Two-egg Clutches Research Articles

Geographic and Local Variation in Nesting Phenology and Clutch Size of the Black Oystercatcher

The Black Oystercatcher (Haematopus bachmani) is a year-round resident with an extensive range from southern California to Alaska. Because of the size of this range (about 28 of latitude) and its simple, nearly linear shape, we hypothesized that the species would exhibit adaptive geographic variation in nesting phenology and clutch size. Museum egg collections provided information for the entire nesting range, while field observations on about 40 breeding pairs on Cleland Island, British Columbia, provided information on local variation. Nesting in Alaska starts about 15 days later than in southern California but trends in between are irregular. Birds on the exposed outer coasts of British Columbia and Washington start to nest about a week later than those in the San Juan Islands and Strait of Juan de Fuca. On Cleland Island the onset and duration of breeding. were similar in 1982 and 1983, and individual females laid similar-sized clutches at similar times in both years. There is no measurable geographic variation in duration of nesting or in clutch size. Clutch size averages 2.4 across the nesting range and was about 2.1 on Cleland Island from 1970 to 1983. Initial and replacement clutches on Cleland Island are similar in size but, in museum samples from throughout the range, three-egg clutches were collected later than two-egg clutches. The weak geographic trends in timing of breeding and their absence in clutch size were unexpected. Our findings may reflect (1) moderate seasonal and latitudinal fluctuations in the species' year-round coastal marine environment and (2) clutch size that is limited primarily by the optimal working capacity of parents. Research topics that can explore these possibilities include: proximate influences on breeding; heritability and individuality of breeding attributes; breeding energetics; and spatiotemporal variation in trophic ecology.

Evolution of Obligate Siblicide in Boobies. 1. A Test of the Insurance-Egg Hypothesis

Obligate siblicide, the unconditional killing of an individual by its sibling, occurs in a small number of bird species. Although clutches of two eggs are frequently laid, obligate siblicide eliminates the younger hatchling before it reaches the age of independence. The phenomenon presents a challenge to adaptationist evolutionary theory because parents produce chicks that are virtually certain to be killed and because surviving offspring actively sacrifice the inclusive-fitness increment represented by the victim. Dorward's (1962) insurance-egg hypothesis explains the production of a second egg as insurance against the first egg's failure. The insurance-egg hypothesis predicts that, if a second egg sometimes produces a fledgling after a first egg fails, then the evolution of insurance-egg production in single-chick species is governed by the ratio of the benefit of the increased probability of producing a hatchling to the cost of egg production. A field study of the obligately siblicidal masked booby (Sula dactylatra) demonstrated that second eggs contribute a surviving hatchling after the first egg's failure in 19.2% of two-egg clutches. The primary source of hatching failure was exceptionally high infertility or early embryonic death. Not all single-chick booby species produce insurance eggs, however, and the prediction of the insurance-egg hypothesis that the occurrence of insurance eggs is associated with the high benefit-to-cost ratio was supported by a compilation of booby breeding parameters. Because females make similar relative investments in each egg, the cost of insurance eggs is assumed to be constant across species, but benefits differ: hatching success in obligately siblicidal species ranges from 51% to 61%, whereas that in single-egg species is at least 85%.

A field study of the breeding biology of the little spotted kiwi (Apteryx owenii) with emphasis on the causes of nest failures

Abstract The breeding biology of the endangered little spotted kiwi Apteryx owenii was studied over a period of seven years on Kapiti Island, New Zealand, Breeding began in spring, with some laying continuing into summer and autumn, Some behavioural displays between members of a pair were recorded. Twenty five of the 31 nests found were underground in burrows dug by the kiwis or in natural cavities. The remainder were in logs or thickets of vegetation. The ratio of one to two-egg clutches was estimated to be 7:1. There was little or no nest preparation before laying, but the incubating birds covered the entrances of the more conspicuous nests with leaf litter. Only the male incubated, and he left the nest unattended for one or two days at a time in the first ten days after laying and for up to six hours at night during full-time incubation. I consider that cooling of the egg is a reason for the long incubation period of between 63 and 76 days. Chicks were not seen to be fed by their parents and left the n...

Research Note: Ovarian Follicular Growth and Maturation in the Domestic Pigeon and Guinea Fowl (Numida meleagris)

Yolks of pigeon and guinea fowl eggs were examined for the number of dichromate-staining rings. Estimates of the time for rapid follicular development using this technique were 5.1 and 5.6 days for pigeons and guinea fowl, respectively. Another measure of follicular development based on the incorporation of Sudan dyes into the yolk yielded an estimate of 6.5 and 8.3 days for pigeons and guinea fowl, respectively. Using one and two-egg clutches, no differences due to clutch position were observed in either species. There was a significant difference (P≤.01) between results obtained with the two techniques for measuring the period of rapid yolk deposition. The dichromate technique can give valuable data relative to yolk deposition, especially in species where it is impractical to daily catch and administer Sudan dyes. But the tendency to underestimate the period of follicular growth means that caution must be exercised in comparing results obtained with that technique and the more labor-intensive Sudan dye incorporation method.The time from the end of yolk deposition until oviposition is defined as the rest period. This interval was 1.5 days for pigeons and 1.7 days for guinea fowl. This is considerably longer than the rest period of Coturnix quail but similar to that found in turkeys and chickens.

What Limits Mourning Doves to a Clutch of Two Eggs?

To examine factors that possibly limit clutch size in Mourning Doves (Zenaida macroura), we exchanged eggs of equal age among nests to create clutches of one and three eggs. We compared the breeding success of parents rearing these clutches to that of parents rearing natural, two-egg clutches. Adults were fully capable of incubating three-egg clutches and brooding three nestlings. Nestlings in enlarged broods grew more slowly than those in control (two-young) broods during the crop-milk stage and generally had less full crops throughout the nestling period. Enlarged broods took 1.3 days longer to fledge. After reaching the growth asymptote, nestlings in enlarged broods had normal carpometacarpus lengths but weighed 26% less than those in control broods. Parents of enlarged broods fledged an estimated 30% more offspring than those of control broods. However, low postfledging survival may negate the advantage to parents rearing three offspring per nest.

Sex ratios of hatchling Mourning Doves

The sex of 306 Mourning Dove (Zenaida macroura) chicks from 153 two-egg clutches laid during March–August 1984 was determined by gonadal inspection. Sex of offspring was unrelated to egg sequence over the entire breeding season. There was, however, a seasonal effect on the sex versus egg sequence pattern, particularly in broods with both sexes: during the middle of the breeding season males predominated in first eggs and females in second eggs but this pattern was reversed late in the season. All other comparisons, i.e., overall sex ratio, seasonal changes in sex ratio, binomial distribution of family types, and the relation between egg size and offspring sex, were nonsignificant.

Intraclutch egg-weight apportionment and chick survival in Caspian terns

Egg-weight apportionment patterns and chick survival were investigated in two-egg clutches of Caspian terns(Sterna caspia) in the Great Lakes. First-laid eggs (A eggs) were typically heavier than second-laid eggs (B eggs) and hatched an average of 1.8 days earlier. The A-egg fraction of total clutch weight increased with total clutch weight. Increased egg weight did not increase the probability of hatching, but did increase the fledging success of A chicks: specifically, A eggs that produced fledglings were significantly heavier than those whose chicks did not fledge. No such relationship held for B eggs. The main effect of B-egg weight was in prolonging life: excluding those few that fledged, egg weight was correlated significantly with age at mortality. Chick mortality appeared to be due mainly to starvation and gull predation. We suspect that greater body size and (or) behaviour associated with body size helps chicks escape risk from gape-limited predators. The hatching of A eggs had a negative effect on the survival of B chicks, but the reverse was not true. B offspring may act as insurance against the loss of A siblings. The potential benefit to parents of investing more in B eggs appears to be constrained by sibling competition with A chicks, which often results in brood reduction. The pattern of egg-weight apportionment in eggs is interpreted as a parental response to the differences in the reproductive values of asynchronously hatching chicks.

A POPULATION STUDY OF TAWNY EAGLES IN THE HWANGE NATIONAL PARK, ZIMBABWE

Summary Hustler, K. & Howells, W. W. 1986. A population study of Tawny Eagles in the Hwange National Park, Zimbabwe. Ostrich 57: 101–106. Tawny Eagle Aquila rapax breeding success has been monitored in the Hwange National Park, Zimbabwe from 1973–1984. There were 1044 breeding pair years with 640 chicks reared; an average of 0,61 chicks/breeding paid year. The average number of pairs breeding each year was 72,4%. Thirty-one percent of all breeding attempts failed; most during nest-building (59%). Rainfall in the latter part of the rainy season appeared to influence early and late laying pairs. Two-egg clutches were most common (76%) and 65% of all one-egg clutches laid failed to produce chicks. Several factors which may influence breeding success are discussed.

Effect of Light Quality on Erratic Egg Laying of Caged Turkey Hens

Abstract The effect of light quality on erratic oviposition (two eggs laid during a 24-hr-period) and the shell quality of erratically laid eggs of turkey hens in cages during 20 weeks of production were examined. Hens were exposed to one of the following sources of light: blue, green, red, or incandescant for 16 hr of light per day (16L:8D). The intensity of light was equalized among treatments on the basis of equal number of photons (3.0 × 1014/cm2 /sec). The frequency of erratic oviposition was similar for all light treatments, ranging between 13.4 to 15.3% of total eggs laid, and most occurred in two-egg clutches. With erratic ovipositions, the first egg was laid during the daily dark period, whereas the second egg was laid in the daily light period, 11.6 to 13.0 hr later. Oviposition time and shell quality were significantly influenced by light treatment. As wavelength increased, shell weight of the erratically laid eggs decreased. Thus, shell quality and the timing of oviposition of erratically laid eggs may be influenced by light color.

Nesting Habitat and Reproductive Characteristics of the Lizard Cnemidophorus sexlineatus (Lacertilia: Teiidae)

Nesting habitat and egg-laying sites of Cnemidophorus sexlineatus were examined for 8 years (1973-1980). Egg clutches (N = 195) were collected from habitats along secondary highways and rural dirt roads in 10 states. Nest construction showed little intraspecific or geographic variation in design or complexity. Nesting sites are characteristically in well-drained, red clay exposures that face S to W. Preferred nesting grounds were near activity burrows. Reproductive characteristics of Cnemidophorus sexlineatus were determined from egg clutches (N = 98) and from autopsied females (N = 90) collected from the Interior Highlands Region in 1973, 1975 and 1980. In 1973 and 1975, females emerged from brumation in early May with regressed ovaries. Follicular enlargement began soon thereafter and occurred sooner in large adult females than in small adults or yearlings. Clutch size (including previously published data) averaged 3.19 (N = 401). A significant positive correlation (r = .69) existed between clutch size (based on counts of oviductal eggs) and snout-vent length. Mean clutch volumes of recently oviposited eggs (< 650 mm3) are not significantly greater in larger clutches (four-egg clutches = ca. 630 mm3) than in smaller ones (two-egg clutches = ca. 600 mm3). Most eggs showed better than a twofold increase in volume during incubation, and two-egg clutches exhibited the greatest range of increase (493.0 to 1376.6 mm3). Two clutches of eggs are commonly produced by adult females, whereas two clutches are infrequent among yearlings. A summer drought in 1980 in the Interior Highlands Region resulted in a dramatic seasonal shift in clutch size. In 1980, two-egg clutches represented 36.5% (27 of 74) of all egg clutches, whereas in a year of higher than average rainfall (1975), two-egg clutches represented 7.4% (2 of 28) of all clutches. The production of significantly smaller clutch sizes (presumably a result of low food resources) agrees with previous studies on the effects of proximal variation in environmental variables on lizard

Brood Reduction in Black-Legged Kittiwakes

Abstract In 1978 and 1979 we studied brood reduction in Black-legged Kittiwakes (Rissa tridactyla) on St. Paul Island, Alaska. In two-egg clutches, first-laid eggs were largest; they were incubated before second eggs were laid and consequently hatched first. Chicks from first-laid eggs begged more frequently, fed more frequently, grew faster, and were more aggressive than their sibs. These sibling differences frequently facilitated the ejection of younger siblings from the nest by first-hatched young. Lower feeding and growth rates and higher aggression levels occurred in broods in which the second chicks subsequently died than in broods in which both chicks fledged. Chick loss was most frequent during inclement weather. These data are consistent with the hypothesis that brood reduction is related to the amount of food that chicks receive.

ASPECTS OF THE BREEDING BIOLOGY OF THE AFRICAN BLACK OYSTERCATCHER

Summary Hockey, P. A. R. 1983. Aspects of the breeding biology of the African Black Oystercatcher. Ostrich 54:26-35. Fifty-five pairs of African Black Oystercatchers Haematopus moquini bred at Marcus Island in 1979–1980. Sixteen pairs laid replacement clutches: the mean interval between loss of the fist clutch and laying of a replacement clutch was 22,2 days. Mean inter-nest distance was 19,4 m. The modal clutch size was 2, with a mean of 1.74. Mean dimensions of 105 eggs were 60,7 × 40,l mm and mean fresh egg mass was 55,s g. There were differences in egg mass and dimensions between eggs in one- and two-egg clutches. Rate of egg loss was high, due mainly to depredation by Kelp Gulls Larus dominicanus promoted by human disturbance. Fledging success was lower at a disturbed site than at undisturbed sites, with highest chick mortality occurring in the first week of life. Mortality of first-year birds of eight days and older was estimated at 48% and 69% in two successive seasons. All juvenile birds dispersed...

The influence of age on laying date, clutch size, and egg size of the white-fronted tern,Sterna striata

Abstract The age at first breeding and the influence of age on laying date, clutch size, and egg size of white-fronted terns were studied at the Kaikoura Peninsula, New Zealand, between 1971 and 1976. Of the 134 banded birds recovered breeding at Kaikoura, 63% were marked as nestlings at Kaikoura; the remainder came from colonies within 104 km. The bird is extremely capricious in its choice of nesting locality, and there are indications that it is not consistently philopatric. A small number bred as 3-year-olds, but the majority did not commence breeding until after they were 6 years old. Most pairs (73%) were of partners with an age difference of 1 year or less. Laying date and egg size varied with the age of the parent, but clutch size showed no significant change in relation to age. Mean egg volume did not vary between one-egg and two-egg clutches, but in two-egg clutches the first egg laid was significantly the larger in length, breadth, and volume. Single-egg clutches were the most common, but as the...

Reproductive Behavior and Male Mating Success in a Neotropical Toad, Bufo typhonius

Bufo typhonius on Barro Colorado Island, Canal Zone, had an explosive breeding system, with most choruses lasting less than one day and all oviposition in a chorus occurring in less than 10 hours. Relatively permanent streams and pools were used for breeding. Several choruses occurred at each site during the four-month breeding season. Some females laid twoegg clutches in six weeks, and some males participated in more than one chorus. Male mating success was determined by direct male-male competition for individual females. Most males less than 38 mm long did not obtain mates. For males over 38 mm, the probability of mating did not increase significantly with increasing size. There was no correlation between male and female size in amplexed pairs. ANIMAL MATING SYSTEMS are molded by ecological factors, particularly temporal patterns of reproduction and the spatial and temporal distribution of limited resources (Trivers 1972, Emlen and Oring 1977). In anuran amphibians the temporal availability of receptive females is an important determinant of the form and intensity of male-male competition and the relative mating success of individual males (Emlen 1976, Wells 1977b). Females of species with prolonged breeding seasons are available on an asynchronous schedule, so each male competes for many females. Females can choose mates from among the available males, and males may influence their choice by defending oviposition sites or other resources needed by females (Wells 1977a, 1977b; Howard 1978). In species with very short breeding periods, many females arrive at the breeding site simultaneously. Males actively search for mates and attempt to displace males already in amplexus. Opportunities for mate selection by females are reduced, so there is little advantage to males in defending resources attractive to females. Because female availability is restricted in time, and because so many are in amplexus simultaneously, it should be difficult for a small number of males to monopolize a large proportion of the females. Hence the variance in male mating success should be lower in explosive breeders than in prolonged breeders (Trivers 1972, Emlen 1976, Wells 1977b, Emlen and Oring 1977). Data now available for two temperate zone ranid frogs, Rana clamitans and R. catesbejana, show that matings are heavily skewed in favor of large males, with a small percentage of males monopolizing a major proportion of the females (Wells 1977a; Howard 1978). Comparable data for explosive breeders are not available, although several recent papers discussed factors that might influence male mating success in these species (Licht 1976, Davies and Halliday 1977, Wells 1977b). In this paper I report relative mating success of individual males in a population of Bufo typhonius, a neotropical toad with an explosive breeding system. I also describe the general reproductive behavior of this species and speculate on ecological factors that favor explosive breeding.

The breeding biology of two Lake Erie herring gull colonies.

The breeding biology of two Lake Erie herring gull colonies was studied from 1973 to 1976, emphasizing effects of clutch size and time of clutch initiation on reproductive success. In 1976, incubation attention of two-egg and three-egg clutches started early and late in the season was measured with a 20-pen event recorder. Chlorinated hydrocarbon residues in eggs were assessed in 2 years. Nest density was greater, clutch initiation more synchronized, and hatching success higher at one of the colonies. Hatching success and fledging success were independent of clutch size bin early nesters were more successful than late nesters. Differences in hatching success between two-egg and three-egg clutches were a function of time of clutch initiation such that the clutch size with the greater proportion of its nests in the early period had a higher hatching success. The reproductive success of the Lake Erie colonies was intermediate among rates reported for other Great Lakes colonies but below those of most eastern North American or European colonies. There were no significant differences in the incubation attention between two-egg clutches and three-egg clutches or between early and late three-egg clutches. Most clutches were incubated greater than 95% of the time although clutches incubated less than 75% realized the same hatching success.

Feeding ecology of the ancient and marbled murrelets near Langara Island, British Columbia

Two field seasons (1970 and 1971) were spent studying the feeding ecology of the ancient murrelet (Synthliboramphus antiquus) and the marbled murrelet (Brachyramphus marmoratus) near Langara Island, Queen Charlotte Islands, British Columbia. The relatively offshore-feeding ancient murrelet nests in colonies, lays two-egg clutches, and has truly precocial young. The inshore-feeding marbled murrelet is apparently a solitary nester, has one-egg clutches, and semiprecocial young. The ancient murrelet feeds predominantly on two species of euphausiid crustaceans, Euphausia pacifica and Thysanöessa spinifera, during the early portion of the terrestrial phase of its breeding cycle (early April to late May), but takes a fish, Ammodytes hexapterus, toward the end of the cycle. It is believed that this change in diet as the season progresses reflects a change in prey availability. The apparent patchy distribution of the prey of the ancient murrelet, which requires more time to locate the food source, has probably promoted the evolution of the 72-h incubation shifts found in this species.During the terrestrial phase of the marbled murrelet's breeding cycle (early May to late August), the fish Ammodytes hexapterus and Cymatogaster aggregata are predominantly taken. During the very early part of its breeding season, Thysanöessa spinifera constitutes a considerable part of its diet.

The Ecological Significance of Clutch Size in the South African Gannet (Sula capensis (Lichtenstein)

Lack (1954) suggests that clutch size in birds is regulated to enable the maximum possible number of chicks to survive to breeding age. In the South African gannet, Sula (bassana) capensis (Lichtenstein), the most dangerous phase of the life history is soon after birds fledge (leave the nesting area). These birds have to learn to fly and fish, apparently without aid from adults. Possibly because of this a high percentage die during the first few weeks of independence. If Lack is correct, it is probable that the body weight of chicks when they fledge has some relationship to their chances of post-fledging survival; evolutionary selection pressures will probably have acted to ensure that the maximum possible number of young leave the nest at the optimum weight for survival. One possible adaptation, that might aid chicks to survive after fledging, could be the regulation of clutch size to ensure that the chicks hatched could be reared to an optimum fledging weight. In addition, or alternatively, the fledging period (time spent in the nest) could be lengthened and chick's growth slowed down. This would spread the parent's feeding burden over a longer period and make the rearing of a chick less strenuous. The chick would probably benefit by receiving a greater total amount of food. If evolution had proceeded in a different direction, it could have led to parents feeding their young after fledging, until they learned to fish. However, at present, this would conflict with the juveniles' behaviour of migrating to the tropics as soon as they can fly. In this paper the chief problem investigated is the significance of the gannet's clutch size. This was investigated at Lambert's Bay, South Africa (320 05' S, 180 18' E) in the following ways. Firstly, I tried to see if birds ever laid two-egg clutches. Secondly, adults were tested to see if they could hatch two eggs and rear two young, and the breeding success of these birds was compared with that of birds with one egg. Thirdly, a comparison was made between the rates of development and the weights at fledging of chicks from single-chick broods and two-chick broods (twins). The twins had been experimentally created in most cases by donating a second egg of equal age to several marked nests; similar experiments had previously been undertaken by several workers with other species, including Nelson (1964) who worked with the North Atlantic gannet (S. (bassana) bassana). Fourthly, the post-fledging survival rates of juveniles were analysed to see which fledging weight seemed to be optimum. The relationship between date of fledging and post-fledging survival, was tested by analysing recovery and recapture data. Possible detrimental effects on parents rearing two chicks were also investigated.

TWO‐EGG CLUTCHES IN ALBATROSSES

TWO‐EGG CLUTCHES IN ALBATROSSES

Variations in Egg Characteristics of the House Wren

THERE are scattered records in the literature showing that the weights of consecutively laid fresh eggs or their measured lengths and maximum diameters may either decrease or increase in different species. In a statistical analysis of 20 clutches in the Laughing Gull (Larus atricilla), Preston and Preston (1953) showed that the length measurement did not differ significantly either among the eggs of the same clutch or between the eggs of different clutches. On the other hand, in three-egg clutches the third egg laid had a significantly smaller diameter than the first two. In two-egg clutches, the second egg appeared to resemble the third more than the second in the three-egg clutches, but the small number of measurements available did not permit definite conclusions. Since a large series of data had been collected at the Baldwin Bird Research Laboratory, near Cleveland, Ohio, between 1925 and 1938, on the eggs of the House Wren (Troglodytes aedon), we decided to determine whether or not there were any consistent changes in dimensions and weight with sequence of laying in this passerine species.