The Condor 94:101-109 0 The Cooper Ornithological Society 1992
NOCTURNAL FORAGING IN THE AMERICAN WHITE PELICAN ’
BLAIR F. MCMAHON AND ROGER M. EVANS Department of Zoology, Universityof Manitoba, Winnipeg,Manitoba R3T 2N2, Canada
Abstract. Nocturnal foraging was examined in American White Pelicans (Pelecanus erythrorhynchos)at the Dauphin River, about 50 km from a breeding colony on Lake Winnipeg, Manitoba, Canada. From two to three times as many pelicans foraged at night as in the daytime, with foraging flocks being larger at night. In contrast, more pelicanswere present at adjacent loafing sites during the day. Capture rates were highest for flocks of up to 100 foragersin the daytime, but showed no relationship to flock size at night. Rates of bill dipping and mean duration of dips were significantlygreater at night, but capture rates were significantly lower. Lower capture rates at night were to some extent offset by the capture of more large fish, probably becausethey were more accessibleat that time. Most of the day-night differencesin captureefficiency appeared to be due to lower visual sensitivity of pelicans at night. Key words: Nocturnalforaging; American White Pelicans; Pelecanuserythrorhynchos; foraging.
INTRODUCTION and Evans 1987) suggeststhat food limitation The American White Pelican (Pelecanus erythro- during the chick rearing period is chronic. rhynchos)is a large, opportunistic fish-eating bird Given apparently high food demands coupled that uses a wide range of foraging habitats, in- with travel constraints on diurnal foraging in pel- cluding rivers, marshes, and lakes. They com- icans, we examined the possibility that pelicans monly breed on relatively inaccessible islands may significantly increase their daily food gath- and reefslocated far from their foraging grounds. ering capacity by foraging extensively at night. Regular foraging trips range up to 100 km or Nocturnal foraging when food requirements can- more (Behle 1958; Johnson and Sloan 1978; not readily be met during daylight hours is known Knopf and Kennedy 1980, 1981; Trottier et al. to occur in other species(e.g., McNeil and Robert 1980; O’Malley and Evans 1984). On a typical 1988). Observations of pelicans foraging at night foraging trip, flocks depart from and return to have been reported (Low et al. 1950, Schaller the colony during daylight hours, with peaks of 1964, O’Malley and Evans 1984, Anderson 1987) travel near midday, when birds make use of ther- but quantitative measures of foraging activity mal updrafts that form as land heats in the sun and successat night are lacking. Examination of (O’Malley and Evans 1982). Individuals remain this potentially important aspect of the Ameri- at the foraging grounds for up to two or three can White Pelican foraging system was the ob- days during the incubation period, decreasingto jective of this study. To gain insight into the one day as energetic demands of young increase importance of nocturnal relative to diurnal for- after hatching (Knopf 1979, O’Malley and Evans aging, data were gatheredand compared for both 1982). time periods. Although white pelicans normally lay two eggs, American White Pelicans normally feed by they rarely fledge more than one young (Knopf moving on the water surface and dipping their 1979, Cash and Evans 1986). Hall (1925) and bill down into the water column. When prey is Anderson (1987) estimated that post-hatching captured, the forager raises its head well above pelicans require an average of 1.1 to 1.2 kg x the water surface to swallow. To assessrelative day-’ of fish prior to fledging. Growth rates of foraging effort directed towards foraging at night, young are strongly influenced by food intake we determined the number of foraging and loaf- (Evans and McMahon 1987, see also Schreiber ing pelicans at a riverine foraging site, and as- 1976), and even when only a single young is sessedrelative rates of bill dipping, prey capture, raised its frantic begging (“convulsions,” Cash and prey size. Becausepelicans could be under visual constraints at night not unlike those faced by humans (Sillman 1973) we also examined the I Received 25 April 1991. Accepted 29 August 1991. extent to which pelicans were able to form and [loll 102 BLAIR F. McMAHON AND ROGER M. EVANS maintain flocks while foraging at night. Ameri- those over 50) and location of foraging and loaf- can White Pelicans foraging during the day are ing pelicans were recorded during a total of 80 known to favor shallow water (Low et al. 1950, vehicle-based surveys at preselectedtimes of day Knopf and Kennedy 1980) often close to shore and night. A flock was operationally defined as where they can concentrate prey for greater ease any group containing pelicans that were within of capture (Behle 1958). We examined forager approximately 5 m of each other (O’Malley and location and water depth to determine if a similar Evans 1982), and for purposes of analyses also preference for shallow near-shore foraging also included solitary birds. Foraging flocks were rec- occurs at night. ognized by the presenceof bill dipping. Pelicans were assumedto be loafing if standing on a solid STUDY AREA AND METHODS substrate,including exposedsandbars or in shal- We studied a heavily used riverine foraging site, low water adjacent to shore, and were not dip- Dauphin River in south-central Manitoba, Can- ping. Distance to shore (nearest 10 m) was noted ada (5 1”58’N, 98”07’W). The study site was ap- for foragersat the time surveys were taken. For- proximately 50 km from a breeding colony on aging-site water depthswere obtained with a me- Lake Winnipeg (Koonz and Rakowski 1985). The ter stick or calibrated rope attached to a metal river varied from 50 to 2 10 m wide (X = 115 m), weight. thus making it possible to observe pelicans at all Rates of bill dipping, prey capture, and esti- times of day or night from an adjacent bank. mates of prey size were obtained from focal bird Most pelican foraging occurred along a seg- samplesin 1985, supplemented with focal group ment of river extending from the mouth up- (flock) samples in 1986. Focal individuals were stream for 6 km to the base of a seriesof rapids, selected by randomly directing the spotting or with lesser activity along and above the rapids night scopetowards a previously unsampled for- up to 17 km from the river mouth. Maximum aging flock or lone individual. Samples lasted water depth varied from 4 to 5 m above and either until the designatedforager departed from below the rapids and was less than 0.5 m along an area visible to the observer, or until a maxi- the rapids. Sandbarsused for loafing were present mum of 5 min elapsed.Verbal recordsof forager over a region extending from 2.5 to 5.5 km up- location and distance to shore, along with rele- stream from the mouth. To avoid visual prob- vant foraging data were recorded into a cassette lems with vegetation at night, nocturnal obser- recorderfor subsequentanalyses. During dipping vations were conducted only from O-4 and 13- episodes,the times that the bill struck the water 17 km from the river mouth. Daytime obser- and was subsequently raised were noted. The vations extended over the entire study area, but time between these two events is referred to as only data from O-4 and 13-17 km were used “dipping time.” If a pelican’s head was raised in when required to make valid day-night compar- a swallowing motion, “capture” was recorded on isons. tape. The period extending from the moment Observations were conducted daily in late that the bill was raised from the water until the spring from 31 May to 10 June 1985, from 11 prey was fully swallowed is referred to as “swal- to 28 May 1986, and in late summer from 23 lowing time.” August to 5 September 1985. These dates in- Randomly selectedfocal groupswere observed cluded late incubation to early brood rearing, and in 1986 for a maximum of 2 min each during late to post-brood rearing periods, respectively. the day, and until approximately 50% of foragers Either a 16-36 x 50 spotting scope or 10 x 50 had left the flock or the flock passedbeyond vi- binoculars were used for observations made dur- sual range at night. Subsamples within the field ing the day. Nocturnal observations were con- of vision of the spotting or night scopeprovided ducted with a Javelin model #325 infrared scope. data on mean dip rates x bird-l. There was usu- Effective range of the night scope on fog- and ally sufficient time between bill dips within sub- rain-free nights was approximately 300 m, well samples to scan the flock to assessthe number beyond the distance to the opposite bank. Ob- and size of captured prey. servations of nocturnal foraging were taken be- Each captured fish was classedinto one of five tween evening and morning civil twilight as il- size classes: Very Small: less than r/4the length lustrated in Figure 1. of a pelican’s bill, Small: l/4to % bill length, Me- Flock size (estimated to within about 10% for dium: L/zto 3/4bill length, Large: % to one bill PELICAN NOCTURNAL FORAGING 103
-A - dip rate c----Q capture rate 12 -
2 4 6 8 10 12 14 16 18 20 22 24
TIME (hr. 1 FIGURE 1. Relationship between time of day and mean f SE rates of bill dipping and prey capture in (A) late spring 1985, (B) late summer 1985, and (C) late spring 1986. Horizontal half-filled bars indicate civil twilight. No sampling was done between 6:30 and lo:30 hr. Note that bill dip and capture rate scaleson the ordinates differ by a factor of ten. length, and Very Large: exceeding one bill length. Bartlett’s or Hartley’s F,,,,, testsused to check for Small and Very Small fish were often hidden in equality of variances (Neter and Wasserman the gular pouch. In these cases,fish size was es- 1974). Following significant differencesdetected timated by examining the amount and shape of by analysis of variance, multicomparisons were pouch distention. Instances where fish size could done using Duncan’s or Scheffe’stests (Berenson not be determined reliably were excluded from et al. 1983). Whenever variance-stabilizing subsequentsize analysis. For statistical compar- transformations were ineffective, nonparametric isons, normality was first assessedand either Kruskal-Wallis rank tests (A’) were used, fol-
PELICAN NOCTURNAL FORAGING 105
TABLE 3. Percentagecapture of different prey sizes’ more frequently at night and made up a signifi- during the day and night (n prey captured in paren- cantly greater proportion of the identified diet at theses). night than during the day, even when Small and Very Small categories were excluded from the Prey size Day Night Total analysis (x2 = 39.1, P < 0.001). Very Small 67.1 (722) 17.7 (25) 61.4 (747) Small 23.2 (250) 59.6 (84) 27.4 (334) FLOCK SIZE Medium 8.0 (86) 5.0 (7) 7.6 (93) Foraging flock sizes recorded during surveys usu- Large 1.5 (16) 8.5 (12) 2.3 (28) Very Large 0.2 (2) 9.2 (13) 1.2 (15) ally ranged from one to about 200 pelicans, al- though one unusually large daytime group in late I Preycaptures at eachsize are expressedas a percentageof identified sample.See methods for definition of prey sizes. summer reached approximately 1,000 birds. In both late spring and summer, mean flock sizes of foraging birds were significantly larger at night during the late spring in both 1985 (t = 8.34, df than during the day (Table 4). This difference = 265, P < 0.001) and 1986 (t = 4.37, df = 475, paralleled a similar day-night difference in for- P < 0.001). Dip rates did not differ between day ager numbers (Table 2) suggestingthat larger and night in late summer of 1985 (t = 1.73, df flockswere simply a reflection of greaternumbers = 210, P > 0.05) largely because of one high of foragersalong the river. Loafing flock size did rate at 11:OOhr (Fig. 1B). not differ between day and night in late spring. Prey captures occurred during both day and During late summer, loafing flock size was sig- night (Fig. l), but rates were significantly lower nificantly greater at night (Table 4), even though at night (late spring 1985, t = 6.39, P < 0.001; numbers of loafers were greater during the day late summer 1985, t = 2.24, P < 0.05; late spring (Table 2). 1986, t = 8.65, P c 0.001). There was a strong The relationship between flock size and cap- positive correlation (P < 0.01 for each compar- ture rate was variable (Fig. 2). In late spring, ison) between dip and capture rates, especially daytime capture rateswere highestfor flockscon- during the daytime (Fig. 1) (late spring 1985 day, taining 21-100 foragers in 1985 and 2-20 for- r = 0.787, night, r = 0.453; late summer 1985 agersin 1986. There was no detectable relation- day, r = 0.759, night, r = 0.465; late spring 1986 ship between flock size and capture rate at night. day, r = 0.660, night, r = 0.452). With the possible exception of late summer (Fig. In addition to capture rates, size of prey cap- 2B), there was no tendency for large flocks(2 100) tured provided an important measure of foraging to be more successfulat capturing prey. success.During the day, frequency of capture decreasedwith prey size (Table 3). Results ob- WATER DEPTH AND DISTANCE FROM tained at night were more strongly affected by a SHORE greater proportion of instances where prey size Most foragers were located in shallow water (5 1.5 could not be determined (65.4% undetermined m), especially in late spring (Fig. 3). This cor- compared with 54.5% during the day). Although relation between water depth and forager abun- this increased uncertainty at night would pre- dance was statistically significant (P -c0.005) for sumably bias against noting small and very small each sample period (late spring 1985 day, r = fish, there still were more fish recorded for these -0.637, night, r = -0.709; late summer 1985 size classesat night than for any larger size class. day, r = -0.533, night, r = -0.650; late spring Large and Very Large fish, which were easily ob- 1986 day, r = -0.714). Becausewater depth de- served during both day and night, were captured creasedtowards shore, similar correlations were
TABLE 4. Mean f SD flock size of foragersand loafers at Dauphin River in 1985 (n flocks in parentheses).
SCWXl Variable Day Night K P Late spring Foragers 9.8 f 25.5 (95) 28.9 & 40.2 (50) 19.8 5 q day 1.6 a night A 1.2 3 I -L 21 17 .8- .8 177 .4 1 40 25 0 * 4 1 2-20 27-40 41-60 61-80 81-100 2101 FLOCK SIZE FIGURE 2. Relationship between flock size and mean t SE capture rate in (A) late spring 1985, (B) late summer 1985, and (C) late spring 1986. necessarilypresent between numbers of foragers Most prey were captured in water less than 1 m and distance to shore. deep during the day (99.4% of captures) and at Rate of bill dipping increased significantly as night (94.8%). foragers approached shore (P < 0.001 for both day and night in each season).Capture rates also DISCUSSION increased as foragersapproached shore. This re- This study indicates that American White Peli- lationship was significant during daylight hours cans forage extensively at night, at least during in late spring of 1985 (P < 0.01) and during both the breeding season.Significantly, the only claim day and night in late spring of 1986 (P < 0.001). to the contrary (Audubon, in Palmer 1962) re- PELICAN NOCTURNAL FORAGING 107 0 day o-49 50-99 100449 150499 200-249 z250 WATER DEPTH (cm) FIGURE 3. Relationship between water depth and abundanceof foragerson the river, plotted on a semi-log scale,during surveysconducted in (A) late spring 1985, (B) late summer 1985, and(C) late spring 1986. Nocturnal surveyswere not conducted in late spring of 1986. Vertical lines show standard error. ferred specifically to the behavior of white peli- similarly brief, and overall size range of prey cans on wintering grounds, when energy needs captured were similar with small prey being by would presumably be reduced. However, even far the most common. Large numbers of loafers during the breeding seasonit remains to be de- were present at both times. Day-night similari- termined whether white pelicans forage in ties in use of coordinated foraging were also pres- marshes and on open lakes at night. ent at Dauphin River (McMahon 1991). Evi- Several broad similarities emerged between dently, observations of pelican foraging during nocturnal and diurnal foraging behavior. At both daylight hours can in large part be extrapolated times, most foraging was done in shallow water to foraging at night. close to shore, dip rates were well in excessof Superimposedon day-night similarities in for- capture rates, duration of individual dips were aging were several smaller but potentially im- 108 BLAIR F. McMAHON AND ROGER M. EVANS portant differences. The larger number of for- 1973, Barnard 1980). This behavior was asso- agerspresent at night than during the day seems ciated with a relatively high capture efficiency unlikely to be due to differential capture success. during the daytime. Scanning behavior was rare- Although nocturnal foragers obtained relatively ly seen at night (3 instances, McMahon 1991), more large fish, few large fish were caught at any and capture efficiency was correspondingly de- time, and overall capture rates were significantly pressed. depressed at night relative to daytime levels. The presenceof larger foraging flocks at night Fewer foragersin the daytime seemsmore likely may also relate to visual ability. While foraging to reflect constraints imposed by diurnal travel during the day, white pelicans make use of local back to the breeding colony. enhancement, shifting positions to join currently Some day-night differences may have been a successfulforagers (O’Malley and Evans 1982, reflection of relative prey availability. The most McMahon 1991). At night, darkness would al- common species of fish found within or near most certainly reduce the ability of foragers to Dauphin River include: Lake whitefish (Core- monitor each others’ success.Under these con- genus artedii), sucker (Catostomus commersoni, ditions, any benefits of group foraging would pre- C. catostomus),yellow perch (Percajlavescens), sumably best be achieved by remaining close to- walleye (Stizostedion vitreum), fathead minnow gether, in larger flocks. (Pimephalespromelas), and johnny darter (Ethe- ostoma exile) (Keleher 1952; Scott and Cross- ACKNOWLEDGMENTS man 1973; Lysack 1980,198l). Numerous small schoolsof perch, minnows and darters normally We thank the people at Anema Bay for their interest and support throughoutthe study. Dr. Robert Barclay predominate in shallow water during the daytime supplied the infrared scope used for nocturnal obser- (Carlander and Cleary 1949, Emery 1973, Hall vations. J. G. Anderson and F. L. Knopf provided and Werner 1977). This may explain the large helpful comments on the manuscript. Financial sup- numbers of small pelican flocks and the predom- port was provided by grants from the Manitoba Wild- life Branch and an operating grant to RME from the inance of Small and Very Small fish in their diet Natural Sciencesand Engineering Research Council, during the daytime. At night, large nocturnal fish Ottawa. speciesincluding walleye, sucker, and whitefish typically move into shore and remain active in LITERATURE CITED shallow water until dawn (Carlander and Cleary 1949, Lawler 1969, Emery 1973). This corre- ANDERSON,J.G.T. 1987. Foragingbehavior of Amer- spondswith the large pelican flocksthat occurred ican White PelicansPelecanus erythrorhynchos in western Nevada. Ph.D.Diss. Univ. Rhode Island, at night, and agrees with our finding that large Kingston, RI. fish made up a significantly greater proportion BARNARD,C. J. 1980. Flock feedingand time budgets of successfulcatches at night. Swallowing time in the House Sparrow(Passer domesticus L.). Anim. was also greater at night, possibly becauseof the Behav.28:295-309. BEHLE,W. H. 1958. The bird life of Great Salt Lake. greater proportion of Large and Very Large fish Univ. Utah Press, Salt Lake City. captured at that time. BERENSON,M. L., D. M. LEVENE,AND M. GOLDSTEIN. Other day-night differences included rate of 1983. Intermediate statistical methods and ap- bill dipping, which was highest during the night, plications: a computer package approach. Pren- and frequency of prey capture, which was lowest tice-Hall Inc., Englewood Cliffs, NJ. CARLANDER,K. D., AND R. E. CLEARY. 1949. The at night. Reduced visual sensitivity during the daily activity patterns of some freshwater fishes. night may have contributed to both of these ef- Am. Midl. Nat. 41:447452. fects. Night vision capabilities of white pelicans CASH,K. J., ANDR. M. EVANS. 1986. Brood reduction have not to our knowledge been directly assessed, in the American White Pelican (Pelecanuserythro- rhynchos).Behav. Ecol. Sociobiol. 18:413-4 18. but it is known that at least one pelican species CASH, K. J., ANDR. M. EVANS. 1987. The occurrence, (P. occidentalis) has visual pigments similar to context and functional significanceof aggressive most other birds (Sillman 1973). Elevated rates beggingbehaviours in youngAmerican White Pel- of bill dipping at night are suggestive of non- icans. Behaviour 102:119-l 28. visual “probing” for food. Diurnal foragerswere EMERY,A. R. 1973. Preliminary comparisonsof day and night habits of freshwaterfish in Ontario lakes. frequently seen moving their heads as though J. Fish. Res. Bd. Can. 30~761-774. scanning the water prior to dipping their bills in EVANS,R. M., AND B. F. MCMAHON. 1987. Within- a manner suggestiveof visual searching(Pulliam brood variation in growth and condition in rela- PELICAN NOCTURNAL FORAGING 109 tion to brood reduction in the American White M.Sc.thesis, Univ. Manitoba, Winnipeg, Mani- Pelican. Wilson Bull. 99: 190-20 1. toba, Canada. HALL, D. J., ANDE. E. WERNER. 1917. Seasonaldis- MCNEIL, R., AND M. ROBERT. 1988. Nocturnal feed- tribution and abundance of fishes in the littoral ing strategiesof some shorebird speciesin a trop- zone of a Michigan lake. Trans. Am. Fish. Sot. ical environment. Proc. Int. Omith. Congr. 19: 106:545-555. 2328-2336. NETER.J.. AND W. WASSERMAN. HALL, E. R. 1925. Pelicans versus fishesin Pyramid I- I 1974. Aoolied__ linear Lake. Condor 27: 147-160. statisticalmodels. Regression,analysis of variance JOHNSON,R. F., JR., AND N. F. SLOAN. 1978. White and experimental design. Richard D. Irwin, Inc., Pelican production and survival of youngat Chase Homewood, IL. Lake National Wildlife Refuae. North Dakota. 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Parkes [eds.], Rep. 80-30: l-l 18. Avian biology,Vol. III. AcademicPress, New York. LYSACK,W. 1981. 1980 Lake Winnipeg fish stock TROTTIER,G. C., R. J. BREMEMAN,AND N. A. YOUNG. assessmentprogram. Manitoba Dept. Nat. Res. 1980. Status and foraging distribution of White Rep. 8 l-20: l-67. Pelicans, Prince Albert National Park, Saskatch- MCMAHON,B. F. 1991. Foragingbehaviour of Amer- ewan. Can. Field-Nat. 94:383-390. ican White Pelicans (Pelecanuserythrorhynchos).