Intermittent Breeding Research Articles

LONG LIVING AND REPRODUCTION SKIPPING IN THE FAT DORMOUSE

We studied the reproductive strategy of a population of fat dormice (Myoxus glis) breeding in nest boxes in a beech forest on the southern Alps, Italy. In eight years of data collection (1991-1998), we observed births only in 1991, 1992, and 1995. We did not observe young during the autumn dispersal in the remaining five years. Reproduction was not correlated with high rainfall, low temperature in summer, or with population density. Reproduction was only observed in years of beech mast seeding. Body condition of adults when they emerged from hibernation did not differ between years with and without repro- duction, suggesting that the adults may avoid breeding when the young have a low prob- ability of surviving their first winter. A capture-recapture analysis on individually marked adult fat dormice identified a very high yearly survival rate (0.86-0.92), close to that of large herbivorous mammals and much higher than that typical of other rodent species with similar body size. No influence of sex on survival or recapture probability was detected. The long life span expectancy of an adult dormouse (averaging nine years) is the prerequisite for the infrequent breeding strategy adopted by this rodent species to cope with an unpre- dictable food source. A high survival rate despite the small body size may have been possible because of the long hibernation time ( .6 months). Long life span and intermittent breeding make the life history of fat dormice unique among rodents.

Intermittent breeding in the short‐tailed shearwater Puffinus tenuirostris

1. At one colony of short‐tailed shearwaters in Bass Strait, Australia, all birds breeding have been recorded individually each year for over 50 years. Among individuals known to be alive and to have bred before, 14% of each sex were not present at their breeding colony, on average, in any one year. 2. A further 15% of males and 13% of females (a significant difference), known to have bred before, were present but not associated with an egg in any one year. 3. Intermittent breeding was associated with pair bond breakdown and with a reduced ability to raise offspring, even when years of absence were allowed for. 4. The frequency of attendance, laying and successful rearing of progeny to fledging increased with age in the early years of breeding. However, this was followed by a decrease, except in the case of laying frequency, which continued to increase throughout life. 5. Our analysis indicated that intermittent breeding in short‐tailed shearwaters did not result from individuals implementing trade‐offs between the effort required for breeding success and their breeding life span. Rather, we suggest that individuals of higher quality are able to breed more frequently than others without any compensatory reduction in either their annual breeding success or their overall breeding life span.

Survival, breeding probability and reproductive success in relation to population dynamics of Brandt's Cormorants Phalacrocorax penicillatus

The size of the breeding population of Brandt's Cormorants Phalacrocorax penicillatus on Southeast Farallon Island, off the coast of central California, USA, shows large annual fluctuations and has declined by 50% since the 1970s. We investigated patterns of variation in adult survival, breeding probability, resighting probability, juvenile survival and reproductive success, to determine the extent to which variation in demographic processes reflected variation in environmental conditions (e.g. food availability) versus densitydependence, using observations of breeders and non-breeders from 1976 to 1995. Resighting and breeding probabilities varied significantly among years, and both were positively correlated with an annual index of prey availability — the abundance of juvenile rockfish. Adult survival between years was significantly correlated with changes in juvenile rockfish abundance and differed between sexes (0.77, males; 0.71, females). Female, but not male, survival appeared age-specific: older females showed reduced survival, especially during El Niño years. Intermittent breeding appeared common in this population: 54% of males and 49% of females estimated to be alive in a given year were not observed breeding in that year. Reproductive success varied in relation to the juvenile rockfish index but not in relation to population size. The return rate of juveniles, an index of survival during the first three years of life, was negatively related to sea surface temperature, both in the year of hatching and in the third year of life, when individuals first return to the colony to breed. We conclude that this population is strongly susceptible to environmental fluctuation. All parameters of survival and reproduction deteriorate under poor environmental conditions, with no evidence of buffering, and there was no evidence of density-dependence. Thus extrinsic, not intrinsic, forces appear most important in explaining recent population fluctuations.

Are Adult Nonbreeders Prudent Parents? The Kittiwake Model

Understanding evolutionary consequences of intermittent breeding (nonbreeding in individuals that previously bred) requires investigation of the relationships between adult breeding state and two demographic parameters: survival probability and subsequent breeding probability. One major difficulty raised by comparing the demographic features of breeders and nonbreeders as estimated from capture–recapture data is that breeding state is often suspected to influence recapture or resighting probability. We used multistate capture–recapture models to test the hypothesis of equal recapture probabilities for breeding and nonbreeding Kittiwakes and found no evidence of an effect of breeding state on this parameter. The same method was used to test whether reproductive state affects survival probability. Nonbreeding individuals have lower survival rates than breeders. Moreover, nonbreeders have a higher probability of being nonbreeders the following year than do breeders. State-specific survival rates and transition probabilities vary from year to year, but temporal variations of survival and transition probabilities of breeders and nonbreeders are in parallel (on a logit scale). These inferences led us to conclude that nonbreeders tend to be lower quality individuals. The effect of sex was also investigated: males and females do not differ with respect to survival probabilities when reproductive state is taken into account. Similarly, there is no effect of sex on transition probabilities between reproductive states.

ARE ADULT NONBREEDERS PRUDENT PARENTS? THE KITTIWAKE MODEL

Understanding evolutionary consequences of intermittent breeding (non- breeding in individuals that previously bred) requires investigation of the relationships between adult breeding state and two demographic parameters: survival probability and subsequent breeding probability. One major difficulty raised by comparing the demographic features of breeders and nonbreeders as estimated from capture-recapture data is that breed- ing state is often suspected to influence recapture or resighting probability. We used mul- tistate capture-recapture models to test the hypothesis of equal recapture probabilities for breeding and nonbreeding Kittiwakes and found no evidence of an effect of breeding state on this parameter. The same method was used to test whether reproductive state affects survival probability. Nonbreeding individuals have lower survival rates than breeders. Moreover, nonbreeders have a higher probability of being nonbreeders the following year than do breeders. State-specific survival rates and transition probabilities vary from year to year, but temporal variations of survival and transition probabilities of breeders and nonbreeders are in parallel (on a logit scale). These inferences led us to conclude that nonbreeders tend to be lower quality individuals. The effect of sex was also investigated: males and females do not differ with respect to survival probabilities when reproductive state is taken into account. Similarly, there is no effect of sex on transition probabilities between reproductive states.

The price of eggs: increased investment in egg production reduces the offspring rearing capacity of parents

Understanding the selective pressures shaping the number of offspring per breeding event is a key area in the study of life-history strategies. However, in species with parental care, costs incurred in offspring production, rather than rearing, have been largely ignored in both theoretical and empirical studies until relatively recently. Furthermore, the few experimental studies that have manipulated production costs have not yet teased apart effects that operate via the parental phenotype from effects on the quality of the resulting young. To examine whether increased egg production influences parental brood rearing capacity independently of effects operating via egg quality, we experimentally increased egg production in gulls and then examined their capacity to rear a control clutch. We found that the capacity of parents to rear the control brood was substantially reduced solely as a consequence of having themselves produced one extra egg. The paradox that, in many species, parents apparently aim for fewer young per breeding event than the experimentally and theoretically demonstrated optimum, has partly arisen from the failure to take into account the constraints imposed by production costs.

Saving the Kakapo: the conservation of the world's most peculiar parrot

SummaryWe review the conservation history and describe the current status of the Kakapo Strigops habroptilus, a large New Zealand parrot which has been reduced to only 54 individuals through predation by introduced mammals, and is now threatened with extinction. Unique amongst parrots, Kakapo are both flightless and nocturnal. They have an unusual mating system in which females nest and raise their young unaided by males, after mating at traditional “courts” at which males display visually and vocally. Mating occurs naturally only in seasons of heavy fruiting of podocarp trees. A decline in range and abundance of Kakapo followed the introduction of alien mammals last century, and culminated in their reduction to a single breeding population on Stewart Island. Following a severe episode of predation by feral cats Felis catus, all known birds from this last population were translocated to a series of cat-free offshore islands. Adult survival on these island sanctuaries has been high (c. 98% per annum), but productivity has been low, with only six young (including a single female) raised to independence since 1982. Reasons for this low productivity are the naturally intermittent breeding of Kakapo, the low numbers of nesting females, high rates of egg infertility (~ 40%), and the early death of most nestlings through starvation or suspected predation by Polynesian rats Rattus exulans. These rats are present on both of the island sanctuaries where nesting has occurred. The Kakapo sex ratio is biased in favour of males (34:20) and only 8 of the 19 adult females are known to have laid fertile eggs in the past 10 years. Management of all remaining birds is now highly intensive, involving radio-tagging of all individuals, the provision of supplementary food, attempts to manipulate matings, nest surveillance, and protection against rat predation.

Intermittent breeding in the Herring GullLarus argentatusand the Lesser Black‐backed GullLarus fuscus

Thirty‐three percent and 37% of adult Herring GullsLarus argentatusand 34% and 40% of adult Lesser Black‐backed GullsLarus fuscus, all with some previous breeding experience, failed to breed in 1993 and 1994, respectively. About half of these intermittent breeders of both species failed to breed in both years. Common amongst the intermittent breeders were smaller individuals, birds with low survival chances and relatively unsuccessful breeders.

Intermittent breeding in Cory's Shearwater Calonectris diomedea of Selvagem Grande, North Atlantic

Every year, 10% of the breeding population of Cory's Shearwater Calonectris diomedea of Selvagem Grande, between the Canaries and Madeira (30d̀09'N, 15d̀52'W), fail to breed. These “sabbatical” periods can last up to 7 years but typically last 1 year in 58% of the cases, with an average of 1.7 years. Both sexes are involved, and most are young breeding birds. The frequency and duration of the absences decrease with the increase of breeding experience. During the year preceding the absence, the breeding success, mate fidelity and nest site fidelity of the breeding birds are the same as those of their sabbatical congeners. During the sabbatical absence, the former mate is also absent in just less than half of the cases. Of those present, two‐thirds are at the same nest site and one‐third at another nest site. On the return of the “missing” birds, the fidelity both to the former mate and the former nest site is low, and more so if the length of the absence has been over 1 year, because the former mate and the former nest site are then seldom available. Divorce is frequent and the breeding success is low. Comparisons with other Procellariiformes reveal few differences in the intermittent breeding except for the proportions involved, which are six times greater for some species than for others.

Lifetime breeding studies in fully fed and dietary restricted female CFY Sprague-Dawley rats. 1. Effect of age, housing conditions and diet on fecundity

The capacity of female CFY Sprague-Dawley rats to bear multiple litters when breeding was commenced at increasing ages was assessed under the following conditions: continuous housing with male (GC), intermittent breeding of group (GI) and individually housed (I) fully fed animals, restriction of food intake (DR), refeeding of dietary restricted rats (DRA). The percentage of females pregnant, number of litters produced and total litter size were assessed. The number of litters or percent pregnant was unaffected by individual housing but there was a significantly greater decline in total litter size with increasing age of mother when compared to (GI) rats. The slope of the regression line in (I) was identical to that for (GC) females and may reflect the burden of continuous breeding or the stress of individual housing. Both litter size and overall fertility were decreased by dietary restriction but these females did retain their capacity to breed as late as 937 days in contrast to 720 days for any group of fully fed animals. There was no decline in litter size with increasing age in (DR). Returning (DR) rats to full feeding resulted in a rate of decline in litter size identical to that for (GI). All groups of (DRA) produced a maximum of three litters, commencement of breeding at 750 days resulting in production of a litter as late as 952 days.

Modelling Annual Changes in Numbers of Breeding Fulmars, Fulmarus glacialis, at a Colony in Orkney

(1) During a long-term study of breeding fulmars in Orkney, the number of birds breeding each year has increased on average, but with some considerable variations between years. (2) In a species with deferred maturity, low fidelity to natal colony and which is faithful to both mate and nest site, the factors causing changes in breeding numbers are likely to be annual survival of established breeders and annual recruitment of new breeders. (3) Recruitment could not be measured directly, but an index was determined, based upon the number of unringed breeding birds. However, in the observed data there were some occasions in which not all of the unringed birds breeding in one year necessarily bred the following year. There is some evidence to suggest that this may be due to intermittent breeding and/or emigration, probably involving inexperienced breeders. (4) A new index of 'recruitment', which could be negative, was calculated from the observed data. A model was developed to simulate breeding population size over a series of years, using observed annual values of survival and 'recruitment' as input parameters. (5) Simulations from the model were realistic in terms of variation between years and correlations between variables. (6) The model was therefore run using particular observed values for both 'recruitment' and survival. It was found that 'recruitment' had a much greater effect upon the population size than did survival of breeding adults. (7) The model was then used to predict the size of the breeding population one year in advance, with reasonable accuracy. (8) The simulations highlight the need for further information about the behaviour and movements of potential breeding recruits.

FACTORS AFFECTING THE AGE OF FIRST BREEDING OF THE KITTIWAKE RISSA TRIDACTYLA

SUMMARYAt a Kittiwake colony in Northumberland, 80% of those birds which returned to their natal colony to breed were males and these supplied 52% of all male recruits. More females breed away from their natal colony than males. There was no differences in the proportions of young fledged from sites in the centre or at the edge of the colony, or by parents of different experience, which returned to breed. Kittiwakes breed for the first time at ages from 3 to 8 years, but most at 4 or S years old. Males arrive back at the colony at an earlier age than females and breed for the first time one year earlier. Males obtaining sites at the centre of the colony first breed at an earlier age than those at the edges. Neither the age nor the area of first breeding appear to be transmitted from parent to offspring. Males breeding first aged 4 years or younger produced more young than those which first bred aged 5 years or older, despite their partners laying smaller clutches. This difference was most marked among those males recruited to sites in the centre of the colony. The advantage of this earlier breeding is counteracted by a lower survival rate among those males which start to breed at the younger ages. In all breeding Kittiwakes, annual reproductive output increases with experience while annual survival rates decrease. Once they had started to breed, many birds failed to breed in one subsequent season. Nearly 60% of these cases of intermittent breeding occurred in the year following first breeding. Intermittent breeding was most frequent among young birds and among females. It is suggested that each breeding involves a cost to the individual in terms of reduced survival, and that deferred and intermittent breeding are means of guarding survival. A model is proposed whereby the age at which a bird starts to breed, the nesting site which it obtains, and its subsequent breeding strategy result in each individual producing an optimal number of reproducing offspring in its lifetime, relative to its quality.

Survival of eggs of Eretmapodites chrysogaster Graham (Diptera: Culicidae).

Studies on egg survival in the African mosquito Eretmapodites chrysogaster showed that about half of the from 10- and 17-day-old colonies survived 5 days of desiccation after maturation, but only about 25% of the eggs from a 24- day colony survived drying for this period. However, a larger proportion of the eggs from the older females was capable of surviving 20 days of dry storage. This was more marked in eggs of the third oviposition than in those of the first oviposition from old females. The younger females produced more viable eggs. As the age of the eggs increased, the duration of the hatching period decreased. The relatively limited ability of the eggs to survive in dry containers may nevertheless represent an important dry season survival mechanism when occasional showers permit intermittent breeding.

BREEDING OF THE FULMAR FULMARUS GLACIALIS

Summary..1. A study of individually marked Fulmars during three seasons has shown that both sexes can breed in successive years. The evidence for the theory of intermittent breeding is examined and definitions are given of the various categories of birds which compose the population during the breeding season. It is suggested that the supposed non‐breeding adults which are seen, often in moult, in July are failed‐breeders which have lost their egg or nestling.2. No marked individual changed its nest‐site between years, although the number and identity of sites occupied each July varied considerably.3. No changes of mate occurred among the six marked pairs which nested in two consecutive years.4. There is some evidence that both parents remain near the colony during the week before and after hatching.5. Analysis of the 90 known sites of a colony subject to avian but not wild mammalian predation has revealed a wide range of habitat selection and a strong tendency toward close concentration of nests.6. There is some evidence that the breeding season, as measured by date of hatching, varies from year to year.

Intermittent Breeding of the Fulmar (Fulmarus glacialis(L.)), with some General Observations on Non‐Breeding in Sea‐Birds

Summary. 1 There is a large non-breeding population of Fulmars and other Pro-cellariiform birds, which suggests that they do not breed annually. 2 In non-breeding Fulmars obtained in northern Labrador in July it was found that the length of the middle claws varied from 16.0 to 10.6 mm., due to wear. As pelagic birds can only wear down their claws when breeding, this observation strongly suggests a non-breeding population of mixed age-groups. 3 Ovaries from four representative non-breeding females were sectioned, and histological confirmation obtained of the relation of claw-length to age. Resorbed remnants of ovulated follicles prove that all these birds had bred in one or many previous years, and thus that their reproduction is intermittent. 4 A case of follicular atresia accompanied by the extrusion of yolk into the stroma is figured. 5 When breeding is inhibited in the Fulmar and certain other birds the annual (post-nuptial) moult occurs prematurely. A short discussion is given of other effects of inhibited breeding in marine birds.