Hatching asynchrony and food allocation within ...

Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbus podiceps MARKR. L. FORBES' A N D C. DAVISON ANKNEY Department of Zoology, University of Western Ontario, London, Ont., Canada N6A 5B7

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Received January 19, 1987 FORBES, M. R. L., and ANKNEY, C. D. 1987. Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbuspodiceps. Can. J. Zool. 65: 2872-2877. Broods of Pied-billed Grebes, Podilymbus podiceps, breeding near Minnedosa, Manitoba (55'16' N, 99'50' W), in the summer of 1984, hatched from five to eight eggs over 3-7 days. We found a negative correlation (partial r = -0.56, df = 11,p < 0.05) between the length of the hatching interval and the number of chicks surviving to 35 days of age, after controlling for clutch size. The mean number of sibling aggressions (= i 1.6, n = 20, 2 SE = 0.4) was higher during foraging bouts with below-average rates of food delivery by parents, as opposed to bouts (.i = 0.5 aggressions, n = 18, 2 SE = 0.2) with above-average rates. Also, larger chicks initiated and won more disputes over positions near adults (Fisher's exact test, p < 0.05, n = 40) than did their smaller sibs. Thus, size hierarchies among siblings influenced food allocation within Pied-billed Grebe broods. Food distribution, however, was nonrandom for 12 of 18 foraging bouts in which there were no apparent size differences between attendant chicks. Signals of hunger status from begging chicks, and parental regulation of which chicks maintained positions near them, also affected food allocation among siblings. Finally, handling times of prey, captured by adults and fed to chicks, depended on brood age (partial r = -0.21, df = 183, p < 0.05) after controlling for prey size. For Pied-billed Grebes, noticeable size hierarchies among chicks remained throughout most of brood development and may have been advantageous during times of low food availability, or may have enabled parents to forage more effectively on a wide range of prey types and sizes. FORBES,M. R. L., et ANKNEY, C. D. 1987. Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbus podiceps. Can. J. Zool. 65 : 2872-2877. Des portCes de Grkbes a bec bigarrC, Podilymbuspodiceps, des environs de Minnedosa au Manitoba (55'16' N, 99'50' W) ont donne cinq a huit oeufs eclos en 3-7 jours durant 1'CtC 1984. Aprks avoir apporte les corrections nkcessaires pour compenser le nombre d'oeufs par portee, il ressort une correlation negative ( r partiel = -0,56, dl = 11, p < 0,05) entre la longueur de l'intervalle d'Cclosion et le nombre de petits qui survivent jusqu'a 1'8ge de 35 jours. Le nombre moyen d'agressions au sein d'une portie (i = 1,6, n = 20, 2 E.T. = 0,4) est plus ClevC lorsque les parents apportent une quantite de nourriture infkrieure a la moyenne que lorsque cette quantitC est superieure a la moyenne (i = 0,5, n = 18,2 E.T. = 0,2). De plus, les oisillons de grande taille initient et gagnent plus de disputes pour maintenir une position favorable pres des parents (test exact de Fisher, p < 0,05, n = 40) que ne le font les oisillons de la mime portCe qui sont de plus petite taille. Ainsi, une hierarchic basCe sur la taille influence la distribution de la nourriture chez les oisillons de la portee. Cependant, 18 episodes d'alimentation ont CtC observks chez des oisillons de tailles apparemment semblables et, en 12 de ces episodes, la repartition de la nourriture ne s'est pas faite au hasard. L'intensite de la faim chez les oisillons quemandeurs et le contrale de la position des oisillons par les parents affectent aussi l'allocation de la nourriture. Enfin, le temps nkcessaire a la manipulation des proies capturkes par les parents et transmises aux oisillons varie avec 1'8ge de la portCe ( r partiel = -0,21, dl = 183, p < 0,05), une fois les effets de la taille des proies pris en consideration. Chez ces Grkbes, la hiCrarchie basee sur la taille se maintient durant presque toute la periode de dCveloppement de la portee et est peut-itre avantageuse en periode de disette; peut-itre permet-elle aussi aux parents de recolter de la nourriture plus efficacement en utilisant une plus grande gamme de types et de tailles de proies. [Traduit par la revue]

Introduction Size hierarchies within avian broods, created by hatching asynchrony, are thought to be adaptive (e.g., Lack 1968; Hahn 1981), by allowing optimal allocation of food or parental investmerit among chicks. During times of low food availability, larger chicks have monopolized parental feedings (Lack 1968) and thus caused the starvation of their -younger siblings. In more extreme cases, smaller chicks were victims of siblicide or infanticide (O'Connor 1978; Stinson 1979; Mock 1985), but the putative result was the same: a trade-off between the quantity and quality of offspring. Other authors have argued that hatching asynchrony is a widespread avian adaptation that reduces the probability of total nest failure (e.g., Hussell 1972; Clark and Wilson 1981). These authors viewed size hierarchies as a consequence and perhaps a cost of hatching asynchrony. We studied parent-offspring interactions within Pied-billed Grebe (Podilymbuspodiceps) broods and assessed whether size 'present address: Department of Zoology, Erindale College, University of Toronto, Mississauga, Ont., Canada L5L lC6.

hierarchies among siblings and sibling aggression determined food distribution. We also examined other factors, pa~ticularly signals from begging chicks and parental behavior, that may have influenced food allocation among siblings. Finally we assessed whether size hierarchies among siblings allowed parents to exploit better a wide range of prey types and sizes. 9

Study area and study species Our study area, of approximately 64 km2,was centered about 10 km southeast of Minnedosa, Manitoba (55'16' N, 99'50' W). This area, of glacial origin, is characterized by many shallow eutrophic ponds (Ferguson-and Sealy 1983). Pied-billed Grebes that breed in prairie pothole regions usually nest one pair per pond (Faaborg 1976; Sealy 1978) and rear their broods on the pond used for nesting. Thus, individual families are easily observed throughout the breeding period. Pied-billed Grebes are semiprecocial (Nice 1962, cited in O'Connor 1984), i.e., chicks are down-covered and nidifugous, but they are fed by their parents. Finally, Pied-billed Grebe broods hatch from average clutches of four to seven eggs (review by Sealy 1978) over 2 or more days (Glover 1953; Chabreck 1963).

FORBES AND ANKNEY

Materials and methods


Thirty-eight nests were located in the shoreline emergent vegetation of 30 semipermanent ponds (Stewart and Kantrud 197 1) in the summer of 1984. Eight of these nests were suspected renests, i.e., they were discovered after first nests were found destroyed. Chicks subsequently hatched from only 13 nests. Nests were visited daily during the egg-laying period to determine laying schedules and clutch sizes. Nests were also visited daily during the hatching period, or twice daily if only one egg was remaining, to determine the number of days over which entire broods hatched. i.e., the hatching interval. Chick survival was later recorded as the number of chicks surviving to 35 days after mid-hatch, where mid-hatch was the date by which half of the brood had hatched. We chose 35 days because chicks were generally independent of parental care and were practising upwind flights by that brood age. The following are definitions of some terms used in this paper. An attendant chick was a chick accompanying, but not necessarily being fed by, a foraging adult. A feeding group was the number of attendant chicks (not necessarily equal to the number of chicks known to be alive that day). Feeding group size was extremely unstable, as chicks frequently entered and left the foraging group(s). Brood age was the number of days between date of mid-hatch and date of observation. A foraging bout was an observation of an adult(s) diving and (or) feeding a chick or group of chicks. Foraging bouts ended when the adult(s) ceased diving and feeding chicks, or moved out of sight. Observations were also terminated if the size of the feeding group changed, thereby keeping the effects of feeding group size constant within bouts. Because size of the feeding groups changed frequently, recorded foraging bouts were usually short. All observations were made during light to moderate (0 to approx. 20 k d h ) wind conditions on sunny to partly sunny (0-75% cloud cover) days between 07:OO and 18:OO. Foraging rate was the number of dives made by an adult(s) divided by the total observation time. Only foraging rates of adults that were feeding prey to chicks were included. Brood division occurred whenever the two adults occupied different regions of the pond simultaneously, each adult being accompanied by some brood members. Brood divisions were important, inasmuch as chicks from one subgroup could not readily affect food allocation within the other subgroup. Sibling aggression consisted of either chases or pecks directed at one chick by another chick during the time between adult dives. Pause time was the time (in seconds) between dives of actively foraging adults (rather than adults also engaged in preening or loafing). Thus, pause time was time largely devoted to prey handling by adults and chicks. Foraging bouts of adults feeding chicks were observed with 7 X 35 binoculars and a 20X spotting scope from a car, from a muskrat-hut blind (Nuechterlein 1985), or from an elevated blind (Pettingill 1970). Foraging success, i.e., the percentage of dives that resulted in prey capture, were recorded, as were the pause times of foraging adults. Whenever possible, individual chicks were identified on the basis of their size relative to adults (i.e., < I/,X , Y4- l/z x , '/2-3/4 X , 3/4- 1 X ) or siblings, and (or) sketches of their striped head patterns (Fig. 1). The striped head pattern is often distinctive for individual chicks. Food allocation among siblings was also recorded; occasionally prey were identified to suborder or more specifically (Pennak 1978; Preston 1982; Merritt and Curnmins 1984), and whenever possible, these prey were "sized" relative to the foraging adult's bill. Culmen length for adult Pied-billed Grebes ranged from 23 to 33 mm (Forbes 1986). In addition, the color of the bare loral area (Fig. 1) and the behavior of an attendant chick were concurrently recorded whenever differences in coloration of the loral areas of chicks within a feeding group were noticed. Finally, instances of brood division by pair members (Simmons 1974) and occurrences of sibling or adult-chick aggression were noted. Statistical analyses We calculated a partial correlation (Zar 1984) between hatching interval and chick survival, after we controlled for clutch size. We also regressed (Sall 1982) the number of dives made by foraging adults against total observation time to determine a mean foraging rate over all

FIG. 1. Drawing of the head and neck region of a 1-day-old chick outlining the extent of striping on the head and neck, and showing the unfeathered loral area. The loral area is the region indicated by the unbroken line; it encircles the lore, eye-ring, and to a lesser extent the proximal ends of the upper and lower mandibles. observation times. We then compared foraging success, feeding group size, and the number and incidence of sibling aggressions for bouts with above- and below-average foraging rates, using the unpaired t-test and the X2 test (Zar 1984). Fisher's exact test (Zar 1984) was used to determine whether success during aggressive interactions between siblings depended on the relative size of the initial aggressor. Fisher's exact test, which is more accurate than the X2 test for contingency analyses with small cell frequencies (Zar 1984), was also used to determine if the color of the loral area of attendant chicks depended on their begging and (or) receiving food from parents. We used Cole's (1962) "closed sequential design" to test if food allocation was nonrandom within feeding groups consisting of two chicks of similar size. Such groups tended to last longer without changes in composition than groups with chicks of noticeably different sizes. Food distribution would be nonrandom if one chick either received food more often than another, or received its food before the other chick did. Cole's design also tests the latter possibility. Only bouts for which there was a 15-min lag before re-onset of foraging by adults were used for comparisons between bouts with above- and below-average foraging rates. We arbitrarily chose a 15min lag so that attendant chicks were likely to be hungry. This restriction also applied to analyses of food allocation within broods. Finally, we tested if pause time was independent of brood age, after we partialled out the effect of prey size.

Results Pied-billed Grebe broods included in our sample hatched from five to eight eggs over at least a 3-day interval (i = 5.3 days, n = 13, SE = 0.3, Table 1). We found a negative correlation (partial r = -0.56, df = 11, p < 0.05) between hatching interval and chick survival, after controlling for clutch size. Presumably, greater initial size disparities among chicks resulted in fewer chicks suhiving to fledge. Mean foraging rate for 38 foraging bouts is described by the following equation: log(y) = - 1.74 + 0.85 log(x); r2 = 0 . 5 3 , ~ < 0.05, where y is the number of dives made by adults and x is the observation time (seconds). Neither mean feeding group size

CAN. J. ZOOL. VOL. 65, 1987

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TABLE1. Nesting data for 13 Pied-billed Grebe nests from which young hatched -

Nest

Clutch size

-

---

Hatching interval (days)

No. of eggs hatcheda

TABLE3. Relationship between size of the initial aggressor and success during sibling disputes over position

No. of chicks surviving

Initial aggressor Result


Successa Failure Total

Larger sibling

Smaller sibling

Total

29 0 29

8 3 11

37 3 40

"Chick securing a position close to a foraging adult; Fisher's exact test, p < 0.01.

NOTE:Data are ordered by clutch size and secondarily by hatching interval. 'The numbers of eggs hatched on the first day are in parentheses.

TABLE2. Comparisons of mean size of feeding groups ( + 2 SE) , mean observation time, mean foraging success, mean number of sibling aggressions (using unpaired t-test), and incidence of sibling aggression (using X 2 test) between bouts with above- and below-average foraging rates by Pied-billed Grebe adults Foraging rate

Feeding group size Observation time (s) Foraging successa No. of sibling aggressions No. of bouts with aggression

Above average (n = 20)

P

Below average (n = 18)

2.7k0.4

ns

2.4k0.2

159.7k37.6

ns

157.7k27.8

93.3% (1.3k0.18)

**

49.4% (0,7820.14)

0.5k0.2

**

1.6k0.4

8

ns

11

NOTE:See text for determination of above- and below-average foraging rates. **, p < 0.01; ns, not significant ( p < 0.05). "Percentages were arcsine transformed for analy ses and values in parentheses are radians.

nor mean observation time was significantly different between bouts with above- and below-average foraging rates (Table 2). Foraging success, however, was lower for below-average foraging rates (Table 2). Thus, relative foraging rate described the relative rate of food delivery, i.e., the number of prey items brought to attendant chicks by adults per unit time. The number of sibling aggressions was higher for bouts with below-average rates of food delivery (Table 2). Moreover, 60% of foraging bouts with below-average rates of food delivery were characterized by at least one aggressive interaction between siblings (Table 2), compared with 40% of bouts with above-average rates of food delivery. Although this result was nonsignificant, the incidence, as well as the number, of sibling aggressions was higher for bouts with low rates of food delivery. Larger chicks were more likely ( p < 0.01) to initiate and win disputes over position (Table 3) than were their smaller siblings. Position referned to proximity to a foraging adult and was

directly related to food acquisition. However, even in the absence of noticeable size differences among siblings, food distribution within the brood was usually nonrandom. For 18 foraging bouts with two attendant chicks of similar size, 12 showed nonrandom food allocation (Table 4). Clearly, factors other than size hierarchies among chicks also influence food distribution within Pied-billed Grebe broods. Color changes in the loral area of chicks appeared to be one such factor. This area can become infused with blood, thereby changing color from flesh ochre to crimson red (color terminology from Smithe 1975). These color differences depended on whether chicks were begging and (or) receiving food from adults (Table 5), and probably signalled the degree of hunger of chicks. Brood division by parents appeared to be facultative rather than obligate (in sensu Simmons 1974), as Pied-billed Grebe adults periodically amalgamated the brood during the day and foraged or loafed together with the chicks. More often, however, adults divided the brood and thereby may have reduced the competitive interactions among siblings. Although parents were often separated during the day, they communicated with each other principally through hen-flicker calls, described by LaBastille (1974) for the similar Atitlan Grebe (Podilymbus gigas), and duets (Nuechterlein and Storer 1982). Parents also influenced distribution of food within the brood through aggression directed at chicks. During 175 h of field observation, nine separate instances of adults pecking and (or) chasing their chicks, which were S 2 5 days old, were witnessed. During five of these interactions, the size of the ousted chick was not determined. On three occasions, however, the largest chick was chased away from the feeding group. All three of these ousted chicks, which were identified on the basis of head pattern, had maintained positions close to foraging adults several days earlier. On one occasion, an adult grabbed a chick by the nape and shook the chick violently (see Horsfall (1984) for similar cases of this behavior in European Coots, Fulica atra). Then the adult chased away this smallest brood member. Such regulation of composition of the feeding group through parental aggression was not observed in broods until 15 days or more after the mid-hatch date. Thus, parental intervention can override sibling hierarchies in terms of food distribution within grebe broods. Pause time was related to prey size (Fig. 2) by the following equation: log(y) = 0.67 + 0.22(x); r2 = 0.32, n = 185, p < 0.05, where y is the pause time (seconds) and x is the prey size relative to the foraging adult's bill. Moreover, pause time was negatively correlated (partial r = -0.2 1, df = 183, p < 0.05) with brood age, after partialling out relative prey size. Thus, adults tending older broods processed prey of a certain size more

FORBES AND ANKNEY

TABLE4. Food allocation between two chicks of similar size for 18 foraging bouts No. of prey items given to:


Chick A"

Sequence of feeding of chicks A and B

Chick Ba

7A 13A, l B , 7A 10A, l B , 2A 12A l A , 3B, 2A, l B , 5A, 2B, 1A 12A, lB, 3A 4A, lB, 2A 7A, 1B 12A 3A, 2B, 6A, l B , 4A, l B , 2A 2A, lB, 12A 17A, 3B 4A, lB, 3A, 4B 4A, 3B, 3A, 2B, 6A, 2B, 12A 6A, l B , 8A l A , 2B, 7A, 6B, l A , l B , l A , lB, 2A, lB, l A , lB, IA, 1B 13A, 3B, l A , l B , 5A, 2B, 2A, l B , l A , IB, 2A, IB, 7A, 2B, 4A 2 A , 2 B , 5 A , 3 B , 2 A , 1B,2A, 1 B , 2 A "A, B, arbitrary designations for chicks. ole's test, *, p < 0.05; ns, not significant @ > 0.05).

TABLE5. Coloration of loral area of attendant chicks" in relation to their activity (begging and (or) receiving prey vs. neither begging nor receiving prey) Attendant chick(s) begging and (or) receiving prey from parents Coloration of loral area Bright crimson Less extensive, less bright, or lacking Total

Yes

No

Total

27

0

27

32

36

32

63

4b 31

"Broods of two or more chicks in which the loral area of at least one chick was brilliant crimson. Brood age 5 14 days. Fisher's exact test, p < 0.001. 'One chick observed to lack the coloration, but it also rejected prey brought to it by an adult.

quickly than did adults tending younger broods. Perhaps adults make decisions about food allocation on the basis of prey size relative to chick size, or the capacity of chicks to handle prey. We saw small chicks reject large salamanders brought to them by adults, but they subsequently accepted smaller prey when these were offered.

Discussion Many species of birds show brood reduction, with laterhatched chicks being most susceptible to starvation (e.g., Strehl 1978; Cash and Evans 1986) or infanticidal and siblicidal tendencies of parents and older siblings (e.g., O'Connor 1978; Stinson 1979). Few studies, however, have demonstrated a relationship between length of the hatching interval and chick survival within a species (Shaw 1985; this study). Such a relationship indicates that variation around an existing degree of

Relative Size of Prey FIG. 2. Relationship between size of four important prey types and pause time of provisioning adults. Prey were sized relative to the adult's bill. Plotted points are the intersections between modal size for that prey taxon and mean pause times. Vertical lines represent 2 SE. Sample sizes are shown above. Prey types (from left to right) are dragonfly naiads, dytiscid larvae, leeches, and salamander larvae (Ambystoma tigrinum diaboli). References used to identify prey items were: Pennak (1978), Preston (1982), and Merritt and Cumrnins (1984).

hatching asynchrony is important in determining how many chicks survive to fledge. We do not claim that the potential for brood reduction is the only adap~iverationale fok hatching asynchrony, especially given that Pied-billed Grebes, like other grebes, are susceptible to heavy nest losses (Glover 1953; Ferguson and Sealy 1983; this study). Possibly, hatching asynchrony is also adaptive for grebe broods because it reduces the likelihood of total nest


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failure, as suggested for altricial birds (e.g., Hussell 1972; Clark and Wilson 1981), or because the incubation pattern, during the egg-laying period, maintains egg viability (see Arnold et al. ( 1987) for recent and compelling evidence). Models incorporating other such arguments will likely predict observed hatching asynchrony for grebe broods better than single-factor models will. Presently, there is little information on variation in hatching asynchrony within species. We are not sure whether the hatching intervals we report are representative for broods of Piedbilled Grebes. Our data span only one breeding season, during which the Minnedosa area experienced semidrought to drought conditions. Forbes (1986) showed that monthly totals of precipitation (Environment Canada 1984) for May, July, and August 1984, were >2 SE less than estimates of means based on 10 years of data before 1984. Total precipitation for June 1984, however, was average, based on the 10-year mean. It is noteworthy that conditions of low water in prairie regions affect other aspects of grebe population dynamics, such as breeding density and nesting success (Sealy 1978; Ferguson and Sealy 1983). Importantly, hatching asynchrony does establish initial size disparities among brood members in many species of birds (e.g., Lack 1968; O'Connor 1978), and these size disparities can influence allocation of parental investment within broods (e.g., Werschkul 1979; Ryden and Bengtsson 1980). Our finding that sibling aggression was higher when rates of food delivery were low is consistent with traditional views about the proximate mechanisms of brood reduction (Lack 1968). That larger grebe chicks were more aggressive and more successful in aggressive encounters than their smaller sibs supports the results of other researchers (Hahn 1981; Inoue 1985; Ploger and Mock 1986)that size hierarchies among siblings are dominance hierarchies. So size disparities and aggression among grebe siblings (see also Nuechterlein 198 1) do influence position and behavior of chicks, which can result in nonrandom allocation of food. There are likely many factors, however, that determine food distribution among siblings. For example, we found that food allocation among attendant chicks of similar size was usually nonrandom. Differences in the color of the bare loral area of chicks appeared to signal the hunger status of chicks. Nuechterlein (1985) similarly identified one signal function of color changes in a bare patch of skin on the crown of downy Western Grebe chicks (Aechmophorus occidentalis). Differences in begging behavior or intensity between chicks do exist, and may interact with sibling hierarchies in determining food allocation. Adult regulation of which chicks accompany them on foraging trips seemed extremely important in determining food distribution to young. Regulation can occur through brood division (Simmons 1974; Ferguson and Sealy 1983), temporary chick abandonment (M. R. L. Forbes, personal observation), and parental aggression directed at larger chicks (Ferguson and Sealy 1983; this study). That adult European Coots greatly influence food allocation through preferential feeding, regardless of size differences among siblings (Horsfall 1984), diminishes the importance of hatching asynchrony in producing competitive disparities among siblings. Nonetheless, we noted that size hierarchies, and presumably differences in competitive ability among siblings, were present throughout most of brood development. Perhaps size hierarchies among Pied-billed Grebe siblings result from both hatching asynchrony and slower growth of later-hatched chicks, as shown for other species

(Werschkul 1979; Inoue 1985), although growth rates of firstand last-hatched chicks of some species, such as larids, did not appear different (Abraham 1 986). To determine whether sibling hierarchies are adaptive, we need to assess all costs associated with, and all benefits accrued from, those hierarchies. Age and size disparities among grebe young might be beneficial if they better enable adults to exploit a wide range of prey types and sizes. We found that prey handling times for adults feeding chicks depended on both prey size (as has been shown for adult birds foraging singly, e.g., Draulans (1982) and references therein) and brood age. One consequence of this relationship is that adults accompanied by larger, older chicks processed prey of a given size more quickly than did adults accompanied by smaller, young chicks. When large prey were available, adults could have foraged more optimally if they had passed those prey to one large chick and resumed foraging, rather than partitioning the prey among smaller, less capable chicks. To summarize, size hierarchies among grebe young can affect food allocation within the brood, especially when food delivery rates are low. Perhaps during times of food shortage, adults intervene less in food allocation because they spend more time foraging. We suggest that parental regulation, through brood division and aggression directed at chicks, was operative over most of the brood-rearing period, and was likely more important than size hierarchies among chicks in determining food distribution. Parental regulation may enable parents to raise as many young as possible, even during times when food is not in shortage. Also, other factors such as chick signals of hunger status are likely to interact with size hierarchies in determining food allocation. Whether the pattern of size differences between siblings, initially produced by hatching asynchrony, remains unchanged throughout brood development has yet to be tested for Piedbilled Grebes. Nevertheless, size disparities among Pied-billed Grebe siblings were present throughout brood development, and may have been advantageous by allowing optimal allocation of parental investment among chicks.

Acknowledgments This research was funded through Natural Sciences and Engineering Research Council of Canada support to C.D. A. and grants in aid of research from the Sigma Xi Foundation to M.R.L.F. We are especially grateful for the logistic support of the Delta Waterfowl and Wetlands Research Station, and for permission from various landowners in the Minnedosa area to visit their ponds. We are indebted to R. Book and S. Lim for their artistic work in Figs. 1 and 2, respectively. T. Thomas and S. Lusk provided able field and technical assistance, while S. L. Johnson and M. W. Austen helped with proofreading. Finally, two anonymous reviewers provided some helpful comments which we incorporated into this paper. ABRAHAM, D. M. 1986. Observations on the breeding biology of Sabine's Gulls (Xema sabini). Can. J. Zool. 64: 898-903. T. W., ROHWER, F. C., and ARMSTRONG, T. W. 1987. Egg ARNOLD, viability, nest predation and the adaptive significance of clutch size in prairie ducks. Am. Nat. In press. R. M. 1986. Brood reduction in the American CASH,K. J., and EVANS, White Pelican, Pelecanus erythrorhynchos. Behav. Ecol. Sociobiol. 18:413-418. CHABRECK, R. H. 1963. Breeding habits of the Pied-billed Grebe in an impounded coastal marsh in Louisiana. Auk, 80: 447-452.

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