Synchronous Broods Research Articles

Sex-Biased Nestling Mortality Is Influenced by Hatching Asynchrony in the Lesser Black-Backed Gull Larus fuscus

Many birds hatch their eggs asynchronously while other species are more synchronous. Different hatching patterns may produce different sex-ratios through sex-biased chick mortality, but the relationship between sex-ratios and hatching pattern has received little attention. An extreme food shortage in a Lesser Blackbacked Gull colony allowed us to examine the timing of mortality of nestlings relative to sex and to hatching order. This was carried out in experimentally-created synchronous and asynchronous broods, where the latter mimicked the normal hatching spread. Unambiguous sexing was facilitated by use of molecular methods. In asynchronous broods, last-hatched female chicks survived significantly longer than last-hatched male chicks, whilst the males significantly out-lived the females among the earlier-hatched chicks. There was no difference in survival time of the sexes in synchronous broods. This suggests that chick sex-ratio is, at least partly, a function of asynchrony. Production of skewed sex-ratios via sex-biased mortality should be considered when evaluating the merits of asynchrony and synchrony.

Hatching asynchrony reduces the duration, not the magnitude, of peak load in breeding green-rumped parrotlets ( Forpus passerinus )

The peak load reduction hypothesis suggests that hatching asynchrony in altricial birds is adaptive because it reduces parental workload during the most energetically costly time in brood rearing. By staggering the ages of their offspring, parents may ensure that all nestlings do not reach maximum energy demand simultaneously. To test the hypothesis, we used the doubly labeled water technique to measure the energy expenditure of green-rumped parrotlets (Forpus passerinus) that reared experimentally manipulated synchronous and asynchronous broods. Peak metabolic rates of the two experimental groups did not differ, but parents of asynchronous broods metabolized significantly less energy than did parents of synchronous broods throughout the first half of the brood-rearing period. Our results suggest that hatching asynchrony in parrotlets substantially shortens the temporal duration of high brood energy demand, but does not reduce the magnitude of peak energy demand.

Hatching Asynchrony and Sibling Size Hierarchies in Gulls: Effects on Parental Investment Decisions, Brood Reduction and Reproductive Success

We manipulated the within-brood mass hierarchy of Lesser Black-backed Gulls Larus fuscus, controlling for chick quality (related to laying order) by swapping last hatched (c-) chicks in experimental broods with aor bchicks from residual broods to create either synchronous broods (all 3 chicks in the brood approximately the same mass) or asynchronous broods (same within-brood difference in mass between largest and smallest chicks as that of unmanipulated broods), whilst also controlling for variation in characteristics of parental quality (clutch size, egg size and timing of laying). Subsequent chick growth and survival for each brood were monitored in comparison with naturally asynchronous, unmanipulated broods in order to assess the effect of hatching asynchrony per se on chick growth and survival. There were no differences among treatments in fledging success or growth characteristics of surviving chicks. However, chick mortality occurred significantly later in synchronous broods and this was a consequence of the smaller within-brood hierarchical differences in size and body condition of these chicks. Apparently, parents could not regulate resource allocation in synchronous broods, and invested a greater amount of parental investment in chicks that later died. The results indicate that (1) the degree of hatching asynchrony may be optimized with respect to the prevailing food supply and/or female nutritional condition, and (2) the degree of hatching asynchrony is more important than initial within-clutch differences in egg or chick quality in determining the outcome of sibling rivalry over parental investment.

HATCHING ASYNCHRONY IN GREAT TITS: A BET-HEDGING STRATEGY?

Previous studies have shown that asynchronous hatching in birds tends to result in fewer, but higher quality, offspring than does synchronous hatching. However, very few studies have considered the consequences of such a pattern for the recruitment of offspring as subsequent breeders. We recorded nestling survival and fledgling body mass in experimentally synchronous and asynchronous broods of the Great Tit, Parus major. Using six different juvenile survival models, taken from published studies of the species, we calculated the expected number of offspring recruited with each hatching pattern and survival function. The sex of fledglings was included in the analysis. When comparing the estimates of surviving juveniles between synchronous and asynchronous broods, we found no significant difference in the means with any of the six models. However, the distributions of recruitment were different: the expected number of recruits was more variable with synchrony than with asynchrony. This suggests that hatching asynchrony in the Great Tit may serve as a bet-hedging strategy. A larger proportion of asynchronous broods than of the synchronous broods experienced brood reduction, but fledgling body masses for the reduced synchronous broods were lower than those for the reduced asynchronous broods. This suggests that synchronous hatching inhibits adaptive brood reduction in the Great Tit. There was considerable individual variation in the level of brood reduction within the same area and season. We argue that conditions also may be unpredictable at a small scale (parental abilities, territory quality, or parasite load). In consequence, high-quality parents may benefit from synchrony, but low-quality parents benefit from asynchrony, within the same year.

Is There a Sexual Conflict over Hatching Asynchrony in American Robins?

-Many hypotheses have been proposed to explain the occurrence of hatching asynchrony in altricial birds. According to one hypothesis, parents would benefit from hatching asynchrony because the offspring would spend less energy on sibling rivalry. Another hypothesis states that there is a sexual conflict over hatching spread, with one parent trying to minimize investment at the expense of its mate. The latter hypothesis was suggested from a study where males survived better following asynchronous than following synchronous hatching of the brood, with opposite results for females. I tested this hypothesis by manipulating hatching spread in another altricial bird with biparental care, the American Robin (Turdus migratorius), and by observing food provisioning late in the nestling period. The hypothesis was supported because males seemed to contribute less, and females more, to asynchronous broods. Males did not take less care than females of the smaller nestlings within the brood, but when the brood had partially fledged it was mostly the female that fed the young that remained in the nest. Provisioning of the parents combined did not seem to be less with asynchronous than with synchronous hatching, lending no support to the siblingrivalry reduction hypothesis. Received 14 October 1996, accepted 31 March 1997. THE YOUNG OF MOST ALTRICIAL BIRDS do not hatch simultaneously but over a period of one or more days. Many hypotheses have been proposed to explain hatching asynchrony (Magrath 1990, Stoleson and Beissinger 1996). For instance, the cost to the parents may be reduced because less energy is spent on sibling rivalry (the Sibling Rivalry Reduction Hypothesis; Hahn 1981), and any offspring mortality that occurs may take place at an early stage of breeding so that little energy is wasted (Lack 1954). Hatching asynchrony also may help to spread out the peak demand for food of individual offspring (Hussell 1972, Mock and Schwagmeyer 1990). Evidence suggests that parents need to work less with asynchronous than with synchronous hatching in some species (Fujioka 1985, Mock and Ploger 1987) but not in others (Bryant and Tatner 1990). The best way of assessing the costs of breeding would be to follow the subsequent survival and fecundity of the parents. When this was done in a study of Blue Tits (Parus caeruleus), there was no evidence that parents benefitted from hatching asynchrony (Slagsvold et al. 1994, 1995). In the study of Blue Tits, data for the two parents were combined. However, males and females seemed to differ in their re' E-mail: tore.slagsvold@bio.uio.no sponses, males having improved survival with asynchronous hatching, females with synchronous hatching. We therefore suggested that a sexual conflict exists between the sexes over the degree of hatching spread, with one parent trying to minimize investment at the expense of its mate (the Exploitation of Mate Hypothesis; Slagsvold et al. 1995). In Blue Tits, the conflict apparently is won by the female because hatching is rather synchronous despite a large clutch size. The female alone incubates, and by delaying incubation until most eggs have been laid, she can reduce the hatching span. Why would a female benefit from synchronous hatching of her brood? One possibility is that with synchronous hatching, the male may contribute more because otherwise the whole brood would starve to death. Breeding success may be more variable with synchronous than with asynchronous hatching (Amundsen and Slagsvold 1997) such that synchronous broods may be more sensitive to the amount of male parental care. The male may invest less if the brood contains some small, undernourished young with low survival prospects. Relative to the female, then, he may give greater priority to molting and territory defense than to parental care (e.g. Svensson and Nilsson 1997). In addition, males may be less certain of paternity of later-hatched young and hence reduce investment in these offspring. In support of these

HATCHING ASYNCHRONY, BROOD REDUCTION, AND FOOD LIMITATION IN A NEOTROPICAL PARROT

A number of hypotheses for hatching asynchrony suggest that the size hierarchy among nestlings produced by hatching asynchrony is adaptive and confers benefits to parents. We assessed the costs and benefits of asynchronous hatching in the Green-rumped Parrotlet (Forpus passerinus), a small Neotropical parrot that hatches large clutches very asynchronously. We manipulated eggs to create broods of four, six, or eight young that hatched synchronously or asynchronously. In a second experiment, we tested whether food limits offspring survival by experimentally feeding later hatched young in large asynchronous broods. We also examined the premise that food varies unpredictably by sampling seeds throughout several breeding seasons. Experimentally synchronized broods generally fledged as many or more young than asynchronous broods. Synchrony particularly outperformed asynchrony in broods of eight, where food demands should have been greatest. Nestlings had a higher probability of fledging from synchronous broods than from asynchronous broods, from small rather than medium or large broods, and if they were early hatched rather than later hatched. Most mortality in asynchronous broods occurred within 12 d of hatching, and a significantly greater proportion of later hatched chicks died with empty crops than did early hatched chicks. Later hatched chicks grew more slowly than their earlier hatched nestmates, but at fledging they were as heavy or heavier than earlier hatched chicks. Chicks from asynchronous broods were slightly heavier at fledging than synchronous chicks, but there was no correlation between fledging mass and the likelihood of being resighted in subsequent years. Cormack-Jolly-Seber model estimates revealed no significant differences in annual survival rates between young fledged from synchronous and asynchronous broods. Female chicks fledged from synchronous broods were recruited into the study population at a lower rate than those from asynchronous broods. Older chicks from reduced broods were less likely to fledge than chicks from broods that fledged all their young. Parents of large synchronous and asynchronous broods provisioned their young at similar rates and did not differ significantly in their subsequent survival. Females that raised experimentally synchronous and asynchronous broods showed no significant differences in the likelihood, timing, or success of their next breeding attempt. A marginally higher proportion of last-hatched chicks that received supplemental food survived to fledging than last-hatched control chicks, but feeding had no effect on penultimate chicks. Seed densities showed a high degree of autocorrelation over spans of 30–50 d. Asynchronous hatching appears to result in the mortality of the smallest young, due in part to the inequitable distribution of food among nestmates, rather than to food limitation, and as a direct result of the size disparities among nestmates. Thus, parrotlet parents appeared to derive no detectable short- or long-term benefits from the staggered hatching of their young through increased nestling growth and survival, reduced parental efforts, or increased parental survival. Although other adaptive benefits from hatching asynchrony are possible that were not tested directly in these experiments (e.g., “insurance” that some nestlings will survive), they seem insufficient to account for the extreme hatching asynchrony observed in the parrotlet. Instead, benefits to egg survival derived from the early onset of incubation may offset the costs of asynchronous hatching.

The function of hatching asynchrony in the blue-footed booby

The blue-footed booby (Sula nebouxii) commonly hatches two eggs 4 days apart; then the senior (first-hatched) chick aggressively dominates the other and sometimes kills it. Two hypotheses explaining the function of the hatching interval were tested by creating broods with synchronous hatching: the facultative brood reduction hypothesis of Lack (1954) and the sibling rivalry reduction hypothesis of Hahn (1981). The results contradicted most predictions of both hypotheses: synchronous broods formed an aggressive hierarchy similarly to asynchronous broods (controls), and subordinate chicks grew poorly (Fig. 1) and died frequently, similarly to junior chicks in control broods. However, compared with synchronous broods, asynchronous broods showed less aggression (Fig. 2), diminished food allocation to subordinate chicks (Fig. 3) and less total food consumption (30% fewer feeds at age 0–10 days). These behavioral comparisons took into account the different ages of chicks in different treatments. The results suggest that natural asynchrony makes brood reduction more efficient and decreases the costs of sibling aggression to parents, in terms of their future survival or fecundity, as proposed by Mock and Ploger (1987). Further, in exaggeratedly asynchronous broods (8-day hatch interval) junior chicks suffered more aggression (Fig. 4) and grew more slowly than junior chicks in control broods. This result supports the hypothesis of optimal hatch asynchrony of Mock and Ploger (1987).

Costs and Benefits of Hatching Asynchrony in Blue Tits Parus caeruleus

We studied the significance of hatching asynchrony in blue tits Parus caeruleus during a 4-year period at Oslo, Norway, by comparing breeding success of broods manipulated to hatch over a shorter or longer period than average. Nestling mortality was high and mainly caused by starvation. Mean body mass of fledglings on day 14 was significantly higher for asynchronous than for synchronous broods. No significant difference was found in the mean number of young fledged between the treatment groups, nor in the number of offspring recruited into the local breeding population, even in a year with heavy nestling mortality. However, the number of recruits was few. The results are consistent with the view that asynchronous hatching ensures high quality of some offspring (the offspring quality assurance hypothesis). This benefit is achieved even when no brood reduction occurs. We suggest a potential cost to hatching asynchrony (the diminishing return hypothesis): in asynchronous broods, early hatched offspring may require so much food that their own subsequent survival is reduced or at least no further improved. Resources may therefore be wasted on first hatched offspring that might have improved the growth of younger siblings. Female parents had lower post-breeding survival with asynchronous than with synchronous hatching; the opposite result was found for male parents. We propose the exploitation of mate hypothesis to explain this result. The hypothesis presumes there is sexual conflict over the amount of parental investment in current versus future reproduction, and over the investment in particular offspring within the brood. We suggest that with asynchronous hatching, females have to invest more to keep less competitive, late-hatching nestlings alive.

The polygynandrous mating system of the alpine accentor, Prunella collaris. II. Multiple paternity and parental effort

Parentage was determined by DNA fingerprinting for 110 young from 38 broods. There was no intraspecific brood parasitism and no young were fathered by males from outside the polygynandrous group. Nineteen (50%) broods were fathered by one male (15 by alpha, four by beta), 17 by two males (14 alpha and beta, two alpha and gamma, one beta and gamma) and two by three males (alpha, beta and gamma). A male's paternity share of a brood increased with his share of the matings, measured as the proportion of time he gained exclusive access to the female. This relationship was the same for alpha and beta males, which suggests that their copulations were of equal potency. Alpha males gained a larger share of the paternity by guarding fertile females and their overall paternity within the group tended to increase with female nesting asynchrony, although not significantly so, and to decrease with more competing subordinate males. Males were more likely to help feed nestlings if they gained a greater share of the matings with the mother. There was no difference between alpha and beta males in the relationship between the probability of helping and mating share. Given a choice between two synchronous broods, males preferred to help where their mating share was greatest. When alpha and beta males helped at the same nest their share of the feeds reflected their share of matings, not their dominance rank. When only one male helped at a nest, alpha males decreased their amount of help with decreased mating share, whereas beta males did not. This difference may arise because alpha males have greater chances of mating with other females in the group, so opportunity costs of helping are greater for them. How these different helping responses influence female preference for alpha versus beta males is discussed.

Food-dependent benefits of hatching asynchrony in American kestrels Falco sparverius

Food supply and hatching asynchrony were manipulated for 90 broods of American kestrels (Falco sparverius) during 1989–1991. We measured the growth and mortality of nestlings within four treatment groups (asynchronous, synchronous, food-supplemented, unsupplemented) to test the brood reduction hypothesis of Lack (1947, 1954). Fledging success did not differ between synchronous and asynchronous broods when food was poor but consistent with the brood reduction hypothesis, nestlings died at a younger age in asynchronous broods. When food was supplemented, mortality did not occur in the synchronous broods but youngest nestlings still died in asynchronous nests despite apparently adequate food for the brood. Oldest nestlings in asynchronous broods fledged with a greater mass than their younger siblings, also consistent with Lack's hypothesis. Average nestling quality in synchronous broods was very dependent on food levels. Synchronous young that were supplemented were, on average, the heaviest of any treatment group but young from unsupplemented synchronous broods were the lightest. Overall, patterns of mortality and growth for kestrels support the brood reduction hypothesis when food is limited, but not when it is abundant. This food-dependent benefit of asynchrony in the nestling period is a prerequisite for facultatively adjusted hatching spans during laying.

Advantages of Hatching Synchrony in the Pied Flycatcher Ficedula hypoleuca

Many bird species begin incubation before the clutch is complete, which leads to asynchronous hatching of the young. Several hypotheses have been proposed to explain this hatching pattern, most of them assuming that asynchronous hatching is of adaptive value. Pied Flycatcher broods were induced to hatch either synchronously or asynchronously in order to compare the breeding success resulting from these two hatching patterns. Experimental maximization of hatching synchrony led to an increase in the production of young. Synchronously hatched young also had longer tarsi and higher fledging weights than young from asynchronous broods. Synchronous hatching was favourable in both years of study, even though one was cold and rainy and the other warm and dry. Survivors in asynchronous broods in which some mortality had occurred were not in better condition than young in synchronous and unreduced broods, in contrast to what the brood reduction hypothesis predicts. Thus, this study does not support Lack's (1947, 1954) original presentation of this hypothesis as an explanation for the presence of the semi-asynchronous hatching pattern occurring naturally in Pied Flycatchers. However, the raising of experimentally synchronous broods might induce a larger parental effort perhaps affecting parental survival probability or subsequent fecundity. It is not possible to decide whether the absence of complete synchrony in the Pied Flycatcher is due to hormonal constraints, or whether some factor, such as predation on the nest contents (eggs, young or adults), may also make hatching asynchrony adaptive.

Selection by sexual conflict for evenly spaced offspring in blue tits

IN animals with parental care, parents rearing offspring of variable ages are typically assumed to exert less effort than those rearing even-aged offspring. This is because spaced births spread out peak loads in the combined food demands of all offspring1–3. Creating a mixed-size sibship also helps establish a hierarchy among the young which reduces the costs of sibling rivalry4 and can help efficient elimination of young if food becomes short5,6. We manipulated hatching spread within broods of the blue tit Parus caerulem and studied postbreeding survival rate of the adults. We report here that, contrary to current theory, female parents suffer less when the young are even-aged than when they are of variable ages, whereas the opposite result was found for male parents. Apparently, the male contributes more in synchronous broods, thus lightening the female's total investment burden. In blue tits this sexual conflict over hatching pattern is won by the female because she alone incubates. By delaying incubation until most eggs have been laid, she reduces hatching span.

Food Supply and Hatching Spans of Birds: Energy Constraints or Facultative Manipulation

Clutches of altricial birds generally hatch over a period of hours or days. Numerous hypotheses have been generated to explain hatching asynchrony, but most experiments have focused on the nestling stage to determine some post—hatch benefit such as increased reproductive success. We concentrated on proximate causes in the prelaying period, which have largely been ignored. In northern Saskatchewan, we censused prey species (small mammals) in early spring and measured the asynchrony of 273 clutches of the American Kestrel (Falco sparverius) over 4 yr. Hatching asynchrony was quantified in two ways: time for all the eggs within a clutch to hatch and mass hierarchies among nestlings. In good food years, the hatchling spans of clutches were more synchronous than in poor years. Similarly, parents on good territories or females in good physical condition had more synchronous broods than parents under food stress. In an experiment, we supplemented food to kestrels during the prelaying period. Parents that were supplemented laid larger eggs and hatched those eggs more synchronously, but control pairs were more successful at raising young to the fledging stage. Synchronous hatching when food is abundant is consistent with the hypothesis of facultative manipulation of hatching spans in relation to nestling food supply, but not with the energy constraint hypothesis.

Hatching asynchrony, sibling hierarchies and brood reduction in the Chinstrap penguin Pygoscelis antarctica

We studied patterns of chick growth and mortality in relation to egg size and hatching asynchrony during two breeding seasons (1991 and 1992) in a colony of chinstrap penguins sited in the Vapour Col rookery, Deception Island, South Shetlands. Intraclutch variability in egg size was slight and not related to chick asymmetry at hatching. Hatching was asynchronous in 78% (1991) and 69% (1992) of the clutches, asynchrony ranging from 1 to 4 days (on average 0.9 in 1991 and 1.0 days in 1992). Chicks resulting from oneegg clutches grew better than chicks in families of two in 1991. In 1992, single chicks grew to the same size and mass at 46 days of age as chicks of broods of two, suggesting food limitation in 1991 but not in 1992. In 1991, asymmetry between siblings in mass and flipper length was significantly greater in asynchronous than in synchronous families during the initial guard stage, but these differences disappeared during the later creche phase. In 1992, asymmetry in body mass increased with hatching asynchrony and decreased with age. Only the effect of age was significant for flipper length and culmen. Asymmetries at 15 days were similar in both years, but significantly lower in 1992 than in 1991 at 46 days of age. There were relatively frequent reversals of size hierarchies during both phases of chick growth in the two years, reversals being more common in 1991 than in 1992 for small chicks. In 1991, survivors of brood reduction grew significantly worse than chicks in nonreduced broods. In both years, chicks of synchronous broods attained similarly large sizes before fledging as both A and B chicks of asynchronous broods. In 1991, chick mortality rate increased during the guard stage due to parental desertions, decreased during the transition to creches (occurs at a mean age of 29 days) and returned to high constant levels during the creche stage, when it is mostly due to starvation (in total 66% of hatched chicks survived to fledging). In contrast, in 1992, mortality was relatively high immediately after hatching and almost absent for chicks older than 3 weeks (87% of chicks survived to fledging). Mortality affected similarly one- and two-chick families. In 1991, asynchronous families suffered a significantly greater probability of brood reduction than synchronous families, but this probability was not significantly related to degree of asymmetry between siblings. No association between asynchrony and mortality was found in 1992. These results show that there is food limitation in this population during the creche phase in some years, that asynchronous hatching does not facilitate early brood reduction and that it does not ensure stable size hierarchies between siblings. Brood reduction due to starvation is not associated to prior asymmetry and does not facilitate the survival or improve the growth of the surviving chick. Asynchronous hatching may be a consequence of thermal constraints on embryo development inducing incubation of eggs as soon as they are laid.

Hatching Asynchrony and Feeding Rates in Yellow Warblers: A Test of the Sexual Conflict Hypothesis

T. Slagsvold and J. T. Lifjeld have proposed that hatching asynchrony in bird species in which only the female incubates is a strategy used by females to extract more parental investment from their mates. We examined this hypothesis by comparing feeding rates of female and male yellow warblers (Dendroica petechia) tending asynchronously and synchronously hatched broods. Regardless of brood size, feeding rates during the early, middle, and late nestling periods were similar for females tending asynchronously and synchronously hatched broods. Nevertheless, there was a general trend for females tending asynchronous broods to have lower feeding rates, which suggests that females may prefer hatching asynchrony. This is consistent with the sexual conflict hypothesis. Male feeding rates exhibited trends similar to those of females. That is, although the result is not statistically significant, males tending asynchronous broods, regardless of brood size, generally made fewer feeds than males tending synchronously hatched broods. However, males tending asynchronous broods of five did make significantly fewer feeds during the early nestling period than males tending similar-sized synchronous broods. This was also true when the data were combined across brood sizes. This suggests, contrary to the sexual conflict hypothesis, that males, especially those tending larger broods, should favor hatching asynchrony. Consequently, the sexes do not appear to be in conflict as to when incubation should begin, but rather females and males prefer, or at the very least are neutral with respect to, the initiation of incubation prior to clutch completion.

Avian Hatching Asynchrony: Brood Classification Based on Discriminant Function Analysis of Nestling Masses

Studies of hatching asynchrony in altricial birds have used several methods to determine if a brood hatches synchronously or asynchronously. The most commonly used methods include making frequent nest visits during the time of hatching and using nestling mass or other measures of size on a given day to estimate nestling age or to describe the extent of the size hierarchy within broods. We made daily nest visits to determine if House Wren (Troglodytes aedon) broods hatched synchronously or asynchronously, and then compared classification of these broods based on discriminant function analysis with that based on mass differences of nestlings. The best misclassification rate based on dis? criminant function analysis was 17.5%, whereas the rates based on mass differences (23.1%) and estimates of nestling age (27.5%) were higher, but not significantly higher. The most frequent error occurring in discriminant function analysis was misclassifying asynchronous broods as synchronous broods, whereas the most frequent error based on mass differences of nestlings was misclassifying synchronous broods as asynchronous ones. Discriminant function analysis based on coefficient of variation of nestling mass and number of nestlings was about as effective (error rate = 17.5%) as was classification using individual masses. We recommend that investigators who wish to categorize broods objectively as synchronous or asynchronous based on nestling masses or other measures of size should use a discrim? inant function analysis.

An Experimental Study of Brood Reduction and Hatching Asynchrony in Yellow Warblers

The purpose of this study was to examine Lack's (1947, 1954) brood-reduction hypothesis concerning hatching asynchrony, and its importance in explaining the evolution of hatching asynchrony in Yellow Warblers (Dendroica petechia). The study was conducted during the summers of 1988-1990 on a population of Yellow Warblers breeding in the dune-ridge forest at Delta Marsh, Manitoba, Canada (58011'N, 98?19'W). I examined the brood-reduction hypothesis experimentally by inducing a sample of females to delay incubation until clutch completion. This was achieved by removing eggs as they were laid, and placing them in a cooler at 200C until clutch completion. As a result, eggs in each manipulated clutch hatched synchronously (hatch spread between firstand last-hatched nestling ?24 hr). Subsequently, I compared the fledging mass and survival rate of nestlings in asynchronously (unmanipulated broods, hatch spread > 24 hr) and synchronously (manipulated) hatching broods. Consistent with the hypothesis, nestling mortality was concentrated in last-hatched nestlings in asynchronous broods and random with respect to hatch order in synchronous broods. Also in agreement with the hypothesis, fledging mass and survival rates of first-hatched chicks were generally higher in asynchronous broods. However, contrary to the hypothesis, fledging mass and survival rates were similar for lasthatched chicks in asynchronous and synchronous broods. Also consistent with the hypothesis, fledging success (no. fledged/clutch) in broods of five tended to be higher in asynchronously hatched broods, regardless of food availability. Finally, fledging rate (no. fledged/no. hatched) was significantly greater in asynchronous broods (four and five nestlings combined) when compared to synchronous broods, especially when food was limiting.

Experimental taphonomy of embryo preservation in a Cenozoic brooding bivalve

To distinguish between alternative explanations for the presence of synchronous broods in the Miocene-Pliocene bivalve, Transenriella species. we performed in situ burial experiments of the Recent species T. corfusa. All Recent Transennella species are asynchronous brooders; a single brood contains all or most developmental stages. Specimens from Miocene—Pliocene deposits of California suggest that some members of this taxon were synchronous brooders, i.e., all the embryos of a brood develop simultaneously with only one developmental stage represented at any time. The presence of synchronous Transennella broods in the Miocene—Pliocene could indicate that an evolutionary change in mode of reproduction has occurred in this genus. Alternatively, asynchronous brooding in this taxon may he conservative and preferential preservation of later stages of development, or seasonal variation in reproduction, could result in a taphonomic overprint. Our burial experiments indicate that the earliest stages of development are almost entirely lost; however. there is enough preservation of the later stages of development to distinguish the two modes of reproduction. Additionally. we discovered a single fossil specimen with an asynchronous brood. Based primarily on this specimen, and observations from the burial experiments, we conclude that the fossil synchronous broods are an artifact of preservation and asynchronous brooding in Transenella is conservative.

The Value of Male Parental Care and its Influence on Reproductive Allocation by Male and Female Dunnocks

The variable mating system of the dunnock enabled us to measure how different amounts of male help influenced reproductive success. Male help became increasingly valuable as brood size increased, but the addition of one male's help increased success more than the addition of a second male's help. Removal experiments confirmed that differences in reproductive success were caused by differences in male help. Females laid a larger clutch when they expected two males' full-time help than when expected male help was less. We used data on the value of male care to predict how males should allocate their effort betwenn two synchronous broods

Hatching asynchrony and brood reduction in the jackass penguin: an experimental study

The brood reduction hypothesis for asynchronous hatching was tested for the jackass penguin, Spheniscus demersus, by comparing the reproductive success of two-chick broods with different degrees of asynchrony, and by identifying the behavioural basis of competitive asymmetries between siblings. By interchanging newly hatched chicks between nests, broods of synchronous and extended asynchronous siblings were created. There was no significant difference in fledging success between unmanipulated normal asynchronous (mean number of chicks per brood=1·14) and synchronous (1·44) broods; however, extended asynchronous (0·6) broods lost significantly more chicks than normal asynchronous ones. Last-hatched chicks in normal asynchronous broods were more likely to die than their siblings, and all last-hatched chicks in extended asynchronous broods failed to fledge. Chicks in normal asynchronous broods were not significantly heavier at fledging, but did attain higher maximum body weights before fledging, and fledged sooner than chicks in synchronous broods. This appeared to be due to higher begging success and lower sibling competition during feeding by asynchronous chicks. In terms of chick survival, the brood reduction hypothesis does not provide an explanation for hatching asynchrony in the jackass penguin. Asynchrony may, however, provide an advantage with respect to fledgling quality and post-fledging survival, as predicted by extensions of both the brood reduction and the sibling rivalry reduction hypotheses.