SEXUAL DIMORPHISM AND SEX RATIO IN THE YELLOWSHOULDERED WIDOWBIRD EUPLECTES MACROURUS SOROR

Savalli, U.M. 1994. Sexual dimorphism and sex ratio in the Yellowshouldered Widowbird Euplectes macrourus soror. Ostrich. 65: 297–301.

Yellowshouldered (Yellowbacked) Widowbirds Euplectes macrourus soror are sexually dimorphic in plumage and size. At Kakamega, western Kenya, adult males were all black except for the yellow shoulders, contrary to previous reports that yellow-mantled individuals were predominant in this area. Males were larger than females, with wing length the best single measure to distinguish sex (since immature males are streaky brown, like females). Immature males had shorter wings and tails than did adult males, but did not differ in other measures of size. The sex ratio of netted birds was near 1:1, but nearly half of the males were immature. The breeding sex ratio was nearly even in one year but heavily male biased the second, suggesting that not all females may breed.     

Establishing body-size indicators in a study of female spruce grouse Dendragapus canadensis

Seven measurements were taken from 72 adult and yearling spruce grouse Dendragapus canadensis hens collected over a 10-year period from north-eastern Ontario, Canada. A principal components analysis (PCA) indicated keel ridge length, keel total length, and femur length, these being the most representative measurements of general body size. Skeletal mass, caecal length, wing length, and foot length had lower character loadings on the first principal component axis (PC1), suggesting they are unreliable measurements of body size. Measurements taken from bones (i.e. keel, femur) were more representative of general body size than external morphometric measurements (e.g. wing and foot length) which ornithologists traditionally use to estimate body size. Pre-laying adults were the smallest individuals and pre-laying yearlings the largest in the population studied. A discriminant functions analysis (DFA) of the seven measurements, with separated body mass as an eighth variable, demonstrated distinct separation between pre-laying and post-laying groups of birds on the basis of separated body mass. Keel ridge length was further implicated as an effective variable in the discrimination of birds from different laying groups.     

MINUTES OF THE ANNUAL GENERAL MEETING OF THE SOUTH AFRICAN ORNITHOLOGICAL SOCIETY,HELD AT CAPE TOWN, 26 JUNE 1959

Brown, C. J. 1989. Plumages and measurements of the Bearded Vulture in southern Africa. Ostrich 60: 165–171.

Four different age classes of the southern African Bearded Vulture Gypaetus barbatus are recognized and their plumages described: juvenile (3–24 months old), immature (24–45 months), subadult (45–60 months) and adult (60+ months). There was no significant difference in size between adult male and female birds. Adults were larger than juvenile birds in bill width, beard length, wingspan and mass, and had a higher aspect ratio and wing loading, while juvenile birds were larger than adults in the length of their outer rectrices, tail area, wing breadth and wing area. These features are considered to be adaptive to young birds inexperienced in flying. Immature and subadult birds were intermediate in size between juveniles and adults. Bearded Vultures differ from other large raptors in two sets of physical characteristics, (a) those adapted to cold, mountainous habitat, e.g. feathered head and face, unusually long wings, a high aspect ratio and a particularly long tail, and (b) those adapted to their diet of mainly bones, e.g. wide gape, beard and relatively long talons for carrying food.     

Seasonal pattern in age, sex and body condition of Barn Owls Tyto alba killed on motorways

The Barn Owl Tyto alba was the most common owl killed on motorways in northeastern France. The possible causes of this mortality and the age, sex and body condition of the road-killed birds in 1991–1994 have been investigated. The number of birds killed on roads was highest in the period from early autumn to late winter, i.e. during the non-breeding period, and showed a pattern similar to that of the temporal difference between sunset, which varies with day length, and peak of traffic, the occurrence of which is constant throughout the year. An autumnal mortality peak, concomitant with the post-fledging dispersal, was mainly of immature birds, especially females. A second mortality peak in late winter was composed mainly of mature birds, with an equal proportion of males and females. From autumn to winter, there was no significant change in body mass in the different age and sex categories of birds killed on roads, except for mature males which had a significantly lower body mass in winter. From early autumn to late winter, the mean body mass of immature owls killed on motorways did not differ significantly from that of captive immatures fed ad libitum. This suggests that the immature birds were in good body condition. In contrast, the body mass of road-killed mature females was significantly lower than that of captive mature females over the same time periods. In mature males in late winter, a drop in body mass in both road-killed and captive birds suggests an endogenous seasonal phenomenon. Except for mature females, Barn Owls killed on roads in 1991–1994 were in good body condition. This does not support the idea that only birds in poor body condition were killed. We conclude that the mortality of Barn Owls on motorways in autumn and winter was probably related to the concomitance between the peak of traffic and the onset of hunting activity and the large number and dispersal of immature individuals during the same period.     

Morphology of the Hooded crow Corvus corone cornix in relation to age, sex and latitude

The morphological variation of the Hooded crow at Trondheim, Norway, was studied, based on a sample of 734 birds collected during a six year period. Mouth colour, plumage colour, skull thickness and feather length were found to be characters which could readily be used to separate juveniles from adults. Females aged 15–19 months had a thinner skull roof than older female birds. Low coefficients of variation were found for the lengths of the third primary and of the tail feathers. A discriminant analysis showed that of the various body dimensions studied bill height and bill length distinguished the sexes most precisely. A high degree of sexual dimorphism was also found to exist in body weight and in the thickness of the skull roof.
Those body structures which develop at about the same stage during the juvenile growth period were associated with the same principal component, viz. the lengths of bony structures, parts that develop early on in life, were intercorrelated (wing bones, tarsus, bill basis and the width of foramen magnum). The lengths of the primaries and of tail feathers were also intercorrelated, structures which develop late.
The mean body weight of the Hooded crow population studied in Norway was intermediate between that of the Hooded crow in Germany and of the Carrion and Hooded crows in England and Scotland. No such differences were found in wing length. Norwegian Hooded crows have shorter tails than German ones, but their bills are much larger, in particular for the females. Therefore, the degree of sexual dimorphism in bill size seems to be reduced at high latitudes.     

Lipid levels in the South Island pied oystercatcher (Haematopus ostralegus finschi)

Wing lengths and some major body components were compared among the sexes and age classes of a sample of 48 South Island pied oystercatchers (Haematopus ostralegus finschi). Immature males were significantly lighter in wet weight and fat‐free weight than either immature females or adults, and were significantly lighter than adults in lean dry weight. No statistical differences were found in wing length, total lipids, or fat measure for birds in this sample. In a larger sample of 182 birds, fat measure varied similarly in the age classes, from a low in autumn to a peak in spring just before vernal migration. Massive pre‐migratory fattening typical of many small passerines does not occur in this species, but even so the average fat reserves in spring adults are sufficient for an approximate flight range of 2140 km, well in excess of need. It is hypothesised that the amount of fat deposited before vernal migration is a compromise between the energy cost of acquiring and transporting extra fat and the advantages of arrival at the breeding grounds with sufficient fat reserves for early achievement of breeding condition.     

Reproductive endocrinology of the Wandering Albatross Diomedea exulans in relation to biennial breeding and deferred sexual maturity

The reproductive endocrinology of the Wandering Albatross Diomedea exulans was studied at South Georgia to investigate the potential endocrine correlates of biennial breeding and of the acquisition of sexual maturity. Gonads of breeding birds and of known-age immature birds of both sexes were examined by laparoscopy throughout the period that they were at the nest site. Blood samples, subsequently analysed to determine concentrations of luteinizing hormone (LH), prolactin, progesterone, testosterone and oestradiol-17/i, were obtained from samples of breeding birds of both sexes at regular intervals from first arrival until the chicks fledged nearly a year later. Before laying in December, breeding birds had mature testes and ovarian follicles and high concentrations of LH, prolactin and sex steroids. Gonadal regression and a rapid drop in hormone levels (except for LH in females) occurred in early incubation (January). Testes (and follicles to a lesser extent) enlarged in mid-incubation, coinciding with high levels of LH and increases in prolactin and testosterone. Gonads finally regressed completely near hatching time. LH, prolactin and testosterone remained at low levels throughout chick rearing (April to November), but females had several periods of active progesterone and oestradiol secretion, and progesterone was detectable in males only late in the chick-rearing period. Although some changes in hormone levels are difficult to explain, the patterns are fairly typical of temperate birds. The persistence of progesterone secretion in both female breeders and non-breeding ‘immature’ birds is viewed as part of a mechanism inhibiting an ovary from becoming vitellogenic. Although testis size and testosterone concentrations increased with age in immature males (of ages 4–10 years), birds of 5 years and older are probably physiologically mature, even though breeding does not start until they are 7 years of age and only half an age group has bred by an age of 11 years. Immature females (of age 4–7 years) had undeveloped follicles, very low oestradiol concentrations but high progesterone levels, providing further support for the role of this hormone in inhibiting gonadotropin secretion. The condition of the female is therefore probably decisive in determining when a pair first attempts to breed but it is unknown what factors initiate normal ovarian development.     

Temperature trends and recent decline in body size of the stone marten Martes foina in Denmark

We studied a sample of 131 skulls of the stone marten Martes foina that were collected in Denmark between 1858 and 1999. Data were available for 37 years, but collection effort was not uniform throughout the study period and annual sample size varied between 1 and 27. We used principal component analysis (PCA) to combine the information of four skull measurements into a single variable (PC1). PC1 was then corrected for factors that significantly affected it (sex and longitude), and residual PC1 was used for further analysis in which we calculated trends in PC1 values during the study period. During the study period there was an increase in mean annual temperature in Denmark, but this increase was not continuous, as there was slight decrease in temperature between 1947 and 1965.We found that skull size (and by implication body size) of the stone marten in Denmark had two periods of decrease and these two periods coincide with the periods of increase in mean annual temperature. These results may indicate that body size of the stone marten is sensitive to the change in ambient temperature, either due to a change in food availability that was caused by the increase in temperature, or decreased its size in accordance with Bergmann's rule.     

Natural selection on tail and bill morphology in Barn Swallows Hirundo rustica during severe weather

An unusual six-day period of cold, rainy weather caused mortality among Barn Swallows Hirundo rustica in southwestern Nebraska, USA, in May 1996. We compared birds that died during the cold to those still alive when the severe weather ended. Among males, survivors had significantly longer culmens and significantly less variance in outer-tail asymmetry than non-survivors. Among females, survivors had significantly longer outer tails and significantly less variance in outer-tail length, overall body size and outer-tail asymmetry than non-survivors. Larger birds in general and those with less asymmetry in wing and outer tail tended to be favoured during this weather event. Long tails may reflect condition in females and, along with high levels of symmetry in wing and outer tail, improve foraging efficiency during extreme conditions. Males with long tails did not appear to suffer survival costs. Larger size probably allows more fat to be stored and may confer thermal benefits to swallows during late spring cold snaps. Similar mortality events have apparently occurred in the study area on only one other occasion since 1875.     

Age and seasonal differences in the biometrics of dunlins (<Emphasis Type="Italic">Calidris alpina</Emphasis>) migrating in spring through the Pripyat River floodplain,southern Belarus

Analysis of the biometric parameters of dunlins is based on data of 496 adult birds and 214 birds in their second year of life captured during the spring migration in the years 2002–2014 in the floodplain of the Pripyat River in southern Belarus. The average size of the dunlins caught in this area shows that the majority of birds correspond to the parameters of the nominate subspecies (C. a. alpina). We have not found any statistically significant differences between the age groups in all morphometric parameters except for the length of the wing, which in the birds in their second year of life is slightly smaller than in adults. Both the adult dunlins and the yearlings have two peaks in the distribution of biometric parameters, such as the bill length to the nostril, the bill length to the feathering, the total length of the head with the bill, and the length of the wing, which is associated with differences in the size of males and females. We have noted an increase in the average bill length to the nostril, bill to feathering length, and the total length of the head with the bill, as well as the wing length in the adult birds captured during the spring migration. These values are especially significant in the last five days of May. The body mass index of the dunlins passing through the floodplain of the Pripyat River increases intensively during the migration, which can partly be attributed to the later migration of large birds (females). However, the main reason for the increase in the body mass index is that the birds migrating later in spring have higher energy reserves compared with the earlier migrants. At the end of the migration period, in the floodplain of the Pripyat River, the body mass indices of dunlins are very high. This suggests that birds leaving the stopover site in southern Belarus in early June have sufficient energy resources to reach the nesting places in one nonstop flight.     

Impaired flight ability during incubation in the pied flycatcher

During the breeding season, many female passerine birds increase in body mass before egg laying, maintain a relatively high body mass during incubation, and then drop back to the original level during the chick-rearing period. The post-hatching reduction in body mass, which can be as large as 10–20%, has been suggested to represent an adaptive mass loss to reduce wing loading, thereby increasing parental flight efficiency when chicks have hatched and have to be fed. Here we present the first study of changes in flight ability from incubation to chick rearing in birds. Wild female pied flycatchers Ficedula hypoleuca flew more slowly during incubation than during chick rearing; a 7% reduction in body mass after the chicks had hatched was associated with a 10% increase in vertical take-off speed. Furthermore, the flight muscle size of the females tracked the reduction in wing load, suggesting that muscle size was adaptively reduced when no longer needed. Since incubation-feeding by males reduces the time females have to spend outside the nest foraging, our results suggest that in addition to increasing female nutritional status and reducing incubation time, incubation-feeding will also reduce predation risk during the period when females face impaired flight ability.     

Changes in diet and morphology of Finnish goshawks from 1960s to 1990s

We studied the morphology of the goshawk in northern Finland by measuring skin and skeletal characters of 258 museum specimens dated between 1961 and 1997. We predicted a decrease in the size of male goshawks from the 1960s because availability of their main prey, grouse, has decreased since then and grouse have been replaced in the diet by smaller prey during the breeding season. Based on the assumption that winter is the most critical period for females, we predicted that female size should have increased because their winter diet consisted of more and more mountain hare, which is a prey generally larger than grouse. Analyses revealed that male size has indeed decreased since the 1960s, while adult females have increased in size. Our data suggest that these morphological shifts were the result of selective pressures due to changes in diet. We also found changes in the (size-independent) shape of the hawks. Relative wing and tail lengths of adult hawks became longer between 1980 and 1990 compared with the 1960–1970 period, while relative juvenile wing and tail lengths tended to decrease. As a result of these morphological changes size dimorphism between the sexes increased from the 1960s to the 1990s. Received: 18 January 1999 / Accepted: 14 July 1999     

THE WINTERING AND MOULT OF RUFFS PHILOMACHUS PUGNAX IN THE KENYAN RIFT VALLEY

Some 5700 Ruffs were ringed in the southern Kenyan rift valley during 1967–79, mainly at Lakes Nakuru and Magadi. These have produced 15 recoveries outside East Africa, 14 in Siberia between 73° and 154°E and one in India. Adult males returned to Kenya mainly during August, and females during late August and early September. Females greatly outnumbered males at all times. Most wintering males departed late in March and early in April, but females not until about a month later. First-year birds appeared from the end of August, but remained in low numbers until late October or November. Most departed during April and May, but a few females oversummered. First-year birds typically accounted for about 25% of the wintering Nakuru females, but about 50% of those at Magadi. At both sites they accounted for a higher proportion of male birds than females. Most of the birds at Nakuru throughout late August to May appeared to be local winterers, and many individuals remained in the area for many months each year. Retrapping indicated that approximately 60% of each season's birds returned the following season. Adult males and most adult females commenced pre-winter wing moult before arrival, but completed most of it in Kenya. Males moulted 3–4 weeks ahead of females, and most had finished before December. Females typically finished during December and early January. Most second year birds timed their pre-winter moult similarly to older adults. Suspension was recorded in over 15% of all moulting birds examined. Adult pre-summer moult involved most or all of the tertials, some or all of the tail feathers, most of the inner wing coverts and the body and head plumage. It occurred mainly during January to March (males) or February to April (females), although tertial renewal commonly began a month earlier. Males showed no sign in Kenya of the supplementary prenuptial moult. First-year birds moulted from juvenile into first winter body plumage during late September to November. They underwent a pre-summer moult similar in extent and timing to that of adults, and again about a month earlier in males than females. Spring feathers acquired were often as brightly coloured as those of adults. About 15% of first-year birds renewed their outer 2–4 pairs of large primaries during January to April. Adult and first-year birds fattened before spring departure, commonly reaching weights 30–60% above winter mean. Weights of adult males peaked early in April, those of adult females early in May, and those of first-winter females later in May. Weights were relatively high also during August and September. This was due to the arrival of wintering birds carrying ‘spare’ reserves, and also apparently to the presence of a late moulting fattening passage contingent. The wing length of newly moulted adults was about 3 mm longer than that of newly arrived first-year birds, but there was no evidence of an increase in the wing kngth of adults with successive moults. Adult wing length decreased by 4–5 mm between the completion of one moult and the middle stages of the next. The migrations and annual timetable of Kenyan wintering Ruffs are discussed, and their moult strategy is compared with that of other Holarctic waders.     

Sex-specific differences in compensation for poor neonatal nutrition in the zebra finch Taeniopygia guttata

Individuals can compensate for poor early nutrition by accelerating their growth rates once diet improves, but if malnutrition occurs at a key stage of development only certain body structures may compensate fully. This degrees of compensation is predicted to differ between the sexes and also between species with different life history strategies. In this paper we determine how males and females in a short-lived bird species, with only slight size dimorphism, differ in their abilities to compensate for a poor start in life. Here, zebra finch Taeniopygia guttata chicks from 93 broods were reared on either a standard quality (SQ), or low quality (LQ) diet for the first 15 d of life, followed by a standard diet for the rest of their lives. Thus, the period of nutritional manipulation was relatively short compared with previous studies. Nestlings on a LQ diet showed slower body mass and tarsus growth rates than those on SQ diet but this was reversed after the diet of the LQ birds was improved. LQ birds of both sexes were able to fully compensate in terms of body mass and beak colour, but not tarsus length. Body size and beak colour are sexually selected traits in male zebra finches. By adulthood LQ females had significantly shorter wings than other birds, apparently directing resources into sex-specific structures instead of feather and skeletal growth. Thus, our experiment showed that the sexes differed in how they phenotypically compensated for a poor start in life. Males in particular invested sparse resources into structures associated with mate acquisition, as proposed by life history theory for a species that 'lives fast and dies young.' We predict that this relatively short period of malnutrition during early development will have long term, sex-specific, fitness consequences for these birds.     

The moult of the Little Stint Calidris minuta in the Kenyan rift valley

Moult data were collected during 1967–80 from some 6900 Little Stints in the southern Kenyan rift valley.
Adults typically moulted from summer to winter body and head plumage during September and early October, soon after arrival. The complete pre-winter wing and tail moult began in most adults between mid-September and early October. Some birds finished by December, but others continued until February and March. Individual duration was usually between 100 and 150 days. Adults which completed this moult early often remoulted outer primaries between January and early April.
Young birds acquired first-winter body plumage during October and early November. Some 90% had a complete pre-winter wing and tail moult. This usually began between December and early February, and finished during March or early April, taking about 70–100 days. In about 10% of young birds, flight feather moult was restricted to the outer primaries and inner secondaries. Birds adopting this strategy typically began moult late, during January or February. Short periods of suspension were common during pre-winter wing moult, particularly in adults. The difference in moult speed between adult arid first-winter birds was attributable in the primary, secondary and tail tracts to differences in numbers of growing feathers.
Practically all birds completed a pre-summer moult involving the entire body and head plumage, most of the tertials, some or all of the tail feathers and many wing coverts. Most birds began this moult between early February and late March, and finished between mid-April and early May. It was typically later and more rapid in first-year birds than adults. In late birds, the onset of pre-summer moult was linked to the final stages of pre-winter moult.
The wing moult of the Little Stint in different wintering areas is discussed. First-winter moult strategy is compared with that in other small Calidris species.     

Sex determination in Turdus amaurochalinus (Passeriformes: Muscicapidae): morphometrical analysis supported by CHD gene

Sex determination is important for conservation and population studies, particularly for reproduction programs of threatened species and behavioural ecology. Turdus amaurochalinus, Creamy-bellied Thrush, only exhibits sexual dimorphism during the breeding season, when males are considered to show intense yellow bills, and females and immature males show dark brown bills. The objectives of this study were: 1) to determine the sex of individuals using genetic techniques, and 2) to test the hypothesis that sex dimorphism can be detected by morphometry. This study was carried out at Parque Nacional da Restinga de Jurubatiba, a preserved area located on the North coast of Rio de Janeiro State. The birds were captured using ornithological nets, singly marked with metal rings, weighed, measured and had blood samples collected before being released. The sex of 42 T. amaurochalinus individuals was determined using the CHD gene marker. A total of 20 males and 22 females were identified from June to August, with peak capture frequency in June. Turdus amaurochalinus females and males differed significantly in morphometrical measures. The most important traits to distinguish males from females were wing length (Student t-test = 4.34, df = 40, p = 0.0001) and weight (Student t-test = 2.08, d f = 40, p = 0.044): females were heavier and had significantly shorter wing length than males. Females and males were correctly classified in 86% and 75% of cases, respectively, using Discriminant Analysis. The molecular analysis was the most secure method for sex determination in the studied species.     

Diet shift induced rapid evolution of size and function in a predatory bird

A predator’s body size correlates with its prey size. Change in the diet may call for changes in the hunting mode and traits determining hunting success. We explored long-term trends in sternum size and shape in the northern goshawk by applying geometric morphometrics. Tetraonids, the primary prey of the goshawk, have decreased and been replaced by smaller birds in the diet. We expected that the size of the goshawk has decreased accordingly more in males than females based on earlier observations of outer morphology. We also expected changes in sternum shape as a function of changes in hunting mode. Size of both sexes has decreased during the preceding decades (1962?2008), seemingly reflecting a shift in prey size and hunting mode. Female goshawks hunting also mammalian prey tend to have a pronouncedly “Buteo-type” sternum compared to males preying upon birds. Interestingly, the shrinkage of body size resulted in an increasingly “Buteo-type” sternum in both sexes. In addition, the sternum shape in birds that died accidentally (i.e., fit individuals) was more Buteo-type than in starved ones, hinting that selection was towards a Buteo-type sternum shape. We conclude that these observed patterns are likely due to directional selection driven by changes in the diet towards smaller and more agile prey. On the other hand, global warming is predicted to also cause a decrease in size, thus these two scenarios are inseparable. Because of difficulties in studying fitness-related phenotypic changes of large raptors in the field, time series of museum exemplars collected over a wide geographical area may give answers to this conundrum.     

A morphological comparison of alates between monogynous and polygynous colonies of Myrmica kotokui in northernmost Japan

Summary: Social structure and sexual size of two populations of Myrmica kotokui were compared. Most of the colonies were polygynous in one population and monogynous in the other. Mean body size of the queens was larger in the monogynous population than in the polygynous population. Although the polygynous population contained many large queens, their wing length was significantly shorter than that of queens in the monogynous population. Some females in the polygynous population were intermorphic between typical workers and queens, while the females of the monogynous population were clearly divided into worker and queen castes without any intermorphic females. The body size of the males showed a large variation and the mean was not significantly different between the two populations.     

Differential timing of spring passage of Ruby‐throated Hummingbirds along the northern coast of the Gulf of Mexico

ABSTRACT Hummingbird migration has long fascinated researchers due to the limitations of small body size and high metabolic rate on migratory performance. Yet, few data are available concerning hummingbird migration strategies, especially for species that must negotiate major geographic barriers. To address this problem, we investigated the migration ecology of Ruby‐throated Hummingbirds (Archilochus colubris) passing through a coastal banding station in southwest Louisiana following flights across the Gulf of Mexico. Our aims were to describe the phenology of spring migration and explore potential differences in the migration ecology of males and females. During our 10‐yr study (1998–2007), we found that peak hummingbird passage generally occurred in the second half of April and that males preceded females by about three days. However, females arrived in significantly better energetic condition as measured by fat and muscle stores as well as size‐corrected body mass. Most birds did not stay at our study site to refuel, only 2% of individually marked birds were recaptured more than a day after initial capture (range = 1–5 d). Our results suggest that Ruby‐throated Hummingbirds exhibit protandrous migration (i.e., males migrate earlier) and that en route body condition may be a consequence of sexual dimorphism in wing morphology (i.e., lower wing loading in females) that allows females to expend less energy during migration across the Gulf of Mexico.     

Growth and reproduction in the Icelandic common seal (Phoca vitulina L., 1758)

Length, weight and maturity were studied in relation to age in the common seal (Phoca vitulina vitulina L., 1758), collected during the periods 1979-1983 and 1990-2000 in Icelandic waters. The maximum age of common seal observed was 36 years for females and 30 years for males. For common seal females and males, asymptotic length and weight were 161 cm and 93 kg and 174 cm and 97 kg, respectively, showing slight sexual dimorphism in size. The condition of adult females, measured as fat thickness at the lower end of the sternum, was lower in the period 1979-1983 than in 1990-2000 during June-September, the breeding and mating time of the Icelandic common seal. Males reached sexual maturity between 5 and 7 years, whereas 50% of females did so at age 4 years. Including the length and age interaction term in the logistic regression model for the maturity of females significantly improved it. Thus, body size matters in the onset of maturity. The mean birthing date for the Icelandic common seal was found to be in early June. A comparison of animals collected in the two periods 1979-1983 and 1990-2000 did not show significant differences in growth and the average age of sexual maturity for either males or females. The observed decline of the Icelandic common seal population is most probably caused by increased mortality, due to exploitation and accidental by-catch in gill-nets, rather than a decrease in fecundity.