Decline In Egg Size Research Articles

Seasonality Predicts Egg Size Better Than Nesting Habitat in a Precocial Shorebird

Egg size represents a fundamental predictor of chick mass and body condition. Chicks from bigger eggs have significantly increased survival, especially in precocial species, where chicks must forage for themselves and cope with environmental threats, such as bad weather or predators. Therefore, our understanding of the factors influencing egg size is crucial both from the perspective of their breeding ecology as well as of their conservation. However, studies simultaneously addressing multiple factors and quantifying their influence on egg size in large samples are rare. Here, we test the effect of seasonality, clutch size and nesting habitat on egg size, measured as volume, in a ground-nesting shorebird, the Northern Lapwing Vanellus vanellus, using a sample of 4384 eggs from 1125 clutches in South Bohemia, Czech Republic, during the period between 1988 and 2018. We report a significant decline in egg size over the breeding season, on average bigger eggs in larger clutches with a significant difference between 2-egg and 4-egg clutches, and no direct effect of nesting habitat. From our review of the same predictors across 15 Northern Lapwing populations throughout Europe it is apparent that replacement or late clutches have on average 3–7% smaller eggs than first or early clutches. Nesting habitat only rarely affects egg size and there are no significant differences in egg size between 3-egg and 4-egg clutches. Earlier studies showed that chicks hatching from bigger eggs early in the breeding season performed better, and that there was higher food abundance available for chicks at that time. This fact, together with the documented seasonal decline in egg size, sends an important message to conservationists and policymakers that early breeding attempts may play a pivotal role in safeguarding shorebird breeding productivity.

Inter- and Intra-specific variation in egg size among reef fishes across the Isthmus of Panama

Effects of planktonic food supplies and temperature on pelagic fish larvae are thought to be the primary environmental determinants of adaptive variation in egg size. Differences between the Atlantic and Pacific coasts of Panama in primary production (higher in the Pacific due to upwelling) and temperature (less seasonal in the non-upwelling Caribbean) allow testing such ideas. We compared the volumes, dry weights and energy content of eggs of 24 species of reef fishes from the two sides of the isthmus during the cool and warm seasons. Both egg volume and egg dry weight were good predictors of egg energy content among species, although not within species. Caribbean species produced larger eggs than their close relatives in the Pacific. In the Pacific, eggs were significantly larger during the cool upwelling season than during the warm, non-upwelling period, with a similar but weaker seasonal pattern evident in the Caribbean. The production of larger eggs in the low-productivity Caribbean is consistent with the hypothesis that species produce larger eggs and offspring when larval food supplies are lower. Parallel patterns of seasonal variation in eggs size and the greater strength of that relationship in the Pacific indicate that temperature drives seasonal variation in egg size within species. The decline in egg size with increasing temperature, a general pattern among ectotherms, may be a physiological side-effect, due to differing effects of temperature on various metabolic processes during oogenesis or on hormones that influence growth and reproduction. Alternatively, the seasonal pattern may be adaptive in these fishes, by affecting larval performance or maintaining a particular timeline of major events during embryonic development.

Long-term decline in egg size of Atlantic puffins Fratercula arctica is related to changes in forage fish stocks and climate conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 457:1-10 (2012) - DOI: https://doi.org/10.3354/meps09813 FEATURE ARTICLE Long-term decline in egg size of Atlantic puffins Fratercula arctica is related to changes in forage fish stocks and climate conditions Robert T. Barrett1,*, Erlend B. Nilsen2, Tycho Anker-Nilssen2 1Department of Natural Sciences, Tromsø University Museum, 9037 Tromsø, Norway 2Norwegian Institute for Nature Research (NINA), PO Box 5685 Sluppen, 7485 Trondheim, Norway *Email: rob.barrett@uit.no ABSTRACT: Due to major shifts in the marine ecosystem, many seabirds in the NE Atlantic have experienced short- and long-term breeding failures and population changes. One such seabird is the Atlantic puffin Fratercula arctica, the populations of which have declined in Norway at an annual rate of 2% over a ~30 yr period. Parallel to this decline, we found a significant decline in egg volume at 2 widely separated Norwegian colonies situated in different marine ecoregions, a decline that is unprecedented among auks. In birds, egg production is costly for the female and varies in response to the amount of energy available for reproduction. Whereas proximate factors determining egg size are largely unknown, food availability prior to egg-laying is considered important. Using generalised least-square models, we demonstrate that the declines in egg size were driven by inter-annual changes in abundances of capelin Mallotus villosus and early life stages of herring Clupea harengus and haddock Melanogrammus aeglefinus, all of which are important prey of Atlantic puffins in these colonies. Furthermore, changes in climatic variables, such as sea temperature and the North Atlantic Oscillation, suggest that other prey may also have played a role in determining puffin egg size. The decline in egg size may reduce the quality of future breeders and hence have negative effects on puffin populations. KEY WORDS: Atlantic puffin · Egg size · Fish stocks · Climate Full text in pdf format Information about this Feature Article Supplementary material NextCite this article as: Barrett RT, Nilsen EB, Anker-Nilssen T (2012) Long-term decline in egg size of Atlantic puffins Fratercula arctica is related to changes in forage fish stocks and climate conditions. Mar Ecol Prog Ser 457:1-10. https://doi.org/10.3354/meps09813 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 457. Online publication date: June 21, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Seasonal and annual variation in Chilean hake Merluccius gayi spawning locations and egg size off central Chile

Variability in Chilean hake reproductive tactics off central Chile was assessed by analyzing ichthyoplankton samples from nine oceanographic cruises (1996–2005) and through experimental trials with early life stages (eggs, yolk-sac larvae) during the main (austral spring) and secondary (late summer–early autumn) spawning seasons. Abundant eggs in the plankton (1300–2000 eggs per 10 m 2) and historical adult reproductive data showed the highest reproductive activity in austral spring, with large egg aggregations near shelf break (50–100 m depth). Large, recently spawned eggs (1.15–1.20 mm diameter) were advected nearshore by coastward subsurface flows in the spring upwelling season. Experimental trials indicated that recently hatched larvae (3.4–3.5 mm) consumed their yolk-sac (0.17–0.41 mm 3) in 3–4 days at 10–12 °C; plankton sampling indicated that larval hake remained at mid-depth (50–100 m) without showing daily vertical migrations until completing their caudal fin formation (∼15 mm). During the secondary reproductive peak, hake spawned nearshore, when smaller eggs (0.95–1.13 mm) and recently hatched larvae (2.2–2.6 mm notochord length) occurred in surface waters (0–10 m depth). Their relatively large yolk-sac volumes (0.57 ± 0.11 mm 3) provided endogenous nourishment for at least 5 days at 10 °C, according to experiments. In the field, preflexion larvae occurred mainly in the mixed layer (0–25 m) and started ontogenetic daily vertical migrations at 7 mm. A strong decline occurred after 2002 in the adult Chilean hake biomass (estimated by hydroacoustic surveys) and body size, coinciding with variations in spawning locations (more coastward in early spring 2004 and 2005) and decline in egg size. Thus, recent variations in Chilean hake reproductive tactics may reflect an indirect effect of declines in the parental population size.

Interaction between flight, reproductive development and oviposition in the pine weevil Hylobius abietis

1 The development of reproductive and flight capacity of pine weevils Hylobius abietis during the spring and their dispersal to, and subsequent development at, new clearfell oviposition sites comprise key phases in their life cycle in managed forests. At an old clearfell site where autumn-emerging weevils had overwintered, weevils were trapped as they re-emerged in the spring and tested for their ability to fly and then dissected to determine the degree of wing muscle and egg development. 2 Re-emerging weevils were most abundant in pine growing at the edge of the clearfell and, over most of the trapping period (April to June), their capacity for flight (proportion flying and wing muscle width) was more advanced than in weevils from the clearfell itself, with a similar trend in the degree of reproductive development (proportion with mature eggs and egg volume). 3 In weevils from the clearfell, flight capacity and reproductive development increased concurrently to a peak around mid-May. In weevils from pine, wing muscles were already well developed at the start of trapping, although few of them flew. Their more advanced development was attributed to the increased opportunities for maturation feeding after emergence in the previous autumn. 4 In the spring, weevils reached the canopy of trees for maturation feeding by walking and, to a lesser extent, by flight. Weevils dispersed by flight to oviposition sites in mid-May when most of them were reproductively mature. After arrival, flight ability and wing muscle size declined rapidly but egg production was maintained until most weevils had stopped flying. When wing muscles reached their minimum size, there was a marked decline in egg size, suggesting that wing muscle breakdown is important in maintaining egg production at oviposition sites. Prospects for further wing muscle and reproductive development are discussed.

Revisiting Trends in the Evolution of Egg Size in Hatchery‐Enhanced Populations of Chinook Salmon from British Columbia

AbstractHatchery enhancement has been reported to result in an increase in egg size in coho salmon Oncorhynchus kisutch and a decline in egg size in Chinook salmon O. tshawytscha. Egg size may be directly influenced by selection, a larger egg size evolving as a consequence of hatchery incubation. Alternatively, a smaller egg size could evolve as a correlated response to fecundity selection, and a unidirectional change in egg size over time may reflect selection and an underlying genetic change in the population. To address this question, temporal trends in egg size were investigated for two hatchery‐enhanced populations of Chinook salmon from Vancouver Island, British Columbia. After the effect of female length variation was removed by standardizing egg sizes to a female of common length (the overall mean for each population), there was no temporal trend in egg size from the 1970s to 2008 for any of the hatchery‐enhanced populations evaluated. These results do not support a previous report of genetically based declines in egg size in hatchery‐enhanced Chinook salmon populations from this region.

Adaptive aspects of intraclutch egg-size variation in the High Arctic barnacle goose (Branta leucopsis)

The pattern of intraclutch egg-size variation in barnacle goose (Branta leucopsis) clutches and its adaptive implications was studied in Svalbard, Norway, from 1989 to 1998. Egg size was measured in relation to laying sequence, egg predation and hatching order were recorded to determine hatching success of eggs in different laying sequences, and the time when incubation started was examined. Egg size showed a rather consistent pattern, with a large second-laid egg and declining egg size for the remainder of the clutch. The first-laid egg was usually smaller than the second one, except in clutches with two and three eggs. Predation was highest for the first-laid egg, and last-laid eggs hatched last in most cases, although only one last-laid egg was abandoned. Four of six females started incubation before clutch completion. Both the "nutrient-allocation hypothesis" as well as the "early incubation start hypothesis" may contribute to explaining the expressed pattern of intraclutch egg-size variation. The fitness gains due to allocating fewer nutrients to eggs in unfavourable positions in the laying sequence may explain the small size of the first egg, whereas the multiple benefits of an early incubation start may have led to the decline in egg size later in the laying sequence as a mechanism to counteract hatching asynchrony.

The mammary gland and its origin during synapsid evolution.

Lactation appears to be an ancient reproductive trait that predates the origin of mammals. The synapsid branch of the amniote tree that separated from other taxa in the Pennsylvanian (>310 million years ago) evolved a glandular rather than scaled integument. Repeated radiations of synapsids produced a gradual accrual of mammalian features. The mammary gland apparently derives from an ancestral apocrine-like gland that was associated with hair follicles. This association is retained by monotreme mammary glands and is evident as vestigial mammary hair during early ontogenetic development of marsupials. The dense cluster of mammo-pilo-sebaceous units that open onto a nipple-less mammary patch in monotremes may reflect a structure that evolved to provide moisture and other constituents to permeable eggs. Mammary patch secretions were coopted to provide nutrients to hatchlings, but some constituents including lactose may have been secreted by ancestral apocrine-like glands in early synapsids. Advanced Triassic therapsids, such as cynodonts, almost certainly secreted complex, nutrient-rich milk, allowing a progressive decline in egg size and an increasingly altricial state of the young at hatching. This is indicated by the very small body size, presence of epipubic bones, and limited tooth replacement in advanced cynodonts and early mammaliaforms. Nipples that arose from the mammary patch rendered mammary hairs obsolete, while placental structures have allowed lactation to be truncated in living eutherians.

The Establishment of a New Brown Noddy Anous stolidus Breeding Colony off South-Western Australia

SummaryA new breeding colony of the Brown Noddy, which started on Lancelin Island in south-western Australia around 1991, expanded exponentially between 1994–95 and 1996–97. This growth in the number of breeding pairs resulted from the immigration of noddies from another breeding area, probably the Houtman Abrolhos 275 km to the north. In the 1996–97 breeding season there were two distinct waves of laying. The initial laying period, which had occurred in the previous two seasons, peaked in early November and was more synchronous than the second one. The second wave peaked in early December and resulted from laying by new arrivals with probably some re-laying from earlier losses. There was a significant reduction in clutch volume after the first 15 days of laying. Factors which may have produced a decline in egg size later in the season are discussed.

The Role of Female Age in Determining Egg Size and Laying Date of Thick-Billed Murres

In many bird species, egg and/or clutch size increases with female age, and declines with laying date. The declines with date have been considered time-saving adaptations to seasonal environments. At Coats Island, N.W.T., Canada in 1994 and 1995, we examined the relationships among female age, laying date and egg size in the Thick-billed Murre Uria lomvia, an Arctic seabird that lays a one-egg clutch. In particular, we tested a model that views the seasonal decline in egg size in this species as a time-saving adaptation: the model predicts that, irrespective of their age, late-laying birds will reduce egg size. Laying dates became earlier with increasing female age to 8 years, because young birds initiated egg formation late. Egg size increased with age to 8 years, possibly because young birds deposited yolk at a slow rate. Individual effects explained much of the variation in laying date and egg size. Egg size declined with laying date in a random sample of eggs laid by birds of unknown age. However, egg size was independent of laying date among older birds. In contrast, egg size declined with laying date among young birds, but this was an effect of age, not date. Young birds that laid at the same time as older birds laid smaller eggs. These results indicate that the time-saving model does not adequately explain the seasonal decline in egg size in Thick-billed Murres. We suggest that characteristics of individual Thick-billed Murres, largely mediated by age (or experience) during early reproductive attempts, determine the size of egg they lay.

Intraclutch egg‐size variation in the Barnacle Goose Branta leucopsis: an egg‐removal experiment

We carried out an egg‐removal experiment on Barnacle Geese Branta leucopsis to test the hypothesis that tactile contact between the brood patch and eggs accumulating in the nest during laying was causally related to intraclutch egg‐size variation, acting through the development of incubation behaviour. Egg removal had no effect on mean egg mass, relaying interval or clutch size: mean (±s.e.) clutch sizes for first clutches were 4.9 ± 0.9 and 4.7 ± 0.9 eggs for control and experimental females, respectively. We therefore conclude that Barnacle Geese have a determinate pattern of egg laying. Daily removal of all eggs from the first‐laid egg did not affect the within‐clutch pattern of egg‐size variation: in both control and experimental clutches the first egg was relatively small, the second or third egg was largest and there was a linear decline in egg size to the smallest, last‐laid egg. There was no significant difference in either the absolute or relative size of the last‐laid eggs in control and experimental clutches. Tactile contact with eggs in the nest, therefore, is not required for the expression of intraclutch egg‐size variation in Barnacle Geese. In this respect, determinate‐laying Barnacle Geese differ from species with indeterminate laying patterns (e.g. gulls) where egg removal does affect the normal pattern of within‐clutch egg‐size variation. If hormonal changes (e.g. prolactin) associated with onset of incubation are causally related to the intraclutch decrease in egg size, then the stimuli involved appear to differ between determinate and indeterminate layers.

Effect of spawning fast on lipid and fatty acid composition of eggs of captive turbot ( Scophthalmus maximus L.)

Total, neutral and phospholipid levels were determined in cultivated turbot eggs from the beginning, middle and end of the spawning seasons of three females in order to assess if the spawning fast of mature females affects the lipid composition of eggs produced late in the season. Dry weight measurements and fatty acid analyses were also carried out. There were no obvious variations in water content or in the percentage concentrations of neutral or phospholipids in early- and late-season eggs. Percentage dry weight of total lipid increased slightly in each female's late-season eggs, but only significantly so in those from one female ( P<0.05). However, the weight of total lipid per egg fell as the spawning season progressed because of a seasonal decline in egg size. There were significant increases in the levels of 18:3 n−3 ( P<0.05), 22:4 n−6 ( P<0.01) and 22:5 n−3 ( P<0.01) in the neutral lipid fraction of late-season eggs compared to early-season eggs. However, neutral 20:5 n−3 and 22:6 n−3 levels remained fairly constant throughout the season. Percentage concentrations of phospholipid 20:5 n−3 showed a significant decrease ( P<0.05) in late-season eggs, and levels of phospholipid 22:5 n−3 and 22:6 n−3 decreased in later egg batches, though not significantly. Experiments comparing the growth and survival of early- and late-season larvae produced by the same three turbot suggest that early-season larvae do not outperform their late-season counterparts under hatchery conditions.

Temporal and spatial variation in egg size of 13 Panamanian echinoids

Though egg size of marine invertebrates has received a great deal of theoretical attention, and though sea urchin eggs have been studied for nearly a century, practically no data on intraspecific variation of echinoid egg size exist. This study examines temporal variation in egg size of 10 species of regular and three species of irregular echinoids from either side of the Isthmus of Panama. Data on spatial variation in egg volume are also presented for eight of these species. In all species mean size of eggs from different females collected from the same locality on the same day were significantly different. Daily (within-month) variation in egg size was significant in three out of the five species for which sufficiently large samples were collected: Eucidaris tribuloides (Lamarck), Diadema antillarum Philippi, and Lytechinus variegatus (Lamarck). No significant differences between daily means of egg volume were evident in D. mexicanum A. Agassiz and in Echinometra viridis A. Agassiz. Monthly means of egg volume were significantly different in seven out of the 13 species studied: Lytechinus variegatus, L. williamsi Chesher, Tripneustes ventricosus (Lamarck), Echinometra viridis, E. vanbrunti A. Agassiz, Clypeaster rosaceus (Linnaeus), and C. subdepressus (Gray). Between-year variation was also significant in all of these species, except Lytechinus variegatus. All Caribbean species with significant monthly variation show a decline in egg size after September, but no obvious environmental fluctuation can be linked to this change. Echinometra lucunter (Linnaeus) showed no significant variation between months of the same year, but exhibited differences between years. No significant monthly or annual variation was observed in Eucidaris tribuloides, E. thouarsi (Valenciennes), Diadema antillarum, D. mexicanum, or Leodia sexiesperforata (Leske). When temporal variation at each locality was taken into account, only one species out of eight, Clypeaster subdepressus, showed spatial variation. Correlations between size of eggs collected at a particular time and the intensity of spawning by the population at that time were generally not significant, suggesting that size of mature eggs is not determined by reproductive state of the parental population. Correlations were also attempted (in three species of Echinometra) between egg size and body weight, gonadal content, and number of eggs carried by the mother. None of these variables was a good predictor of egg size. Only in E. viridis was there a significant correlation between egg size and gonadal content. Though its causes remain obscure, intraspecific variation in egg size found in these echinoids could be exploited to study the possible effects of this parameter on larval life history.

Should Egg Size and Clutch Size Decrease with Age?

We analyse a model in which females gather resources for eggs in a pre-reproductive phase, and are then, in the reproductive phase, subject to mortality during the production of successive clutches of offspring. We find that if clutch size is constrained, egg size should decline with maternal age. Where egg fitness shows diminishing returns with egg size, and maternal mortality is age-independent, the decline in egg size will be linear, and the rate of decline will be proportional to maternal mortality. All females with the same survival prospects follow the same trajectory of decline, but larger females enter the trajectory at a larger egg size. Similarly, if egg size is constrained, clutch size declines with maternal age, the rate of decline increases with maternal mortality, and larger females lay larger clutches initially. Support for the models is found in a wide variety of species, and in particular in a number of recent studies of butterflies.

Seasonal variation of egg size and number in a Daphnia pulex population

Seasonal variation of egg size and number was examined in a Daphnia pulex population inhabiting a vernal pond. In this population, size at maturity declines at midseason, probably as an adaptive response to size-selective predation by larvae of the salamander Ambystoma. The larger early season individuals produce more and larger eggs than the smaller late season individuals. Age at maturity does not vary between seasons. Laboratory experiments indicate that temperature may affect egg size, egg number and size at maturity. However, field data suggest that temperature accounts for only a small fraction of the total variation in egg size and number. Indirect measures of nutrition indicate that food limitation does not cause the seasonal decline in egg size and number. The seasonal change in reproductive traits is well correlated with changes in invertebrate and vertebrate predation. Examination of predator feeding preferences and their impact on Daphnia mortality indicate that variation of reproductive traits is most likely a complex adaptation to changing predation regimes.

The Adaptive Significance of Egg Size and Laying Date in Thick‐Billed Murres Uria Lomvia

The effect of egg size and laying date on the fledging mass of Thick—Billed Murre chicks was examined at Cape Hay, Bylot Island, and Coburg Island, Northwest Territories, Canada, to test the idea that late—laying females face a trade—off between the time of egg laying and egg size. There was a seasonal decline in egg volume, and egg volume was correlated with mass and size (wing length) of chicks soon after hatching. Partial correlations showed that larger eggs hatch into heavier, but not larger chicks. Chics hatched early in the season fledged at heaver masses than late—hatched chicks. Hatching mass contributed significantly to this effect, but other factors (e.g., energy intake) were probably also important. A model, based on a multiple regression of the effects of hatching mass and hatching date on fledging mass, shows the possible combinations of egg size and timing of breeding while maintaining a constant fledging mass. Data on the replacement interval and size of replacement eggs suggest that females that lay late may do better (in terms of fledging mass of their chick) to minimize the delay in laying by producing a small egg earlier, rather than delay laying further in order to produce a larger egg. This is because they are unable to accumulate nutrients at a fast enough rate to make a delay worthwhile. This suggests that the seasonal decline in egg size is an adaptive response to a seasonal deterioration in environmental conditions.

FACTORS AFFECTING BREEDING OF RAZORBILLS ALCA TORDA ON SKOKHOLM

SUMMARYA study of the breeding biology of the Razorbill was carried out on Skokholm (South Wales) during 1971‐73. Birds ringed or colour ringed before the study began provided additional information upon the effects of age on breeding. Mean laying date was delayed in 1972, compared with 1971; the effect is attributed chiefly to stormy weather which upset colony attendance. Eggs were also smaller in 1972. A seasonal decline in egg size (volume) was noted in all three years, attributed mainly to the later laying of young birds. Egg size increased with age, at least up to the fifteenth year. Eggs lost totalled 30% of those laid; 73% of this total was due to predation by Herring Gulls and of Jackdaws. Most losses (45%) occurred during the first 10 days after laying. Of lost eggs, 25% were replaced, usually 14 days after the loss of the original; only eggs laid and lost early in the season could be replaced.Only 7% of the chicks which hatched failed to fledge. Most (62.5%) chick losses occurred in the first week of nestling life, when chick weight was related to egg size. Afterwards, both growth rate and fledging weight were independent of egg size. The chicks fledging early in the season were heavier than later chicks. Failure to fledge was mainly due to a breakdown in behaviour between parent and young, rather than to predation. Breeding success was highest for birds breeding early in the season, most of which were older, more experienced breeders. These laid early enough to replace an egg if it was lost; they produced large eggs, and their chicks were therefore both heavier than average during the critical first 7–10 days of life, and fledged at a high weight. Thus experience accumulated with age, and the ability to lay early in the season are important for successful breeding in the Razorbill.

FACTORS AFFECTING THE EGG SIZE OF RED‐BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

SUMMARYThe factors influencing the egg size of the Red‐billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two‐ and three‐egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one‐, two‐ and three‐egg clutches declined significantly as the season progressed. The size of eggs from single‐egg clutches tended to be smaller than eggs from two‐egg clutches laid at the same time.There were correlations between the proportions of one‐egg and of three‐egg clutches being laid at a given period and the mean egg volume of two‐egg clutches. When the mean egg volume of two‐egg clutches increased there was a corresponding increase in the proportion of two‐ and three‐egg clutches laid. When the mean egg volume of two‐egg clutches decreased there was an increase in the proportion of single‐egg clutches laid.The egg size of the Red‐billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time.Two‐year‐old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females.It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.

Spawning Time and Egg Size of Atlantic Mackerel, Scomber scombrus, in Relation to the Plankton

The mean date of peak spawning of Atlantic mackerel (Scomber scombrus) in the southwestern Gulf of St. Lawrence is July 1 ± 1 wk. This coincides with the maximum abundance of the summer plankton. Mackerel eggs average 1.3 mm in diameter at the beginning of the spawning season in June and decrease steadily to 1.1 mm by mid-August. There is a corresponding trend in the mean size of the plankton, which reach a minimum length in midsummer, and a maximum in winter. This recurring size cycle is as important as the seasonal change in plankton biomass in determining the abundance of suitable food organisms for newly hatched larvae. In an evolutionary sense, the observed reproductive characteristics of mackerel ought to be adaptive since the accurate timing of spawning and the progressive decline in egg size will tend to ensure that the parent stock produces larvae of a suitable size and at the correct time, so they encounter an abundant supply of appropriately sized food organisms. Key words: reproduction biology, plankton size composition, larval feeding, adaptive behavior, recruitment

The interrelation of the size of fish eggs, the date of spawning and the production cycle

The variability in size of pelagic and demersal marine and freshwater fish eggs is examined. The difference between the smallest and largest volumes, based on published figures for the diameters, is large in many species. In marine species with planktonic eggs, the median percentage difference is just over 100%, and this is similar in species with demersal eggs and in freshwater fish.The available evidence suggests that geographical differences in egg size are small, but in marine fish there is a well‐known seasonal decline in egg size. In herring it has previously been shown that egg size in different spawning groups can be correlated with the timing of the production cycle. A similar correlation can be seen in the seasonal shift in time and locality of spawning, and egg size, of the plaice. Sufficient data on seasonal freshwater fish egg variations are not available, but the time of spawning does appear to be linked with the availability of food for the larvae in both lake and stream species.