significant interaction between sample type (diet or benthos) and prey category (Table 3) which confirmed overall 'selectivity' (Thomas

Prey type Hydrobiosidae Mayflies

Aoteapysche Aphrophila

Resource selection coefficient 0.103 ~ 0.072" 0.067 ~,b 0-052~'b 0-043".h 0.039 b -0.022 h 0-35¢

C. J. Veltman et al.

192

Table 5. Ranked order of selection coefficients for prey categories in each sample month (1 = most highly selected) Prey Category

Month January

March

May

July

September

November

8 5 2 3 7 1 4 6

8 6 2 3 4 5 1 7

8 3 1 7 5 4 2 6

8 2 5 1 3 4 7 6

8 2 4 7 3 6 1 4

2 1 4 7 3 5 8 6

Cased caddisflies Aoteapsyche Hydrobiosidae Mayflies Stoneflies Aphrophila Chironomidae Others

flood conditions prevailing at that time, Nesameletus may have congregated at the river margins and become more accessible than other prey for foraging blue ducks. Microhabitats of prey and selectivity of predators One source of apparent selectivity could be microhabitat preferences of invertebrates, making some prey more inaccessible or harder to locate than others. To investigate this, we performed a discriminant function analysis using the fauna of the upper stone surfaces, lower stone surfaces, and diet samples. The first discriminant function (Fig. 3) separated diet samples from benthic samples. This reflected the numerical dominance of cased caddisfly larvae on stones and the predominance of Hydrobiosidae, Aoteapsyche and mayflies in the diet. The second discriminant function separated the upper and lower stone fauna, a contrast between

5

I

I

I

I

I

I

I

I

I

4 3 •

•

•

Cq c 0

o

o

2

o°

•

oo

o~

o

°o

E)

)

1 ~5 C

cO

;o

0 -1

5

•

o

o

o

o

-2 -3 o

-4

I

-6

I

5

I

I

I

I

I

I

I

-4

-3

-2

-1

0

1

2

3

Discriminant function 1

Fig. 3. Discriminant analysis of the relative abundance of invertebrate taxa in blue duck faeces (O), and on upper stone surfaces, lower stone surfaces and boulders in Manganuiateao River (O) (all months combined, arcsine transformed proportions). Ellipses enclose 50'/o of the data points in each group. Ellipse A, faecal samples; B, lower stone surface samples; C, upper stone surface samples; D, boulder samples.

Chironomidae dominance on upper surfaces and mayflies, cased caddisflies and Aoteapsyche on lower stone surfaces or between stones. The discriminant function scores for the 12 boulder samples are also plotted on Fig. 3. Their composition was similar to that of upper stone surfaces, though Chironomidae larvae were even more dominant. The centroid of the faecal samples in discriminant space is almost equidistant from the three types of benthic samples. There was large variation on the second discriminant function in the faecal samples, as much as amongst the benthic samples. This suggests that variability in blue duck diet was related to whether birds foraged on the upper surfaces or the undersides of stones as well as an avoidance of or inability to collect cased caddisfly larvae. Foraging tactics and prey capture In the clear water of the Manganuiateao River, we could clearly see that blue ducks collected prey on upper stone surfaces by grazing, head-dipping and up-ending. Parts of lower stone surfaces were reached using head-dipping, up-ending and diving. The blue ducks foraged mainly (81-99% of scores) in shallow rapids rather than pools within their territories. Diving behaviour was most frequent in March and July when water levels were high and prey living on stones and boulders would have been relatively inaccessible using other foraging methods. We checked the possibility that diet preferences were due to short-term changes in foraging tactics using a canonical correlation analysis to relate diet and foraging data. There were 21 cases when both diet and foraging data could be matched for individual ducks over the course of the study. This was possible whenever we had observed an individual duck and collected foraging data in the hours leading up to deposition of a faecal dropping by the same individual. We eliminated one diet category ('others') and one behaviour category ('pecking') to overcome the problem of using proportional data, and correlated seven diet values with four behaviour values for each case. The first (and only significant) canonical variate exhibited a contrast between diving and the other three foraging tactics (Table 6). Canonical variable loadings for the prey categories contrasted mayfly and stonefly numbers with chirono-

Foraging ecology and conservation o f blue ducks Table 6. Correlations of canonical variables with original variables for foraging behaviour and diet categories

Foraging behaviour Diving Up-ending Head-dipping Grazing

Canonical variable 1 ~).917 0-413 0.646 0.288

Diet categories Canonical variable 1 Cased-caddisflies Aoteapsyche Hydrobiosidae Mayflies Stoneflies Aphrophila Chironomidae

0.345 -0.437 0.177 -0.84 ~.755 -0-100 0.421

mid and cased caddisfly abundance (Table 6). This suggests that mayfly and stonefly larvae were captured mainly during diving activity, but that no other dietary pattern could be related to foraging tactic. DISCUSSION Temporal variations in prey numbers and composition such as occurred in the Manganuiateao River present a potential challenge for avian predators foraging in heterogeneous lotic environments. However, the diets of the blue ducks we studied reflected benthic community composition and were broadly similar to those described by Collier (1991). We did not find any evidence that blue ducks were obligate specialists or even that they consistently preferred certain prey. During the course of our work at Manganuiateao River, blue duck diets were dominated by a number of different invertebrate taxa, most of which were numerically abundant at different times. Chironomidae larvae were frequently the most abundant prey item in blue duck faeces and this was probably a reflection of their relatively high abundance in epilithon on easily accessible upper stone surfaces, particularly after periods of stable flow. We were surprised to find blue ducks apparently avoiding cased caddisfly larvae, since Collier (1991) found that cased caddisfly larvae were consumed in similar proportions to their occurrence in the benthos in the Manganuiateao River (one sampling occasion, several sites combined) and several other rivers in the eastern North Island. Further investigation of these differences is required. Other potential problems for lotic predators may arise in catching fast-moving prey (Peckarsky, 1984) and in discriminating cryptic prey on stone surfaces. Fastswimming mayfly species might be expected to evade predation more easily than relatively sessile cased caddisfly and chironomid larvae (Collier, 1991). However, our discovery that the swimming mayfly Nesameletus was highly selected in July indicates that blue ducks were able to capture fast-swimming prey when other taxa were not available due to the prevailing flood conditions. The correlation between diving activity and the presence of mayfly and stonefly larvae in the diet probably only reflected the fact that blue ducks dived more when water levels were high and that these prey had moved

193

into slower-flowing reaches. Other invertebrates may move temporarily into the hyporheos. Generally, blue duck foraging patterns represented all-purpose foraging tactics for gleaning invertebrates from submerged rocky substrates where prey are patchily distributed. It seems unlikely that the causes of decline in blue duck populations are related to the food supply. The association between stonefly richness and abundance and blue duck presence found by Collier et al. (1993) did not translate to a preference for stonefly nymphs by the foraging blue ducks we studied. A question of scale is involved here - - patterns exhibited at a population level may be nearly undetectable at the individual level (Ives et al., 1994), but we found no evidence at all of any consistent preferences. Such 'preferences' as the birds did show related to which part of a rock was gleaned. This is an encouraging result, since it suggests the choice of future translocation sites need not be limited by community composition of prey at those sites. Blue duck pairs in our study area have almost exclusive use of three to five shallow rapids (Veltman & Williams, 1990), their preferred foraging areas. They spend relatively little time foraging (Veltman & Williams, 1990), may require much less energy than is provided by available prey during non-breeding months (Veltman et al. 1991), and did not consistently prefer particular types of prey (this study). These facts lead us to suggest that other agencies, such as predators, may be causing decline via the high nest mortality and low recruitment rates recorded by Williams (1991). ACKNOWLEDGEMENTS We thank Bronwen McKay, Stephanie Prince, Carla Odlum, Martin Williams, Mike Wakelin, Ian Stringer and Jill Rapson for assistance in the field. Murray Williams banded the birds as part of an on-going demographic study. Murray Williams and Bobbi Peckarsky and two anonymous referees made very useful criticisms of an earlier draft of the manuscript. Financial assistance was received from the Massey University Research Fund and the Department of Conservation.

REFERENCES

Caughley, G. (1994). Directions in conservation biology. J. Anita. Ecol., 63, 215~4. Collier, K. J. (1991). Invertebrate food supplies and diet of blue duck on rivers in two regions of the North Island. N. Z. J. Ecol., 15, 131-8. Collier, K. J. & Lyon, G. L. (1991). Trophic pathways and diet of blue duck Hymenolaimus malacorhynchos on Manganuiateao River: a stable carbon isotope study. N. Z. J. Mar. Freshwat. Res., 25, 181-6. Collier, K. J., Moralee, S. J. & Wakelin, M. D. (1993). Factors affecting the distribution of blue duck Hymenolaimus malacorhynchos on New Zealand rivers. Biol. Conserv., 63, 119-26. Cudby, E. J. & Strickland, R. R. (1986). The Manganuiateao River Fishery. Fisheries Environmental Report, No. 14. Ministry of Agriculture and Fisheries, Turangi.

194

C . J . Veltman et al.

Cunningham, D. M. (1991). Distribution of blue duck in New Zealand from 1980 to 1991. Science and Research Series Internal Report, No. 36. Department of Conservation, Wellington. Eldridge, J. L. (1986). Territoriality in a river specialist: the blue duck. Wildfowl, 37, 123-35. Glasser, J. W. (1984). Evolution of efficiencies and strategies of resource exploitation. Ecology, 65, 1570~8. Graham, A. A., McCaughan, D. J. & McKee, F. S. (1988). Measurement of surface area of stones. Hydrobiologia, 157, 85 7. Harding, M. A. (1990). Observations of fruit eating by blue duck. Notornis, 37, 150~2. Heisey, D. M. (1985). Analyzing selection experiments with log-linear models. Ecology, 66, 1744-8. Ives, A. R., Kareiva, P. & Perry, R. (1994). Response of a predator to variation in prey density at three hierarchical scales: lady beetles feeding on aphids. Ecology, 74, 1929-38. Johnson, D. H. (1980). The comparison of usage and availability measurements for evaluating resource preference. Ecology, 61, 65 71. Jowett, I. G., Richardson, J., Biggs, B. J. F., Hickey, C. W. & Quinn, J. M. (1991). Microhabitat preferences of benthic invertebrates and the development of generalised Deleatidium spp. habitat suitability curves, applied to four New Zealand rivers. N. Z. J. Mar. Freshwat. Res., 25, 187-99. Kear, J. & Burton, P. J. K. (1971). The food and feeding apparatus of the blue duck Hymenolaimus. Ibis, 113, 483-93.

Ormerod, S. J. (1985). The diet of breeding dippers Cinclus cinclus and their nestlings in the catchment of the River Wye, mid-Wales: a preliminary study by faecal analysis. Ibis, 127, 316-31. Ormerod, S. J., Boilstone, M. A. & Tyler, S. J. (1985). Factors influencing the abundance of breeding dippers Cinclus cinclus in the catchment of the River Wye, mid-Wales. Ibis, 127, 33240. Peckarsky, B. L. (1984). Predator-prey interactions among aquatic insects. In The Ecology of Aquatic Insects, ed. V. H. Resh & D. M. Rosenberg. Praeger, New York, pp. 196-254. Scrimgeour, G. J. (1986). Prey selection by torrent fish, Cheimarrichthys fosteri Haast, in the Ashley River, North Canterbury, New Zealand. N. Z. J. Mar. Freshwat. Res., 20, 29-35. Thomas, D. L. & Taylor, E. J. (1990). Study designs and tests for comparing resource use and availability. J. Wild. Manage., 54, 322-30. Veltman, C. J., Triggs, S. & Williams, M. et al. (1991). The blue duck mating system - - are river specialists any different? Act. Congr. Int. Ornithol., 20th, 860 7. Veltman, C. J. & Williams, M. (1990). Diurnal use of time and space by breeding blue ducks. WildJowl, 41, 62 74. Williams, M. (1991). Some social and demographic characteristics of blue duck. Wildfowl, 42, 65-86. Zar, J. H. (1974). Biostatistical analysis, 2nd edn. PrenticeHall, London.