Chick survival in relation to nest site: is the Antarctic petrel hiding from its predator?

In birds, the characteristics of the nest site may affect reproductive success. We found that shelter is an important characteristic of the Antarctic petrel (Thalassoica antarctica) nests because shelter prevents chick predation. However, the benefit of shelter was countervailed by melt water which mainly entered well-sheltered nests. Chick survival was monitored until the chick was left unattended for the first time. Late-hatched chicks had a higher survival probability than early-hatched chicks, possibly because late hatchers swamp the predator, the south polar skua (Catharacta maccormicki). Poorly sheltered nests tended to be occupied by parents with low body mass and late-hatched eggs. The results suggest that both shelter per se and parental characteristics may explain the relationship between predation risk and shelter. We need experiments to study the influence of nest site on reproductive success, and we need to map the frequency of melt water as a cause of reproductive failure.     

Nest positioning by male Daito White‐eyes Zosterops japonicus daitoensis improves with age to reduce nest predation risk

Age‐related improvement in reproductive success is widely observed in birds, and the mechanisms by which productivity is enhanced have received considerable attention. However, little is known about how parental age affects the loss of eggs or nestlings despite the fact that age effects on nesting success are often reported. We examined parental age effects on reproductive success in relation to the avoidance of nest predation in an island subspecies of the Japanese White‐eye, the Daito White‐eye Zosterops japonicus daitoensis. Clutch size and annual number of breeding attempts did not differ between parental age classes. Reproductive success was affected only by male age through an increase in nesting success. Nest failure was attributed only to predation in this species and nest concealment and nest height were important nest characteristics promoting successful fledging. Older males built their nests in more concealed and higher positions than first‐year birds, regardless of vegetation status around the nest. Analysis of individual birds suggested that by shifting the nest to a safer position, male White‐eyes achieved higher nesting success than in the previous year. Of three hypotheses of age‐related improvement in reproductive success considered, our data favoured the hypothesis that as individuals grow older, their breeding competence improves.     

Experimentally-elevated testosterone, female parental care, and reproductive success in a songbird, the Dark-eyed Junco (Junco hyemalis)

In male dark-eyed juncos (Junco hyemalis), experimentally elevated testosterone (T) decreases male parental care and offspring survival, but results in higher overall fitness through greater mating success. To help address the ensuing question of what prevents selection from favoring higher levels of T in male juncos, we manipulated T in female juncos. A previous study demonstrated no effect of experimentally elevated T on female incubation behavior, suggesting that female parental behavior might be insensitive to T. In this study we asked whether experimentally elevated T mediates other female parental behaviors and whether variation in T-mediated parental behavior might influence reproductive success. We videotaped free-living control- and T-females during nesting to quantify brooding behavior when young were 3 days old and provisioning behavior when young were 6 days old. Nest defense was measured by quantifying responses to a mounted predator placed near the nest. Reproductive success was assessed via fecundity, nestling quality, and nest survival. T-females spent less time than control females brooding but did not differ in provisioning rate. T-females performed fewer dives at the predator mount and, unlike controls, failed to increase defense as nesting progressed. T-females also had lower daily nest survival and lower nest success (odds of producing at least one fledgling). We conclude that some aspects of female parental behavior are sensitive to experimentally elevated T while others are not and consider the implications for the evolution of T-mediated characters in both sexes.     

Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests

Parental care should be selected to respond to honest cues that increase offspring survival. When offspring are parasitised, the parental food compensation hypothesis predicts that parents can provision extra food to compensate for energy loss due to parasitism. Chick begging behaviour is a possible mechanism to solicit increased feeding from attending parents. We experimentally manipulated parasite intensity from Philornis downsi in nests of Darwin's small ground finch (Geospiza fuliginosa) to test its effects on chick begging intensity and parental food provisioning. We used in‐nest video recordings of individually marked chicks to quantify nocturnal parasite feeding on chicks, subsequent diurnal chick begging intensity and parental feeding care. Our video analysis showed that one chick per brood had the highest parasite intensity during the night (supporting the tasty chick hypothesis) and weakest begging intensity during the day, which correlated with low parental care and rapid death. We observed sequential chick death on different days rather than total brood loss on a given day. Our within‐nest video images showed that (1) high nocturnal larval feeding correlated with low diurnal begging intensity and (2) parent birds ignored weakly begging chicks and provisioned strongly begging chicks. Excluding predation, all parasite‐free chicks survived (100% survival) and all parasitised chicks died in the nest (100% mortality). Weak begging intensity in parasitised chicks, which honestly signalled recent parasite attack, was not used as a cue for parental provisioning. Parents consistently responded to the strongest chick in both parasitised and parasite‐free nests.     

Nest Defense by Red Jungle Fowl (Gallus gallus spadiceus) Hens: The Roles of Renesting Potential,Parental Experience and Brood Reproductive Value

Reproductive-effort theory predicts that parents of any given age should expend more parental effort (1) as their residual reproductive value declines, and (2) as the reproductive value of offspring increases. An observational and experimental study of nest defense by captive red jungle fowl hens was used to examine these two predictions. Both young and old individuals significantly increased defense of the second nest compared to the first nest within a season; this pattern occurred for the defense of both eggs and chicks. Old hens showed significantly greater defense of both eggs and chicks in each of the nests than did young hens. Both young and old hens were significantly more defensive of chicks than eggs in each of two clutches of a season. Hens also reduced their nest defense significantly at the end of a two to three-day period after their chicks were replaced with eggs, and increased their nest defense after eggs were exchanged for chicks. Hens given four chicks showed more vigorous defense than hens given two chicks. When the brood size of hens with four chicks was reduced to one chick, the hens responded by exhibiting less vigorous nest defense. These patterns of nest defense in jungle fowl were not confounded by parental experience of hens, or differences in offspring quality that are related to time of breeding, maternal age, sire genetic quality or vulnerability of offspring to weather.     

Effects of nest-site characteristics and parental activity on cowbird parasitism and nest predation in Brown-and-yellow Marshbirds

ABSTRACT Nest‐site selection and nest defense are strategies for reducing the costs of brood parasitism and nest predation, two selective forces that can influence avian nesting success and fitness. During 2001–2002, we analyzed the effect of nest‐site characteristics, nesting pattern, and parental activity on nest predation and brood parasitism by cowbirds (Molothrus spp.) in a population of Brown‐and‐yellow Marshbirds (Pseudoleistes virescens) in the Buenos Aires province, Argentina. We examined the possible effects of nest detectability, nest accessibility, and nest defense on rates of parasitism and nest predation. We also compared rates of parasitism and nest predation and nest survival time of marshbird nests during the egg stage (active nests) with those of the same nests artificially baited with passerine eggs after young fledged or nests failed (experimental nests). Most nests (45 of 48, or 94%) found during the building or laying stages were parasitized, and 79% suffered at least one egg‐predation event. Cowbirds were responsible for most egg predation, with 82 of 107 (77%) egg‐predation events corresponding to eggs punctured by cowbirds. Nests built in thistles had higher rates of parasitism and egg predation than nests in other plant, probably because cowbirds were most active in the area where thistles were almost the only available nesting substrate. Parasitism rates also tended to increase as the distance to conspecific nests increased, possibly due to cooperative mobbing and parental defense by marshbirds. The proportion of nests discovered by cowbirds was higher for active (95%) than for experimental (29%) nests, suggesting that cowbirds used host parental activity to locate nests. Despite active nest defense, parental activity did not affect either predation rates or nest‐survival time. Thus, although nest defense by Brown‐and‐yellow Marshbirds appears to be based on cooperative group defense, such behavior did not reduce the impact of brood parasites and predators.     

Adoption Behaviour in Little and Common Terns (Aves; Sternidae): Chick Benefits and Parents' Fitness Costs

Adoption seeking by semi-precocial chicks of some bird species can be adaptive since it provides an alternative tactic to permit poorly cared-for young to survive despite their neglectful parents' behaviour. Moreover, own-nest desertion may enhance inclusive fitness of fugitive chicks by increasing survival prospects of siblings. On the other hand, adoption by breeding adults can be detrimental to foster parents' fitness if they invest resources in promoting survival of others' offspring at the expense of their own. In this study we report on the proximate causes and survival consequences of adoption seeking by chicks, and on the presumed costs of adoption sustained by foster parents in two colonial, ground-nesting tern species: the little tern (Sterna albifrons) and the common tern (Sterna hirundo). Adoptions were frequent in little tern and, notably, in common tern colonies. Chicks that deserted their original brood were poorly fed compared with resident chicks, but crowding in the brood and age rank relative to nest companions did not influence the chances of desertion. Deserting their original brood was risky for little tern chicks since stray chicks experienced higher mortality than resident ones. Common tern chicks deserted their original nest significantly more often than little tern chicks. Adoptees of both species benefited by the same survival prospects as resident chicks. A negative correlation existed between the proportion of food given to strange chicks and seasonal fitness or chick survival rate of breeding pairs. This was not predictable because: (1) foster parents could have increased their parental efforts, thus ensuring the same survival prospects to their own chicks, and (2) chicks of foster parents could have been, in turn, adopted thus obliterating the negative fitness effects of adoption. The sexes were identical with respect to their proneness to provide food to strange chicks. We suggest that adoption seeking by little and common tern chicks has evolved as an alternative tactic that is pursued to counter the effects of poor parental care. Adoption behaviour seems maladaptive since it is accompanied by a reduction in seasonal fitness in both species. Possible alternative explanations for nest desertion and adoption behaviour are also discussed.     

Age-specific reproductive performance of Black Grouse Tetrao tetrix females

As part of a demographic study of Black Grouse in the southern French Alps, we estimated variations in reproductive parameters in relation to age of females. Both yearlings and adults initiated a clutch, but adults raised about eight times more young (yearlings 0.25 chicks/females, adults 1.85 chicks/female). Yearlings laid significantly fewer eggs and initiated incubation later than older females. Success of nests did not differ between the two age classes, but yearlings tended to re-nest less frequently and exhibited higher chick loss than older females. The lower reproductive success of yearlings was primarily due to their lower ability to raise young. At nest sites, nest concealment, as well as vertical and horizontal structure and density of the vegetation, did not differ between yearling and older females, which is consistent with the similarity in nesting success in the two age classes. Results support the hypothesis that the extent of the difference in number of young raised by yearling and older females increases with predation pressure.     

Parental infanticide in birds through early eviction from the nest: rare or under‐reported?

Avian parental infanticide is apparently a rare phenomenon judging from the scant reports in the literature. However, unexplained cases of single nestling disappearance or of apparent fall from the nest are rather common and are routinely attributed to parental eviction of dead nestlings, single chick predation or dislodging of nestlings through errors by nestlings or parents. During long‐term studies of pied flycatchers Ficedula hypoleuca in central Spain 29 cases of eviction from the nest cup of mostly hatchlings or small, heterothermic nestlings were recorded, leading invariably and quickly to death. Parental errors and failed attempts to predate single chicks are unlikely in these cases. Although the incidence of recorded evictions was mostly below 5% of nests, it reached higher levels in years and populations where reproductive success was low. Some cases were preceded by prior egg eviction. In these studies, there were unexplained disappearances affecting 3.7% of eggs laid which amounted to 20% of all losses. During studies of three other passerine cavity nesting species, similar cases were observed. The rarity of reports of parentally evicted nestlings is apparently based on the difficulty of directly observing parents evicting live nestlings but may also be due to observer bias as ejections of dead nestlings, erroneous dislodgings or single‐chick predation events are also rarely observed but are normally given as reasons for single‐chick losses. Field researchers should attempt to determine the real causes of partial brood loss before assuming that parsimony excludes parental infanticide through eviction.     

Costs of large communal clutches for male and female Greater Rheas Rhea americana

The breeding system of the Greater Rhea Rhea americana is almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg-laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.     

Aggression by adult South American Terns toward conspecific chicks

ABSTRACT Infanticide and nonfatal aggression by adults toward unfamiliar chicks have been widely reported in colonial birds, and can be an important cause of chick mortality. We studied intraspecific aggression by adults toward chicks at a South American Tern (Sterna hirundinacea) colony in Patagonia, Argentina, during 2005 to characterize this behavior, evaluate its relationship with nesting density, chick age and microhabitat characteristics, and assess its effect on breeding success. Of 111 chicks in the study area, 45% were attacked at least once. Chicks older than 9 d posthatching were more likely to be attacked than younger chicks, and unattended chicks were more likely to be attacked than guarded chicks (88 vs. 12%). Chicks were also attacked more often when in their own territories (76% of cases), but were less likely to be attacked in territories with more vegetation cover and high‐quality shelters (i.e., vegetation with characteristics that prevented adults from reaching chicks). The number of aggression events was not related to nest density. At least 8% of the chicks in our study area died as a result of adult intraspecific aggression. Our results indicate that intraspecific aggression by adult South American Terns toward chicks is relatively common in the Punta Loma colony and should not be underestimated as a factor affecting their breeding success.     

Exclusion of ground predators improves Northern Lapwing Vanellus vanellus chick survival

Many farmland‐breeding wader species have declined across Europe, probably due to reductions in reproductive output caused by high nest losses as a result of agriculture or predation, or low chick survival between hatching and fledging. Most studies have focused on nest failures, and the factors affecting post‐hatching survival of chicks are poorly known. In an experimental approach, we fenced parts of the arable foraging areas of Northern Lapwing Vanellus vanellus families to quantify chick survival simultaneously in the presence and absence of ground predators. Lapwing chicks were radiotagged to estimate survival probabilities by daily locations, applying multistate capture–recapture models. During the night, chick survival was considerably lower outside fenced plots than within. During the day, chick survival was higher than at night and did not differ between protected and unprotected plots. This suggests that nocturnal ground predators such as Red Foxes Vulpes vulpes were responsible for a significant proportion of chick mortality. Cumulative survival probability from hatching to fledging was 0.24 in chicks within fenced plots, but virtually zero in chicks outside fenced plots. In farmland, temporary electric fences can be effective in minimizing the impact of ground predators and offer a promising short‐term method to increase fledging success of precocial birds.     

Parental Investment Theory and Nest Defence by Imperial Shags: Effects of Offspring Number,Offspring Age,Laying Date and Parent Sex

Nest defence is a common form of parental care employed by birds to improve the survival of their offspring. Theory predicts that parents should adjust their nest defence according to the value of the brood at stake, defending more intensively broods with high survival and reproductive prospects. We evaluated the influence of offspring number, offspring age, laying date and parent sex on nest‐defence intensity (NDI) of the Imperial Shag Phalacrocorax atriceps, a sexually dimorphic seabird with seasonal decline in offspring survival and very limited renesting potential. We also evaluated whether NDI was correlated within pairs and whether NDI of both members of the pair was correlated with incubation and breeding success. To elicit defensive behaviour, we simulated predation attempts using a Kelp Gull Larus dominicanus model. As predicted by theory, NDI was positively correlated with the number of offspring in the nest and offspring age. NDI during chick rearing was higher than that at early and late incubation, while no differences were found between incubation stages. Contrary to our prediction, we did not find differences in NDI according to laying date. NDI for males was higher than females, while NDI was also positively correlated within pairs. NDI was not statistically related to incubation or breeding success. These results suggest that other factors, such as laying date or parental quality and age, play a much larger role in determining the outcome and productivity of a nesting attempt. Our results provide partial support for parental investment theory; while parental defence increased with brood value according to offspring number and age, parental defence was not related to laying date, a factor strongly affecting offspring survival and recruitment probabilities in this species.     

Common loon parents defend chicks according to both value and vulnerability

In many territorial breeders, conspecifics that intrude during the chick‐rearing period pose a threat to survival of young. Defense of young from intruders is costly to parents, so it is likely that intense selective pressure has shaped chick defense so as to maximize parental fitness. We simulated territorial intrusion by exposing adult common loons Gavia immer and their chicks to a decoy and used mixed models to investigate responses. We tested two hypotheses: 1) the value hypothesis, which holds that parents should defend large broods of offspring more strongly because of the greater potential fitness benefits they offer, and 2) the vulnerability hypothesis, which predicts vigorous defense of young offspring, whose small size and limited mobility render them vulnerable to sudden attacks from intruders that approach under water. Under natural conditions, parents spent over 80% of their time within 20 m of chicks younger than two weeks (‘young chicks’) but 66% or less of their time close to chicks four weeks or older (‘old chicks’). Parents of young chicks associated less with the decoy but yodelled and penguin danced more during decoy trials than did parents of old chicks, supporting the conclusion that the parents protected young chicks not by engaging intruders directly but by remaining close to chicks and using vocalization and display to keep intruders at a distance. While these findings lent clear support to the vulnerability hypothesis, the value hypothesis too was supported, as males with two‐chick broods were almost three times more likely to yodel than males with singleton chicks. Age of parents was not associated with any aspect of chick defense, but the paucity of known‐aged parents in the oldest age classes makes future investigation of age effects warranted.     

The effect of parental age,experience and historical reproductive success on wandering albatross (Diomedea exulans) chick growth and survival

Growth and survival of altricial young are influenced by their parents’ abilities to invest in a breeding attempt. As a result, chick growth and survival in one breeding season may be indicative of their parents’ long-term reproductive potential. To determine whether variation in long-term reproductive success is driven by differential breeding investment, parental care and chick growth in wandering albatrosses (Diomedea exulans) were correlated with parental historical reproductive success. Effects of age and breeding experience (determined from past breeding attempts) and pre-laying body condition (mass–size indices) on chick growth and survival also were tested. Longer brooding of chicks increased their survival, but length of chick brooding did not differ between historically unproductive and successful breeders. Past reproductive success also was not correlated with chick growth rates or fledging mass or size. Chick brooding period, chick growth rates, final mass and size were independent of parental body condition. Older and more experienced parents brooded chicks for longer and their chicks grew faster, supporting previous findings that breeding competence is a learnt skill. Chick care and growth characteristics differed more between than within pairs, suggesting that differences in these characteristics are driven by variation among pairs.     

Habituation and habitat changes can moderate the impacts of human disturbance on shorebird breeding performance

Disturbance by humans is widely expected to reduce the reproductive fitness of nesting birds if disturbance reduces nest attentiveness, and unattended eggs experience increased risk of predation or exposure to potentially lethal temperature extremes. Yet, relatively few studies have examined the physiological or behavioural mechanisms whereby disturbance influences reproductive fitness, or the extent to which the costs of disturbance may be reduced through habituation. We compared the behavioural responses, egg temperatures and reproductive success of shore-nesting white-fronted plovers Charadrius marginatus to disturbance at two breeding sites experiencing low versus high human recreational activity, respectively. Daytime nest attentiveness decreased with increasing experimental disturbance at both sites, but this relationship differed between sites; for any given level of disturbance, incubating birds at the more disturbed site had greater nest attentiveness. They achieved this through habituation, allowing a closer human approach before leaving the nest, and returning to the nest faster after a disturbance event. Despite lower average daytime nest attentiveness at the more disturbed site, incubation temperatures did not differ significantly between sites. Nest mortality, mostly by natural mammalian and corvid predators, was significantly lower at the site experiencing high recreational activity. However, chick mortality was significantly greater at the more disturbed site, most likely because of predation by domestic dogs. Chick mortality may have been increased by the habitation of chicks, whose escape responses were much reduced at the more disturbed site. Nonetheless, annual fecundity was substantially higher at the more disturbed site, showing that the overall reproductive fitness of wild birds is not always compromised by human disturbance and urbanization.     

Parenting styles in white‐rumped swallows (Tachycineta leucorrhoa) show a trade‐off between nest defense and chick feeding

Studies about personalities in wild animals usually focus on five categories of behavioral traits that do not easily accommodate all aspects of parental care, a class of behaviors with direct consequence to reproductive success. Parental care can vary consistently between individuals and constitute parenting styles. Here we investigate the consistency of four behaviors of parental care across two breeding seasons of white‐rumped swallows Tachycineta leucorrhoa in southern Brazil. These behaviors are a prospection of potential nest‐sites, nest defense against predators or conspecifics, and chick feeding. If these first three behaviors are consistent, they can be classified according to the standard categories of personality as exploration, boldness, and aggressiveness, respectively. We find that all behaviors, except exploration, are consistent between individuals in the long term. We also show that, besides individual identity, couple identity is an important component explaining variation in behavior. This pattern can arise if behavior influences pair formation or if behavior is a consequence of a common environment affecting the couple. We show that, depending on the parental task, males or females are more consistent, suggesting that sexes are adopting different strategies of parental care allocation. We then investigate if there is a trade‐off between nest defense and chick feeding. We find that birds that defend more also feed the brood less often and suggest that testosterone might be the mechanism modulating this trade‐off. Lastly, we discuss the implications of our results to mate choice and highlight the need for studies linking parenting styles to fitness in swallow species.     

Habitat selection and potential fitness consequences of two early‐successional species with differing life‐history strategies

Habitat selection and its relationship to fitness is a fundamental concept in ecology, but the mechanisms driving this connection are complex and difficult to detect. Despite the difficulties in understanding such intricate relationships, it is imperative that we study habitat selection and its relationship with fitness. We compared habitat selection of least terns (Sternula antillarum) and piping plovers (Charadrius melodus) on the Missouri River (2012–2014) to examine the consequences of those choices on nest and chick survival. We hypothesized that plovers and terns would select habitat that minimized the chance of flooding and predation of eggs, chicks, and adults, but that plovers would also select habitat that would provide foraging habitat for their chicks. We developed an integrated habitat selection model that assessed selection across multiple scales (sandbar and nest scales) and directly modeled the effect of selection on nest and chick survival. In general, the species selected habitat in keeping with our hypotheses, such that predation and flooding, in particular, may have been reduced. Sandbar selection had either a negative or no appreciable effect on nest survival for both species across years. Nest‐site selection in 2012 had a generally positive effect on nest survival and chick survival for both terns and plovers, and this trended toward a negative effect by 2014. This result suggested that early selection decisions appeared to be adaptive, but we speculate that relatively high site fidelity and habitat degradation led to reduced benefit over time. Our results highlight the complex nature of habitat selection and its relationship to fitness.     

Comparative reproductive biology of sympatric species: nest and chick survival of American avocets and black‐necked stilts

Identifying differences in reproductive success rates of closely related and sympatrically breeding species can be useful for understanding limitations to population growth. We simultaneously examined the reproductive ecology of American avocets Recurvirostra americana and black‐necked stilts Himantopus mexicanus using 1274 monitored nests and 240 radio‐marked chicks in San Francisco Bay, California. Although there were 1.8 times more avocet nests than stilt nests, stilts nonetheless fledged 3.3 times more chicks. Greater production by stilts than avocets was the result of greater chick survival from hatching to fledging (avocet: 6%; stilt: 40%), and not because of differences in clutch size (avocet: 3.84; stilt: 3.77), nest survival (avocet: 44%; stilt: 35%), or egg hatching success (avocet: 90%; stilt: 92%). We reviewed the literature and confirmed that nest survival and hatching success are generally similar when avocets and stilts breed sympatrically. In addition to species, chick survival was strongly influenced by age, site, and year. In particular, daily survival rates increased rapidly with chick age, with 70% of mortalities occurring ≤ 1 week after hatch. California gulls Larus californicus caused 55% of avocet, but only 15% of stilt, chick deaths. Differential use of micro‐habitats likely reduced stilt chick's vulnerability to gull predation, particularly during the first week after hatch, because stilts nested in vegetation 2.7 times more often than avocets and vegetation height was 65% taller at stilt nests compared with avocet nests. Our results demonstrate that two co‐occurring and closely related species with similar life history strategies can differ markedly in reproductive success, and simultaneous studies of such species can identify differences that limit productivity.     

Effect of predation by skuas on breeding success of the Cape petrel Daption capense at Nelson Island, Antarctica

Breeding success of Cape petrels at Nelson Island (South Shetland Islands) in 1991/1992 averaged 29%. Predation by skuas accounted for 76% of all nesting failures. Direct attacks prevailed over opportunistic predation. Intensity of observer disturbance alone could not account for the inter-colony variation in predation rate, which tended to increase with colony size, presence of nearby breeding skuas and local availability of other prey species (penguins). Predation was significantly lower on nests that could be attacked only, or more easily, from the air and nests with close neighbours. Selective predation on nests in which relatively narrower eggs were laid was independent of nest-site characteristics and laying date, suggesting an effect of parental age/experience on breeding success through antipredator behaviour. Development of oil-spitting behaviour in growing chicks balanced the parental attendance and effort in nest defence, which declined as chick age increased. Increased predation on lighter but not smaller chicks may reflect the lower ability of such chicks to defend themselves by spitting stomach oil. Seasonal variations of predation rate on Cape petrel nests did not result from predator swamping by numbers of simultaneously active nests, but rather reflected temporal availability of alternative prey. Success of all phases of nesting declined with later laying date. Received: 15 December 1997 / Accepted: 11 April 1998