Ruddy Duck and Bufflehead Dive Duration

James L. Boone
SAIC, 1261 Town Center Drive, Las Vegas, NV 89134
Nevada State Museum, 700 Twin Lakes Drive, Las Vegas, NV 89107

E. A. LaRue
Nevada State Museum, 700 Twin Lakes Drive, Las Vegas, NV 89107

Abstract.--Ruddy and Bufflehead Ducks dive to the bottom of ponds to forage. During the winters of 1983-84, 1984-85, and 1997-98, we measured the length of time that Ruddy Ducks and Buffleheads stayed underwater during consecutive foraging dives. We found that, on average, male Buffleheads stayed underwater about 25% longer than females (P ≤ 0.007). Male and female Ruddy Ducks also differed in the length of time they stayed underwater (P = 0.023), but this result was not convincing. Controlling for water depth and gender, each individual duck stayed underwater for almost the same length of time during each consecutive dive (4 of 5 P >= 0.469); however, on average, different individuals stayed underwater for different lengths of time (all P <= 0.001). Based on estimates of high variance among-ducks, and low variance within-ducks, optimized sampling designs for studying population-level diving differences in Ruddy Ducks and Buffleheads would likely record one dive from each of many ducks rather than record many dives from a smaller number of ducks.

Introduction

Ruddy Ducks (Oxyura jamaicensis) and Buffleheads (Bucephala albeola) are diving ducks that feed by diving to the bottom of a pond, foraging on the bottom, returning to the surface to breathe, and then diving to forage again (Bellrose 1980, Palmer 1976). Optimal foraging theory suggests that because animals have a variety of demands on their time, they should forage in a manner that minimizes the total amount of time they spend foraging and maximized foraging efficiency (Ricklefs 1995). For these ducks, foraging dives are composed of multiple parts (e.g., diving to the bottom; foraging on the bottom, returning to the surface, and breathing), and minimizing any of these components may reduce total foraging time (Tome 1988).

While designing a field study of the diving behavior of Ruddy Ducks and Buffleheads, we needed to determine the variance of certain factors in the statistical design. With knowledge of these variances, we could define the sampling strategy necessary to answer our larger questions while minimizing our sampling efforts (Sokal and Rohlf 1995). To optimize our statistical design for each species, we needed to know two variances. First, we needed to know the variance among ducks: do all ducks dive for the same length of time, or do different ducks dive for different lengths of time? For at least one species of diving duck (Oldsquaw, Clangula hyemalis), nine individuals repeatedly dove and returned to the surface in synchrony as they foraged (Stewart 1967), indicating that the variance among ducks could be low. The second variance we needed to know was the variance within ducks: does each duck dive for the same length of time during each of its consecutive dives? We also needed to know if the sexes behaved alike, and we needed to know if ducks dove for different lengths of time in different depths of water. Because we expected to treat the two species separately, we did not need to consider differences between the species. Knowing which factors in the statistical design were most variable would allow us to focus our sampling efforts, permit us to reduce the error variance in our statistical tests, and help us make more powerful tests of our hypotheses concerning optimal foraging.

We, like other authors, generally made the assumption that ducks are foraging when they repeatedly dive and return to the surface in the same general location. This assumption was occasionally confirmed when a duck brought a food item to the surface. Much has been written about the foraging behavior of diving ducks (e.g., Bellrose 1980, Palmer 1976); however, relatively few researchers have quantified the length of time that wild Ruddy Ducks or Buffleheads stay underwater during foraging dives. Siegfried (1976), Tome (1991), and Hughes (1992) presented means and variances for groups of ducks (e.g., by gender or plumage class within species). While the data presented by these authors was helpful, they did not give information on individual ducks. In addition, Siegfried (1976) and Tome (1991) did not measure water depth, and Hughes (1992) averaged dive times over a range of depths (1.5-5.0 m) that was too wide for our purposes. Other authors reported the water depth, but not the time spent underwater (Cottam 1939, Bergan and Smith 1989). Only Heintzelman and Newberry (1964) reported the mean duration and range of values for an individual, but their sample was 25 observations of a single Ruddy Duck, and they did not present a variance or water depth. Less work has been done with Buffleheads, but Palmer (1976) reported that Buffleheads usually dive for 15-25 s in 2-3 m of water and that they stay submerged for less time in very shallow water. In general, information on variation in dive times among individual Ruddy Ducks and Buffleheads was lacking.

Because the information we needed was not available in the literature, the objective of this study was to estimate the variances associated with different aspects of the duration of foraging dives made by Ruddy Ducks and Buffleheads. In presenting these variances, we also present data on factors that affect the duration of dives and differ somewhat from previously published accounts.

Methods

During the winters of 1983-84 and 1984-85, we measured the length of time that male and female Ruddy Ducks and Buffleheads stayed underwater during consecutive foraging dives. We made observations on two shallow sewage treatment ponds at the Arcata Marsh (0.5-1 m, and 1-2 m deep), and in deeper water at Stone Lagoon (3-5 m deep), all in Humboldt County, California. During the winter of 1997-98, we made additional observations in sewage treatment ponds in Henderson, Clark County, Nevada, in water of depths similar to those in Humboldt County. The cities of Arcata and Henderson both used oxidation ponds for municipal sewage treatment, and in both cases, water depths were relatively uniform throughout each pond. In Stone Lagoon, we measured water depth and restricted our sampling to an area where the water was 3-5 m deep.

We measured at most 12 consecutive dives per duck using a timer and recorded the duration of foraging dives to the nearest second. While observing ducks, we attempted to measure one sequence of dives from each duck rather than repeatedly measure sequences of dives from one animal. We carefully followed individuals while recording observations, and we did not return to the same pond on the same day. Winter duck populations in both areas were large, and although some individuals could have been measured more than once, we are confident that the majority of the data represent different individuals. To reduce the effects that our presence may have had on the behavior of these ducks, we made observations from cover (e.g., parked vehicles, blinds) or at a distance with a spotting scope.

Using the ducks for which we recorded the duration of four or more consecutive dives (except n-i ≥ 3 in deep water), we calculated a mean dive duration (D-bar-i) and standard deviation (SD-i) for each individual. We used the D-bar-i values (i.e., one observation per individual to avoid pseudoreplication; Hurlbert 1984) to calculate a mean and standard error for dive durations for females (D-bar-f ± SE-f) and males (D-bar-m ± SE-m) within each water-depth class.

We analyzed the data separately for each water depth. Using the first four dives (three dives in deep water) from each duck, we compared the duration of foraging dives among genders, individual ducks, and the sequence of dives for each duck in a repeated measures analysis of variance. We used repeated measures analysis of variance because we repeatedly measured the dive duration of each duck. In our model, gender was a fixed factor, while individual ducks and the repeated dives were random factors. We used the mean squares from this analysis to calculate variances associated with the factors gender, ducks, and dives, using the methods of Sokal and Rohlf (1995:294). By calculating these variances, we could determine the relative contribution of each variance component to the total amount of variance in the data, which in turn could be used to optimize the sampling strategy.

Prior to data analysis, we performed tests of normality and equality of group variances. The results of these tests indicated that the data were suitable for parametric analyses. Despite this, we detected four outliers and deleted these from the analysis. Outliers likely resulted from ducks returning to the surface sooner than expected when, for example, a prey item needed special handling, or when ducks stayed underwater for much longer than expected for unknown reasons.

Results

We recorded observations of 43 Ruddy Ducks and 43 Buffleheads (Table 1). Most ducks dove in shallow to mid-depth water (0.5-2 m). Only 10 Ruddy Ducks were observed foraging in deeper water (>3 m) during this study, and Buffleheads were rarely observed diving in deep water. We did not see Buffleheads diving in the shallow pond (<1 m deep) at the Arcata Marsh, but we did see them in shallow water in Henderson.

For Ruddy Ducks (sexes combined), dive duration differed with water depth. Individual ducks dove for an average of 18-20 s in shallow water, 27-32 s in mid-depth water, and 45-50 s in deep water (Table 1). In contrast, Buffleheads dove for similar lengths of time in shallow (21-27 s) and mid-depth (21-26 s) waters. We did not test for differences among water depths.

For Buffleheads, and for Ruddy ducks to a lesser extent, the genders differed in the time spent underwater (Table 2). In both depths of water, male Buffleheads dove for about five seconds longer than females (both P ≤ 0.007). For Ruddy Ducks, females in shallow water dove for a longer period of time than males (P = 0.051), but males dove for longer than females in middle (P = 0.023) and deep waters (P = 0.212).

For both species of ducks, each individual dove for almost the same length of time during each of its consecutive dives (all P > 0.190; P ≥ 0.47 in four of five tests; Table 2). However, individuals dove for different lengths of time (all P ≤ 0.001; Table 2). For Ruddy Ducks, the largest variance was always associated with individual ducks, and the variance increased as depth increased (Table 2). In shallow and deep water, the variances associated with gender and dives were similar, but in mid-depth water, the variance associated with dives was less than half of the variance associated with gender. In shallow water, the variance among dives accounted for 34% of the total variance; however, in deeper water this variance accounted for only 13 to 16%. For Buffleheads in shallow water, the largest variance was associated with gender, but in mid-depth water the largest variance was associated with individual ducks (Table 2). In both cases, the variance among dives accounted for only 12 to 16%.

Discussion

Within each depth of water, individual ducks of both species dove for similar lengths of time on each consecutive dive, and in general, the magnitude of the variance associated with an individual ducks consecutive dives was small compared to the magnitude of the variance among different ducks. Thus, if one were designing a population-level study of diving behavior, the optimal allocation of sampling effort at this level in the statistical design (depending on the cost function chosen) would likely be to record one observation per duck and to maximize the number of individual ducks measured.

In mid-depth water, male and female Ruddy Ducks dove for different lengths of time (P = 0.023), and in shallow water, these differences approached significance (P = 0.051). However, the magnitude of the difference was relatively small compared to the length of time that these ducks stayed underwater (10-17% of dive duration depending on depth). It seems unlikely that these differences are biologically important. Furthermore, in shallow water, female Ruddy Ducks dove for longer periods, on average, than did males. Despite the statistical significance, these data were not convincing: larger sample sizes per water depth, and better control of water depth, would be needed to produce convincing results. Male Buffleheads, however, consistently dove, on average, for 23-26% longer than females. Adult male Buffleheads are, on average, larger and heavier than females (Belrose 1980); perhaps the longer dives by males reflects greater lung capacity or greater blood volume (oxygen carrying capacity). Tome and Wrubleski (1988) found that three species of diving ducks exhale immediately prior to diving, so lung capacity may not be important. Male and female Ruddy ducks are similar in size (Belrose 1980).

In different depths of water, Ruddy Ducks appeared to dive for different lengths of time. For each depth of water, the range of mean dive durations did not overlap with those from other depths of water. Although these differences appear significant, we did not test for the effect of differing depths because we did not carefully measure water depth, we did not attempt to control for differences among ponds, and other factors may have contributed to the apparent difference. For Buffleheads, water depth did not appear to influence dive durations. For Ruddy Ducks, this observation raises questions about optimal foraging behavior that needs further research.

Some anecdotal observations suggest that other factors may affect the duration of foraging dives. We noticed a few cases where apparently minor human disturbances (people arriving at the edge of a large pond, vehicles moving past a pond) appeared to correspond with slightly longer dive durations. In one case, maintenance personnel slowly drove a truck past a pond in which we were recording dives. On three dives before the vehicle arrived, a female Bufflehead dove for 20, 22, and 20 s. The duck did not dive while the vehicle was passing, but after it passed, we recorded two dives of 25 s each, which were then followed by two dives of 22 and 21 s. We also noticed that wind and rain may affect the duration of dives, slightly lengthening the time that Ruddy Ducks stay underwater. In Great Britain, Hughes (1992) found differences in dive times of Ruddy Ducks that were associated with age (plumage class), diurnal versus nocturnal dives, dive bout number, time of day, season, and other factors; however, the significance of the differences are questionable because of problems associated with pseudoreplication (Hurlbert 1982).

These data appear to be the first published detailed measurements of the duration of foraging dives by individual wild Ruddy Ducks and Buffleheads, therefore, there was little material in the literature to which to compare these data. Heintzelman and Newberry (1964) recorded 25 dives for a Ruddy Duck and reported that it stayed "below the water surface a minimum of 17 seconds, a maximum of 28, and an average of 20 seconds" (Bellrose 1980:473). We never recorded this many consecutive dives for one duck, and we never recorded this wide of a range of dive times in a given depth of water. In general, however, the dive times we recorded in certain depths of water were similar to dive times recorded by other authors. Siegfried (1976) found that the mean and standard deviation of the duration of male Ruddy Duck dives was 17.1±3.2 s (mean±SD, n=6) while that for females was 20.4±2.4 s (n=5). These means were similar to measurements we took in water of less than 1 m deep; however, Siegfried's (1976) estimates appear to confound variation among ducks with variation within ducks (i.e., the estimates appear to be based on 10 dives for each duck). Our results recorded from ducks in less than 1 m of water also were similar to those of Tome (1991) who recorded an average dive duration of 21.7±2.9 s (N = 140), but these values confounded within- and among-duck variation because Tome (1991) only measured 12 ducks. The standard deviations reported by Siegfried (1976) and Tome (1991) were slightly larger than we would have expected. These variance estimates may confound sources of variance due to within-duck, among-ducks, and water depths. The estimates of dive time (26.5-30.5 s) presented by Hughes (1992) averaged longer than those presented by Siegfried (1976) and Tome (1991), and may reflect differences related to deeper water. However, Hughes (1992) reported dive times that were not as long as we would have expected. Despite these minor differences, the results of our study and those of or predecessors suggest that Ruddy Ducks generally behave in similar ways in different places. For Buffleheads, our results for females were similar to those of Palmer (1976) who reported that Buffleheads dove for 15-25 s; but for males, we found that average dive times were slightly longer (23-32 s) than those reported by Palmer (1976).

It is clear that before we can understand optimal foraging in these diving ducks, we must account for, and hopefully understand, a variety of sources of variation related to their foraging behavior. In this study, we describe some sources of variation related to the length of time diving ducks spend underwater, we suggest directions for future research, and we point out analytical problems with some of the previous work done in this area. We encourage further work in this area, and we encourage others to become familiar with the work of Hurlbert (1984) and reanalyze their data in light of his suggestions (more recent references can be found in Heffner et al., 1996).

Acknowledgments

We express our sincerest gratitude to Stanley W. (Doc) Harris for honing our ornithological skills and encouraging this research. We also thank Patrick E. Lederle, C. Don Powers, Steven Peterson, and the anonymous reviewers for improving the manuscript.

Literature Cited

Bellrose, F. C. 1980. Ducks, Geese, and Swans of North America. 3rd ed. Stackpole Books, Harrisburg, PA. 540 pp.

Bergan, J. F., and L. M. Smith. 1989. Differential habitat use by diving ducks wintering in South Carolina. J. Wildl. Manage. 53:1117-1126.

Cottam, C. 1939. Food Habits of North American Diving Ducks. USDA Tech. Bull. 643:1-140.

Heffner, R. A., M. J. Butler IV, and C. K. Reilly. 1996. Pseudoreplication revisited. Ecol. 77:2558-2562.

Heintzelman, D. S. and C. J. Newberry. 1964. Some waterfowl diving times. Wilson Bull. 76:291.

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Table 1. Summary statistics for the duration of dives made by male and female Ruddy Ducks and Buffleheads in different depths of water. Entries are the number of individual ducks (N) included in the analysis, the range of individual means (D-bar-i) and individual standard deviations (SD-i), plus the overall means (D-bar) and standard errors (SE) for each gender.

Table 2. Summary of analyses on the length of time that Ruddy Ducks and Buffleheads stay underwater during foraging dives in three depths of water. Results are organized by model factor (Gender, Ducks, Dives). Entries include sample size (df) and significance (P) of the analysis of variance F-tests, the variance (S2) of each factor, and the amount of variance (%) associated with each factor. Buffleheads were not observed diving in deep water.