Parental conflict and brood desertion by females in blue-headed vireos

We investigated sexual conflict over parental care in blue-headed vireos (Vireo solitarius) and documented the first example of unvarying unisexual brood desertion in passerines. Females at all nests (N = 24) that were monitored closely near fledgling, deserted their broods on or near the day of fledging leaving males alone to complete parental care of young. No males deserted. This observational evidence was confirmed with radiotracking of females (2004, 2007) and both pair members (2008). Radiotracked females began visiting distant males 1–4 days before young left the nest, subsequently paired with males 355–802 m away, and laid first eggs in new nests less than 5 days after deserting. In contrast, females suffering nest predation did not desert and renested with the same male. We suggest equal parental care (nest building, incubation, feeding) in the sexes, genetic monogamy, and an adult sex ratio biased towards males has led to female control of brood desertion in this species. Unisexual desertion may be more important in altricial birds than generally realized and we discuss prerequisites to predict its occurrence. One is genetic monogamy, which may be a female tactic that reduces the likelihood of males evolving counter-adaptations to female desertion.     

The dynamics of brood desertion among communally breeding females in the treehopper, Publilia concava

In species that exhibit extended parental care, females sometimes lay eggs communally in order to redistribute the costs of offspring care. Communal egg-laying often involves redundancy in female effort, such that the number of females contributing to reproduction is more than is needed to provide adequate parental care. As a result, a subset of females will often abandon the communal brood, with the time of departure ranging from immediate abandonment after egg-laying (brood parasitism) to delayed abandonment with prolonged care (cooperative breeding). In this paper I approach the parental care dynamics of female-female broods as a desertion game similar to that of mate desertion in species with bi-parental care. I describe a field study of the insect Publilia concava (Hemiptera: Membracidae), a species that exhibits communal oviposition and a full range of egg guarding. This species exhibits full redundancy in female care, with no difference in survival between singly and doubly guarded broods. I find that double guarding is extremely rare in the population, with most communal broods having only one female guard. While this guard was usually the female that initiated the brood, these same females were more likely to abandon when secondary females arrived and when secondary females exhibited longer guarding durations. Paradoxically, the secondary females usually abandoned the broods they visited, resulting in up to 50% of broods with double abandonment. These unguarded broods suffered a 50% reduction in hatching success, reflecting an important risk for primary females that abandon egg masses to secondary females. Overall, P. concava exhibits desertion dynamics similar to mate desertion in vertebrates and it is likely that the theoretical work in this area will be useful for future work that addresses the allocation of parental care among communal breeders.     

Determinants of parental effort: a behavioural study in the Eurasian kestrel,Falco tinnunculus

We recorded behaviour of kestrels (Falco tinnunculus) in western Finland during the courtship (1988–1992), incubation (1989–1991), early nestling (age of young 1–2 weeks, 1989–1992) and late nestling stages (3–4 weeks, 1989–1991) to examine determinants of their parental effort (PE). In males, PE was estimated as the hunting effort (the proportion of budget time spent in flight-hunting) and in females as the food provisioning rate (number of prey items delivered to the nest per hour). The following predictions derived from the parental investment theory were examined. (1) Parents rearing large clutches and broods should invest more in breeding than do parents rearing small clutches and broods. The hunting effort of parents did not increase with clutch or brood size, but males tending large broods had a higher prey delivery rate than males tending small broods (Figs 1–2). (2) PE of parents should increase in the course of the breeding season. In males, this was true only between the incubation and early nestling phases (Fig. 3). (3) The early pairs should invest more in breeding than late ones. This tended to be true during the early (for males) and late nestling phases (for females) (Fig. 4). (4) There should be a negative correlation between PE of mates within pairs, but no evidence for such adjustment was found (Fig. 5). (5) Females mated with bright-coloured attractive males should show higher PE than females mated with dull-coloured males but our results were inconsistent with this prediction. We conclude that PE decisions of kestrels are mainly based on cost-benefit estimates of residual reproductive value, rather than on current investment indicators, like clutch or brood size. This might be beneficial in environments with highly variable survival prospects of offspring caused by pronounced among-year variation in abundance of the main food (microtine rodents). The results also show that hypotheses explaining variation in PE in the short term are not necessarily valid for long-term PE, e.g. tending clutches or broods, which also reflects the demands of female and young.     

Nest desertion is not predicted by cuckoldry in the Eurasian penduline tit

Engagement in extra-pair copulations is an example of the abundant conflicting interests between males and females over reproduction. Potential benefits for females and the risk of cuckoldry for males are expected to have important implications on the evolution of parental care. However, whether parents adjust parental care in response to parentage remains unclear. In Eurasian penduline tits Remiz pendulinus, which are small polygamous songbirds, parental care is carried out either by the male or by the female. In addition, one third of clutches is deserted by both male and female. Desertion takes place during the egg-laying phase. Using genotypes of nine microsatellite loci of 443 offspring and 211 adults, we test whether extra-pair paternity predicts parental care. We expect males to be more likely to desert cuckolded broods, whereas we expect females, if they obtain benefits from having multiple sires, to be more likely to care for broods with multiple paternity. Our results suggest that parental care is not adjusted to parentage on an ecological timescale. Furthermore, we found that male attractiveness does not predict cuckoldry, and we found no evidence for indirect benefits for females (i.e., increased growth rates or heterozygosity of extra-pair offspring). We argue that male Eurasian penduline tits may not be able to assess the risk of cuckoldry; thus, a direct association with parental care is unlikely to evolve. However, timing of desertion (i.e., when to desert during the egg-laying phase) may be influenced by the risk of cuckoldry. Future work applying extensive gene sequencing and quantitative genetics is likely to further our understanding of how selection may influence the association between parentage and parental care.     

Cooperative breeding in purple gallinules: the role of helpers in feeding chicks

Summary The participation of breeders and helpers in the feeding of 21 broods of chicks was studied in a population of cooperatively breeding purple gallinules (Porphyrula martinica). In the breeding group, all birds over the age of 2 months fed chicks. Female breeders fed chicks at the highest rate, followed by male breeders and adult helpers, old juvenile helpers, and young juvenile helpers. The amount of food breeders fed chicks was independent of the number of helpers in the breeding group. However, breeders made fewer feeding visits when they had helpers. Male and female breeders spent similar amounts of time feeding chicks. Helpers spent significantly less time feeding chicks than did breeders. As helpers grew older they fed chicks at a faster rate, made more feeding visits and spent more time feeding chicks. Analysis of variance was used to determine which variables explained the variation in the brood feeding rate (amount of food delivered to an entire brood during one observation). Age of chicks had a significant nonlinear effect, and size of brood and number of helpers had significant linear effects on the brood feeding rate. Chicks in groups with helpers received more food and were accompanied for longer periods of time than chicks in groups without helpers; either or both of these factors may have led to increased chick survival.     

Offspring protection by merlin Falco columbarius females; the importance of brood size and expected offspring survival for defense of young

Summary Nest predation was simulated by presenting a stuffed raven close to nests of merlins. This was done to examine the influence of brood size related factors on female defence intensity. The original clutch size did not affect nest defense after hatching, but brood size was important. It determined the attack frequency and, for each brood size level, the proportion of attacking females. When brood sizes were manipulated, the defense intensity increased and decreased, respectively, in relation to the size of the brood. In addition, broods with high future survival (first broods) were defended more vigorously than broods with low future survival (replacement broods). Hence, expected benefits in terms of fledgling production and chick survival seem to be important determinants of female investment in offspring protection. The lower predation rate among females responding with overt aggression to the raven compared to that of less aggressive females suggests that defence of young is beneficial.     

Costs and benefits of temporary brood desertion in a Neotropical harvestman (Arachnida: Opiliones)

Brood desertion is a life history strategy that allows parents to minimize costs related to parental care and increase their future fecundity. The harvestman Neosadocus maximus is an interesting model organism to study costs and benefits of temporary brood desertion because females abandon their clutches periodically and keep adding eggs to their clutches for some weeks. In this study, we tested if temporary brood desertion (a) imposes a cost to caring females by increasing the risk of egg predation and (b) offers a benefit to caring females by increasing fecundity as a result of increased foraging opportunities. With intensive field observations followed by a model selection approach, we showed that the proportion of consumed eggs was very low during the day and it was not influenced by the frequency of brood desertion. The proportion of consumed eggs was higher at night and it was negatively related to the frequency of brood desertion. However, frequent brood desertion did not result in higher fecundity, measured both as the number of eggs added to the current clutch and the probability of laying a second clutch over the course of the reproductive season. Considering that harvestmen are sensitive to dehydration, brood desertion during the day may attenuate the physiological stress of remaining exposed on the vegetation. Moreover, since brood desertion is higher during the day, when egg predation pressure is lower, caring females could be adjusting their maternal effort to the temporal variation in predation risk, which is regarded as the main cost of brood desertion in ectotherms.     

Common goldeneyes adjust maternal effort in relation to prior brood success and not current brood size

Parental investment theory predicts that parental effort should be related to the reproductive value of the current brood. This depends on both the number of young and the survival prospects of each of them. Thus parents may provide more care to larger broods either because of (1) the direct effect of brood size per se on reproductive value (the “brood size” hypothesis) or because (2) past mortality, reflected in current brood size, predicts future mortality of the brood and hence its reproductive value (the “brood success” hypothesis). Earlier studies have not attempted to distinguish between these alternatives. We tested the hypotheses in the precocial, nidifugous common goldeneye Bucephala clangula, a species with uniparental female care. Maternal effort was measured as the time spent by the female in rearing the brood. We found that brood size itself is not associated with maternal effort, but that females modify their maternal effort according to the mortality already experienced by the brood, supporting the prediction of the brood success hypothesis. We also found that brood mortality varied considerably between broods and that previous mortality predicts future mortality within broods, basic assumptions of the brood success hypothesis. Received: 30 January 1996 / Accepted after revision: 27 October 1996     

Influences of sex,sex composition of brood and hatching order on mass growth in common terns<Emphasis Type="Italic"> Sterna hirundo</Emphasis>

We studied sex-dependent mass growth of chicks of the monomorphic common tern Sterna hirundo which fledged at a colony site in Wilhelmshaven, Germany, between 1997 and 1999. Brood size, brood mates' sex and hatching order (a-, b- and c-chicks) were known for many individuals, which were sexed by molecular techniques. Daily growth rates and age of fledging were independent of sex. However, in all years male chicks reached higher peak mass and fledged up to 5.2% heavier than female chicks. Broods with at least two fledglings showed that besides sex, brood size interacting with sex composition affected chick mass. In mixed broods, brothers had higher peak and pre-fledging mass than sisters they were reared with. Especially in the combination a-daughter and b-son the brothers were heavier. Lowest mass was found in broods with three nest mates of the same sex. A detailed study of 24 three-fledgling broods showed that male c-chicks were heavier than their siblings. The results reveal an advantage for chicks in mixed broods, especially for sons, and more especially if the son was a c-chick. Higher mass and possibly dominance of sons in c-position might be related to higher maternal androgen levels, which are known to increase with each egg laid. The results suggest that even in a monomorphic species, sons might be more expensive to rear, and are discussed with respect to sibling competition, parental effort, survival of sons, as well as to fitness benefits favoring parents producing sons.     

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).     

Parental condition, brood sex ratio and differential young survival: an experimental study in gulls (Larus fuscus)

Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios. Received: 15 February 2000 / Revised: 7 August 2000 / Accepted: 26 August 2000     

Brood dispersal and multiple central place foraging by Lapland longspur parents

Summary Lapland longspur chicks continued to be fed by their parents for 2 weeks after nest departure. Shortly after they left the nest, broods were divided evenly into two units each tended by a single parent. Female-tended brood units dispersed away from the nest at a faster average distance per day than those tended by males. The distance between offspring within a brood unit also increased as chicks got older. For the first 8–10 d after nest departure, parents were multiple central place foragers, making 1–8 foraging trips away from each dispersed chick (i.e. the central place) before moving on to feed another chick in a similar fashion. During the final 5–7 d before independence, chicks were quite mobile and followed parents on their foraging bouts. A simulation model shows that brood dispersal reduced total parental travel time per chick, primarily because each chick moved closer to a foraging site. By comparing models for a variety of brood division and foraging itinerary scenarios, we also show that multiple central place foraging by parents visiting different, separated brood units is less energetically expensive than other reasonable alternatives.Although brood dispersal is usually considered to be an adaptation for avoiding predation, it can also help parents reduce the energetic costs of parental care.     

Asymmetric costs favor female desertion in the facultatively polyandrous northern flicker (Colaptes auratus): a removal experiment

Previous studies of biparental care in birds have focused on the males contribution and experimental removal of males. Woodpeckers, with a high level of paternal care including nocturnal incubation and brooding by males, offer a meaningful system in which to examine the importance of care by females. I studied the reproductive performance of 17 widowed male and 8 widowed female northern flickers (Colaptes auratus) relative to biparental (control) pairs. Of these single parents all widowed shortly after hatching, only one female abandoned its nest. Single parents boosted their provisioning rates to achieve 83% the provisioning rate of control broods, but reared significantly fewer young and young of poorer quality. However, single males, with 85% of the reproductive success of controls, were more successful than females with 43% the success of controls. Among widowed birds, a not significant lower survival was observed, but the chance of re-pairing with the same partner in a subsequent year was only 16% in the natural population, so long-term costs of desertion may be small. Although females seem to have the incentive and ability to desert, a lack of available males may constrain opportunities. This study demonstrates that when parental care roles are reversed in altricial birds, asymmetric benefits can favor female and not male desertion.     

Resource allocation varies with parental sex and brood size in the asynchronously hatching green-rumped parrotlet (<Emphasis Type="Italic">Forpus passerinus</Emphasis>)

When eggs hatch asynchronously, offspring arising from last-hatched eggs often exhibit a competitive disadvantage compared with their older, larger nestmates. Strong sibling competition might result in a pattern of resource allocation favoring larger nestlings, but active food allocation towards smaller offspring may compensate for the negative effects of asynchronous hatching. We examined patterns of resource allocation by green-rumped parrotlet parents to small and large broods under control and food-supplemented conditions. There was no difference between parents and among brood sizes in visit rate or number of feeds delivered, although females spent marginally more time in the nest than males. Both male and female parents preferentially fed offspring that had a higher begging effort than the remainder of the brood. Mean begging levels did not differ between small and large broods, but smaller offspring begged more than their older nestmates in large broods. Male parents fed small offspring less often in both brood sizes. Female parents fed offspring evenly in small broods, while in large broods they fed smaller offspring more frequently, with the exception of the very last hatched individual. These data suggest male parrotlets exhibit a feeding preference for larger offspring—possibly arising from the outcome of sibling competition—but that females practice active food allocation, particularly in larger brood sizes. These differential patterns of resource allocation between the sexes are consistent with other studies of parrots and may reflect some level of female compensation for the limitations imposed on smaller offspring by hatching asynchrony.     

How Kentish plovers,<Emphasis Type="Italic"> Charadrius alexandrinus</Emphasis>, cope with heat stress during incubation

Biparental incubation is frequent among shorebirds and is expected when the survival prospects of offspring increase relative to uniparental incubation. To understand why this occurs, it is important to identify the factors that constrain uniparental incubation. It is assumed that birds choose nesting sites that provide an appropriate microclimate for incubation. Many shorebirds nest in sites with no or little cover, where ambient temperatures at ground level might be >50°C during very hot days. Shorebirds nest in exposed sites because predation risk on incubating adults is higher in covered sites. In hot environments, incubating shorebirds might experience heat stress in exposed sites, and this may compromise nesting success if adults are unable to attend their nests continuously, limiting the possibilities of uniparental incubation and thus the expression of a sexual conflict over incubation. The operative temperatures of Kentish plovers (Charadrius alexandrinus) were recorded in exposed and covered sites, and the thermal behaviour and incubating tactics of pair members were studied in a hot environment. During the hottest part of the day, there was a difference of 10–15°C in the operative temperatures of plovers between covered and exposed sites. Plovers in covered sites did not exhibit any thermoregulatory behaviour indicative of thermal stress, probably because the thermal range encountered by them in such places during most of the daytime was close to the thermo-neutral zone. The frequency with which plovers in exposed sites exhibited thermoregulatory behaviour was related to ambient temperature. Under very hot conditions, incubating birds were probably unable to maintain homeostasis for long periods and pair members resorted to shortening incubation bouts. Female Kentish plovers mainly incubate in the daytime and males during the night. However, the probability of diurnal incubation by males increased with ambient temperature in exposed nests, but not in covered ones. In fact, the frequency of participation in diurnal incubation by males was greater in exposed than in covered sites, suggesting that the participation of males in diurnal incubation may be related to the inability of females to stay at the nest during long periods when the ambient temperature is high. Even after resorting to shortened incubation bouts, the plovers may be unable to attend their nests continuously during heat waves, and the nests may be deserted. The propensity of plovers to desert their nests was affected by proximity to water, with nests located close to water being deserted less frequently. It seems likely that susceptibility to thermal stress changed in relation to proximity to water because in sites close to water it was possible to belly-soak, which would allow a more continuous nest attendance. Therefore, despite the adoption of behavioural solutions to face heavy heat loads, nesting success was vulnerable to these solutions because heat stress during extended periods may constrain parental nest attendance, and this may limit the opportunities for sexual conflicts over incubation.Communicated by J. Graves     

Filial cannibalism in a nest-guarding fish: females prefer to spawn in nests with few eggs over many

In fish, fecundity correlates with female body size and egg-tending males often eat small broods. Therefore, small females may prefer to spawn in nests that already contain many eggs, to ensure the brood is as large as possible. In contrast, large females may prefer nests with few eggs, if high egg number or density has a negative effect on egg survival, or if there are drawbacks of spawning last in a nest. To test the hypothesis that female body size affects nest (and male mate) choice, using the sand goby (Pomatoschistus minutus), we allowed small and large females to choose between two males that were matched in size — one guarding a small clutch and the other a large clutch, respectively. We recorded where females spawned (measure of female preference), the combined brood size, male courtship, egg care and nest building. We also quantified the effect of brood size and egg density on egg survival in a separate data set. Although the combined broods did not exceed the small brood sizes that are at risk of being eaten, both small and large females preferred to spawn in nests with smaller clutch sizes. This preference could not be explained by more courtship or male parental effort, nor by reduced survival of larger or denser broods. Instead, our result might be explained by females avoiding the danger of cannibalism of young eggs by males or the risk of reduced egg health associated with being near the nest periphery.     

Parental care sex differences in the brown bullhead,Ictalurus nebulosus (Pisces,Ictaluridae)

Summary Males were the principal care-givers in the brown bullhead, Ictalurus nebulosus. Direct observations on individually tagged adults in nature revealed that males attended broods at closer distances than females and were the most frequent lone care-givers. However, females often participated in parental care and in some cases assumed the typical male role in the absence of the male. In the presence of her mate, females generally performed different care-giving functions than males. Females attended broods at a distance and chased other fishes more frequently than males. Males remained directly over the brood guarding. A series of mated pairs in each of three reproductive conditions were captured and examined. Different pairs were compared between reproductive conditions for body weight, body condition, gonadal-somatic index, and a gut-contents index. The inital costs of reproduction, as measured by weight loss, condition change, and gonadal-somatic index change, were more severe in females than males. During breeding and subsequent care-giving, males did little or no feeding whereas females increased their feeding during the same time period. However, males appeared to sustain only minor weight and condition losses due to care-giving. I suggest that males were the principal care-givers because the net benefits of remaining at the nest were greater than the net benefits of leaving. Males had little opportunity to mate with more than one female each season, and offspring had little chance of survival in the absence of a care-giving adult. Considerable variation in female behavior occurred which suggests that the net benefits of care-giving were nearly balanced with the net benefits of leaving the brood. The relative importance of feeding versus care-giving by females may vary with slight differences between females and the behavior of their mates as care-givers.Present address: Department of Zoology, Ohio State University, 1735 Neil Avenue, Columbus, Ohio 43210, USA     

Effects of territory quality, food availability and sibling competition on the fledging success of oystercatchers (Haematopus ostralegus)

We investigated the fledging probability of oystercatcher, Haematopus ostralegus, chicks as a function of hatching order, brood size, territory quality and food availability. Sibling dominance was related to the hatching order in both low- (’leapfrogs’) and high-quality (’residents’) territories. Differences in hatchling mass might have aided the establishment of a dominance hierarchy, since breeders produced small late eggs and hatchlings. These mass differences were most pronounced in leapfrogs, and in large broods in years with lower food availability (’poor’ years). Late hatchlings fledged less often and with lower body masses compared to early hatchlings in all situations. Leapfrogs produced smaller broods and hatched their broods more asynchronously in poor years than leapfrogs breeding in years with more available food (’good’ years) and residents breeding in both poor and good years. Large brood sizes resulted in lower survival of hatchlings in poor years. These results favour the ’brood reduction’ hypothesis. However, contrary to the expectations of this hypothesis, hatching order also affected fledging success in residents. Moreover, large brood size resulted in higher survival of hatchlings in good years, particularly in residents. Thus, although large broods experienced losses due to sibling competition in some years, they nevertheless consistently produced more fledglings per brood in all years, both as leapfrogs and residents. We believe this effect is due to parental quality correlating with initial brood size. Most leapfrogs, at best, fledged one chick successfully each year, losing chicks due to starvation. Nevertheless, leapfrog broods were reduced in size after hatching significantly less quickly than resident broods. These results suggest that breeders lay and hatch insurance eggs to compensate for unpredictable losses due to the high predation rates on both nests (ca 50%) and chicks (ca 90%), in accordance with the ’nest failure’ hypothesis. Received: 14 February 2000 / Revised: 27 September 2000 / Accepted: 10 June 2000     

Brood guarding and the evolution of male parental care in burying beetles

Summary Parental behavior that has an impact on the increased survival of offspring, an important factor in the evolution of parental care, can include both guarding and provisioning. The effects of these two components of parental care can be separated and quantified in the burying beetle Nicrophorus orbicollis in which both male and female cooperate to rear young. Although in the absence of competition, reproductive success is reduced by the presence of the second parent in the brood chamber, two parents dramatically reduce the probability that conspecifics will usurp the resource, replace either the male or female, kill the newly hatched brood, and produce a replacement clutch. After the establishment of the burial chamber (but not before) beetles appear to assist their mates in driving off intrasexual competitors. Male assistance in burial does not account for very much of the variance in the speed in which the carcass can be concealed nor are two parents essential to guard against insect predators. There were no significant differences in the duration of parental care by males paired with virgin and non-virgin females suggesting that paternity of the brood for which the male provides care is not a factor determining the length of care. Since male and female reproductive success is limited in Nicrophorus by access to suitable carcasses, many of the typical asymmetries in the costs and benefits of parental care are lacking. However since sperm displacement is not complete, paternity of the replacement clutch, for which the male does not provide care, may be a factor encouraging male desertion before female desertion. Other factors important in the evolution of paternal care, especially the probability of additional reproductive opportunities, are discussed.     

Adjustment of parental effort in the puffin; the roles of adult body condition and chick size

 We examined the adjustment of parental effort of puffins by switching 20-day-old chicks randomly between parents of known body condition. Among unmanipulated birds mass gain (5–20 days) and mass of 20-day-old chicks was positively correlated with the body condition of parents at day 6. During the first 5 days after chick switching 28% (n = 55) of the parents deserted their foster chick. Parents which deserted their foster chick originally had a chick of their own that was smaller than that of those which did not desert their foster chick. Whether parents deserted their foster chick was also negatively related to the size of the foster chick. The mass of the foster chick was more important than the size of the parents' own chick in determining the desertion rate of chicks. The mass gain of the foster chick during the first 5 days after switching was positively related to the body condition of foster parents and also positively related to the mass of the foster parents' own chick, but negatively related to the size of the foster chick. The results suggest that puffins adjust their parental effort according to both their own body condition and the size of the chick. The latter may indicate the chick's prospect of survival and recruitment to the population. Received: 20 January 1996 / Accepted after revision: 27 October 1996