Blue Petrel Halobaena Caerulea Research Articles

New records of Blue Petrel Halobaena caerulea in Uruguay

We report five new records of Blue Petrel Halobaena caerulea in Uruguay; three individuals were found stranded on the coast in 2005, 2016 and 2018, and two were recorded at sea in the economic exclusive zone in 2021 and 2022. These few records suggest that the species' presence in Uruguayan waters is no more than occasional. However, its similarity to, and association with prions, together with the fact that this petrel does not usually approach fishing vessels, could mean that it is under-recorded.

Variation among species and populations in bill shape and size in three planktivorous petrels

Morphological variation in biological structures may be driven by genetic and environmental factors, such as inter- and intraspecific competition for resources. In seabirds, although the bill is also involved in vocalization, olfaction, sexual selection and defence, the main drivers of high morphological plasticity in bill size and shape appear to relate primarily to diet and thus to niche differentiation. Here, we combined geometric morphometrics and comparisons of linear measurements as a precise tool for measuring shape variation in anatomical features, to investigate the differences among species and populations (island groups) in bill shape of three planktivorous petrels (Antarctic prion Pachyptila desolata, blue petrel Halobaena caerulea and thin-billed prion Pachyptila belcheri). Fieldwork was carried out in South Georgia (54° 0′ S, 38° 3′ W), Falkland (51° 42′ S, 57° 51′ W), Diego Ramírez (56° 31′ S, 68° 44′ W) and Kerguelen (49° 20′ S, 69° 20′ E) Islands, from 2010 to 2021. Results show that the bills of Antarctic prions were more robust and shorter, appropriate for filtering large amounts of small prey. Blue petrels and thin-billed prions had narrower and longer bills, effective for catching and tearing large single prey. Also, Antarctic prions and blue petrels from Kerguelen had longer and narrower bills than conspecifics from other colonies, which could potentially be explained by geographic variation in diet. In conclusion, prey availability and diversity appear to be important factors influencing variation in bill morphology. This study highlights the utility of geometric morphometrics for investigating bill shape variation in seabirds. Nevertheless, further studies are needed to better understand selective pressures leading to morphological variation of biological structures.

Call rate, fundamental frequency, and syntax determine male-call attractiveness in blue petrels Halobaena caerulea

In blue petrels (Halobaena caerulea), females are supposed to be particularly choosy and mate choice can take a couple of years. In these lifelong monogamous seabirds, choosing a good mate is crucial and has a strong influence on their fitness. Due to their nocturnal habits, the absence of sexual dimorphism, and the physical barrier between males calling from their burrow and females flying above the colony, vocal signals seem to be one of the main channels for males to communicate with potential mates. In a previous study, we investigated whether acoustic parameters of male calls carry information about morphological characteristics that might be indicators of males’ qualities. Here, we experimentally test whether these acoustic parameters linked to male characteristics are actually attractive to females. To do so, we played back modified calls of males to females in a colony of blue petrels of the Kerguelen archipelago. We found that flying females were more attracted by high-pitched calls, and by calls broadcasted at a high call rate. Previous studies showed a relationship between pitch and bill depth and length. In filter-feeding birds, such as blue petrels, bill morphology influences feeding efficiency. A high call rate is an indicator of sexual motivation and makes the caller easier to locate by potential mates and predators in the hubbub of the colony. We thus hypothesized that producing frequent high-pitched calls appeared to be preferable for a conspicuous sexual signaling although it may increase predation risks. Mate selection process is largely unknown in burrowing petrels due to their cryptic life at the colony. Here, we examined the implication of vocal signals in mate choice in the blue petrel Halobaena caerulea. We used an experimental setting based on a two-choice test to show that male calls are sexual signals attracting females. As expected, broadcasting male calls attracted females. Despite the apparent stereotypy of male calls, their acoustic parameters transmit pieces of information that may influence females’ preference. We found that females are more attracted by high call rate and high-pitched calls. This is the first evidence of the implication and influence of vocal signals in mate choice in burrowing petrels.

Niche switching and leapfrog foraging: movement ecology of sympatric petrels during the early breeding season

BackgroundThe timing of events in the early part of the breeding season is crucially important for successful reproduction. Long-lived animals that migrate large distances independently of each other meet at the breeding sites to re-establish their pair bonds and coordinate their breeding duties with their partners.MethodsUsing miniature light-geolocation and immersion data together with blood stable isotopes, we studied the early breeding season in Thin-billed prions Pachyptila belcheri, Antarctic prions P. desolata and Blue petrels Halobaena caerulea breeding at Kerguelen Islands in the Indian Ocean. These three species exhibit differences in their winter habitat and timing of migration, moult and breeding. We hypothesised that these differences would influence their behaviour during the early breeding season.ResultsIn line with our hypothesis, we found clear differences not only in the timing of colony attendance, but also in the time budgets while at sea and in habitat use. Both early breeding Blue petrels and late breeding Antarctic prions spent about 8 h per day in flight and 15 h foraging. In comparison, Thin-billed prions, which breed in mid-summer, spent less time (5 h daily) in flight and more time (18 h daily) foraging, thus maximizing the time spent foraging during the longest daylight days of the year. While the ecological habitat parameters (sea temperature, wind, productivity) of Thin-billed prions and Blue petrels were relatively stable throughout the year, Antarctic prions showed clear niche switching, caused by leapfrogging between the northernmost winter distribution to the southernmost distribution during the early breeding season. Blood stable isotopes confirmed the habitat switch between the inter-breeding and early breeding periods and highlighted trophic segregation with Blue petrels feeding more on fish and Antarctic petrels more on crustaceans during the early breeding period.ConclusionWe found that the three sympatric petrel species segregated in time and space, both in the winter and the early breeding season. The interplay of timing and distribution meant that the three species show the full range of migratory strategies, from niche-tracking Blue petrels to niche-switching Antarctic prions. The latitudinal distribution resembled the leapfrogging of terrestrial avian migrant species or populations.

Males’ calls carry information about individual identity and morphological characteristics of the caller in burrowing petrels

Acoustic communication in burrowing petrels has been poorly studied. However, as for many other bird species, acoustic communication seems to play an essential role in social interactions during the breeding season of these seabirds. Bachelor males call from their burrow, likely to attract females, but also when vocally challenged by other males. Calling in the breeding colony exposes petrels to high predation risks and thus it should provide an important benefit. The present study focuses on the informative content of males’ calls in the blue petrel Halobaena caerulea and the Antarctic prion Pachyptila desolata, two monogamous petrel species producing a single egg per year. We tested the hypotheses that acoustic parameters of a male's calls 1) reflect phenotypic characteristics, and 2) bear an individual vocal signature. To do so, we first tested on both species the relationships between seven morphometric measurements and 11 acoustic parameters using multivariate analyses. Second, we performed a between‐class analysis and calculated the potential of individuality coding (i.e. the ratio between intra‐ and inter‐individual variabilities) for acoustic parameters in both spectral and temporal domains. Results show acoustic parameters (especially energy quartiles, call duration, and syllable or phrase rate) reflect the caller's body size, bill morphology and wing morphology in both species. Considering the seeming pertinence of wing morphology, we suggest wing area may be a more relevant trait to consider than wing length when studying soaring birds. The results support the idea that energy quartiles, phrase rate and call duration also code for individual identity. Information carried by males’ calls might play a role in social interactions, such as burrow defence (e.g. male‐male competition, neighbour‐stranger discrimination) and/or female mate choice.

Mouse predation affects breeding success of burrow-nesting petrels at sub-Antarctic Marion Island

AbstractWe report the breeding success of four species of burrow-nesting petrels at sub-Antarctic Marion Island where house miceMus musculusare the sole introduced mammal. Feral catsFelis catuswere present on Marion for four decades from 1949, killing millions of seabirds and greatly reducing petrel populations. Cats were eradicated by 1991, but petrel populations have shown only marginal recoveries. We hypothesize that mice are suppressing their recovery through depredation of petrel eggs and chicks. Breeding success for winter breeders (grey petrelsProcellaria cinerea(34±21%) and great-winged petrelsPterodroma macroptera(52±7%)) were lower than for summer breeders (blue petrelsHalobaena caerulea(61±6%) and white-chinned petrelsProcellaria aequinoctialis(59±6%)) and among winter breeders most chick fatalities were of small chicks up to 14 days old. We assessed the extent of mouse predation by monitoring the inside of 55 burrow chambers with video surveillance cameras (4024 film days from 2012–16) and recorded fatal attacks on grey (3/18 nests filmed, 17%) and great-winged petrel chicks (1/19, 5%). Our results show that burrow-nesting petrels are at risk from mouse predation, providing further motivation for the eradication of mice from Marion Island.

The distribution and abundance of Blue Petrels (Halobaena caerulea) breeding at subantarctic Marion Island

ABSTRACTBlue Petrels (Halobaena caerulea) are known to breed at seven locations in the Southern Ocean. Population estimates have been made recently for the two major breeding sites, but accurate estimates are lacking for the remaining locations. We used a systematic survey technique to estimate the size of the population breeding at Marion Island (290 km2), the larger of the two Prince Edward Islands. A combination of colony area and density estimates suggested there were 214 700 Blue Petrel burrows on Marion Island in 2012. Burrow occupancy rates at the mid-incubation stage averaged 82% (range 36–98%), suggesting a total breeding population of 145 000 pairs (95% confidence interval 110 000–180 000). There appeared to be some range expansion since the population was mapped in the mid-1980s. Predation of chicks and eggs by introduced house mice (Mus musculus) could be affecting the recovery of Blue Petrels since feral cats (Felis catus) were eradicated in 1991. Based on our count from Marion Island alone, the Prince Edward Islands support the third largest population of Blue Petrels globally, after Diego Ramirez Islands and the Kerguelen Islands.

Blue Petrels Halobaena caerulea discovered breeding on Gough Island

The Blue Petrel Halobaena caerulea breeds at five subantarctic island groups and at islands off southern Chile in a narrow latitudinal band from 47° to 56° S on either side of the Antarctic Polar Front. We found a colony on Gough Island (40° S, 10° W), central South Atlantic Ocean, more than 700 km north of its known breeding range. Breeding appears to take place later than at colonies farther south. Although the colony is in a fairly frequently visited part of the island, it might have been overlooked rather than representing a recent range extension.

Multi-scale factors influencing seabird assemblages in the African sector of the Southern Ocean

AbstractOceanic fronts are important foraging areas for many top predators, but they also define biogeographical boundaries to animals in the Southern Ocean and play a role in structuring seabird assemblages. Understanding the factors driving patterns in the spatial and temporal distribution of seabirds is important to infer the likely impact of a changing climate. Latitudinal transects south of Africa in two summers indicate that fronts and sea ice extent play key roles in determining seabird assemblages. We observed 37 seabird taxa and found five seabird assemblages. The Subtropical Convergence and pack ice-edge form the strongest biogeographical boundaries, whereas the Sub-Antarctic Front and Antarctic Polar Front are less well defined. As summer progresses, the Southern Antarctic Circumpolar Current Front (the Antarctic Divergence or southern boundary of the Antarctic Circumpolar Current) becomes important, when a distinct seabird assemblage forms north of the retreating sea ice following an influx of great shearwatersPuffinus gravis(O'Reilly), blue petrelsHalobaena caerulea(Gmelin), Kerguelen petrelsLugensa brevirostris(Lesson) and southern fulmarsFulmarus glacialoides(Smith). Seabird assemblages show strong seasonality and are predictable between years. They are structured primarily by latitudinal gradients and secondarily by seasonal variation in sea-surface temperature and ice cover within their latitudinal habitat zones.

Ecological Segregation in Space, Time and Trophic Niche of Sympatric Planktivorous Petrels

The principle of competitive exclusion postulates that ecologically-similar species are expected to partition their use of resources, leading to niche divergence. The most likely mechanisms allowing such coexistence are considered to be segregation in a horizontal, vertical or temporal dimension, or, where these overlap, a difference in trophic niche. Here, by combining information obtained from tracking devices (geolocator-immersion and time depth recorders), stable isotope analyses of blood, and conventional morphometry, we provide a detailed investigation of the ecological mechanisms that explain the coexistence of four species of abundant, zooplanktivorous seabirds in Southern Ocean ecosystems (blue petrel Halobaena caerulea, Antarctic prion Pachyptila desolata, common diving petrel Pelecanoides urinatrix and South Georgian diving petrel P. georgicus). The results revealed a combination of horizontal, vertical and temporal foraging segregation during the breeding season. The stable isotope and morphological analyses reinforced this conclusion, indicating that each species occupied a distinct trophic space, and that this appears to reflect adaptations in terms of flight performance. In conclusion, the present study indicated that although there was a degree of overlap in some measures of foraging behaviour, overall the four taxa operated in very different ecological space despite breeding in close proximity. We therefore provide important insight into the mechanisms allowing these very large populations of ecologically-similar predators to coexist.

Responses of seabirds to the rabbit eradication on Ile Verte, sub-Antarctic Kerguelen Archipelago

Studies on the role of introduced rabbits, Oryctolagus cuniculus, on islands have mainly focused on their negative impacts on vegetation. However, little attention has been paid to their influence on vertebrate communities. On Ile Verte (148 ha) in the sub-Antarctic Kerguelen Archipelago, rabbits are the only mammal that have been introduced. The long-term consequences of their eradication in 1992 on both native, burrowing seabird prey populations and their predator, the brown skua Catharacta skua, were investigated between 1991 and 2005. Densities of breeding petrels were followed on site with three plant communities differing in their soil depth. In addition, the diet and breeding activities of skuas were evaluated on the entire island area. The density of breeding pairs of the most abundant petrel species, the blue petrel Halobaena caerulea, which only nested at the site with deepsoil, increased by approximately eightfold during the 6 years following the rabbit eradication. Of the other species nesting in deep soil, there was an approximately fourfold reduction in the Antarctic prion, Pachyptila desolata, but such a decrease in breeding pair densities was not observed in areas with shallow soils. The SouthGeorgian diving petrels, Pelecanoides georgicus, was the rarest species, nesting only on mineral soils, and for which breeding pairs did not vary through time. The total numbers of fledged chicks of skua on the island significantly increased during the study period, but not the total number of breeding pairs. Thus, brown skuas were not affected by the disappearance of rabbits and rather benefited from an increase of their preferred prey. Blue petrels recovered quickly to sites with deepsoil, benefiting from the rabbit eradication and the cessation of burrow disturbance. The decrease of Antarctic prions could have been the result of an exclusion process from nesting areas on the deep soil site by blue petrels.

Interannual dietary changes and demographic consequences in breeding blue petrels from Kerguelen Islands

Interannual variation in summer diet and demographic parameters of blue petrel Halobaena caerulea were investigated to understand how changes in the marine environment affect the reproduction of an oceanic seabird from the Southern Ocean. Lipids from stomach oil were used as trophic markers of prey consumed by adult birds when they self-fed during long trips, and conven- tional food analyses allowed determination of the food delivered to chicks after long and short adult foraging trips. Biochemical analysis of stomach oil showed little differences in lipid classes and in fatty acid and fatty alcohol compositions, with most oils deriving from triacylglycerol- and wax ester- rich myctophid fishes over the 4 yr study period (1996, 2000, 2001 and 2002). Accordingly, stomach content analysis showed that chick food was dominated by mass by crustaceans and fish, with myc- tophids as the main fish items. Important interannual variations in chick diet occurred, most promi- nently featuring an almost lack of fish in 2002. Summer 2002 was also marked by the lowest fledging success, lowest mass of chicks at fledging and lowest summer survival of adults during the period 1986-2002. In contrast to 1996, 2000 and 2001, summer months in 2002 were also characterized by an intense warm event in Kerguelen waters. Overall, results strongly suggest causal links between cli- matic anomalies, availability of myctophids and dietary shifts with poor reproductive performance and lower survival in blue petrel. Furthermore, they emphasize the importance of lipid-rich prey in seabird nutrition.

Lipids from stomach oil of procellariiform seabirds document the importance of myctophid fish in the Southern Ocean

We investigated the relative importance of myctophid fish and Antarctic krill in the diet of adult flying seabirds of the Southern Ocean. The main prey of short‐tailed shearwaters Puffinus tenuirostris (P. ten.), white‐chinned petrels Procellaria aequinoctialis (P. aeq.), blue petrels Halobaena caerulea (H. cae.), thin‐billed prions Pachyptila belcheri (P. bel.), and Antarctic prions Pachyptila desolata (P. des.) were mostly deduced from the lipid analysis of adult stomach oils. More than 97% of the 125 analyzed oils mainly consisted of wax esters (WEs) and triacylglycerols (TAGs) (>70% of total lipids). WE fatty alcohol (FAlc), WE fatty acid (FA), and TAG‐FA profiles clearly segregated P. aeq. from P. ten., with smaller, but still significant, differences among the three other petrel species. P. aeq. and P. ten. therefore preyed on distinct prey species, whereas H. cae., P. bel., and P. des. had a more similar diet, but still with some prey differences. Comparisons between FAlc and FA patterns of oils with those of potential prey species showed that >93% of FAlc and FA patterns of oil WEs had a high probability of resemblance with the myctophid signatures, and similar results were obtained with the TAG fractions. Almost no stomach oil fit the lipid patterns of subantarctic and Antarctic euphausiids, including those of the WE‐rich Thysanoessa macrura and the TAG‐rich Antarctic krill Euphausia superba. This study thus demonstrates for the first time the importance of myctophids in the nutrition of adult flying seabirds breeding in subantarctic islands and foraging in Antarctic waters during the austral summer.

The potential role of ammonia as a signal molecule for procellariiform seabirds

Procellariiform seabirds (petrels, albatrosses and shearwaters) navigate vast distances over seemingly featureless ocean habitat by mechanisms that are not well understood. These birds have large olfactory bulbs, and the use of smell has been implicated in both foraging and homing behavior. While many olfactory cues relevant to these behaviors have been identified, ammonia is a potentially significant, biogenic, scented compound that has not been studied in this context. Ammonium (NH4+) constitutes a primary waste product produced by many of the prey species on which procellariiforms forage. Nitrogen waste products, including volatilized ammonia (NH3) also scent the terrestrial landscape of sub-Antarctic islands where newly breeding procellariiform seabirds recruit and raise their young. Since an ability to smell ammonia may be relevant to both prey detection and locating colonies or islands, we used a non-invasive, behavioral assay to examine whether this scented compound is detectable by a candidate test species, the blue petrel Halobaena caerulea Gmelin. Our results suggest that these birds can detect volatilized ammonia within a concentration range that they may naturally encounter (10(-11) to 10(-5) M), and point to ammonia as a potential signal molecule in the sub-Antarctic.

Climate and density shape population dynamics of a marine top predator

Long-term studies have documented that climate fluctuations affect the dynamics of populations, but the relative influence of stochastic and density-dependent processes is still poorly understood and debated. Most studies have been conducted on terrestrial systems, and lacking are studies on marine systems explicitly integrating the fact that most populations live in seasonal environments and respond to regular or systematic environmental changes. We separated winter from summer mortality in a seabird population, the blue petrel Halobaena caerulea, in the southern Indian Ocean where the El Niño/Southern Oscillation effects occur with a 3-4-year lag. Seventy per cent of the mortality occurred in winter and was linked to climatic factors, being lower during anomalous warm events. The strength of density dependence was affected by climate, with population crashes occurring when poor conditions occurred at high densities. We found that an exceptionally long-lasting warming caused a ca. 40% decline of the population, suggesting that chronic climate change will strongly affect this top predator. These findings demonstrate that populations in marine systems are particularly susceptible to climate variation through complex interactions between seasonal mortality and density-dependent effects.

Diet and feeding ecology of blue petrels Halobaena caerulea at Iles Kerguelen, Southern Indian Ocean

The food and feeding ecology of the blue petrel Halobaena caerulea was investigated over 4 consecutive chick-rearing periods at Iles Kerguelen. In all years, blue petrels fed on a large diversity of crustaceans and fish, with a small proportion of squid and other organisms. Crustaceans ranked first by number (98%) and second by reconstituted mass (37%). The hyperiid Themisto gaudichaudii and the euphausiid Thysanoessa sp. were the dominant prey items, accounting each for 42% by number, and for 12 and 4% by mass of the diet, respectively. Other important crustacean prey were the Antarctic krill Euphausia superba (2% by number and 10% by mass) and the large shrimp Pasiphaea scotiae ( 1000 km from the breeding colonies, but blue petrels also foraged on their way back to Iles Kerguelen from these distant foraging grounds to feed their chicks. A comparison of the stable carbon and nitrogen isotopic compositions of chick and adult feathers reveals that adult blue petrels fed at the same trophic level during the chick-rearing and moulting period, and that they renew their flight feathers in Antarctic waters.

Predation risk and moonlight avoidance in nocturnal seabirds

Unlike most seabird families, the vast majority of small petrel species are nocturnal on their breeding grounds. Further, they reduce markedly their activity when the light level increases. Moonlight avoidance might be a consequence of reduction in foraging profitability, as bioluminescent prey do not come to the sea surface on bright nights. Alternatively, petrels may avoid colonies during moonlit nights because of increased predation risk. We studied predation on petrels by Brown Skuas Catharacta antarctica lönnbergi at Kerguelen, and the influence of moonlight on behaviour of both skuas and petrels, to test the ‘predation risk’ hypothesis. On the study area, Brown Skuas hunt at night and prey heavily upon the Blue Petrel Halobaena caerulea and the Thin‐billed Prion Pachyptila belcheri. Predation risk was higher on moonlit nights, as skuas caught more prey, and particularly more Blue Petrels when the light level increased. Nightly intakes of Blue Petrel and Thin‐billed Prion by skuas was related to colony attendance of non‐breeders rather than that of breeders. Biometry of prey also suggested that skuas caught a higher proportion of non‐breeding birds than was present at the colonies. Predation risk was thus greater in non‐breeders and on moonlit nights. Colony attendance by non‐breeding Blue Petrels and Thin‐billed Prions was also reduced during moonlit nights. Vocal activity, which is mainly by non‐breeders, was also drastically reduced when the light level increased in the species suffering the highest predation rate. Our results supported the ‘predation risk’ hypothesis, although the ‘foraging efficiency’ and the ‘predation risk’ hypotheses are not mutually exclusive: the former might explain the moonlight avoidance behaviour of breeding, and the latter that of non‐breeding individuals.

Diving Ability of Blue Petrels and Thin-Billed Prions

Capillary depth recorders (Burger and Wilson 1988) are useful tools for studying the foraging ecology of seabirds, and have been used successfully on penguins, alcids, shags and gannets (Burger 1991, Wanless et al. 1991, Adams and Walter 1993, Croxall et al. 1993). Recent studies of Procellariiformes have revealed that diving petrels (Pelecanoides georgicus and P. urinatrix) are able to dive routinely down to 25-40 m (Prince and Jones 1992, Chastel 1994). Furthermore, investigations on albatrosses (Prince et al. 1994), and on the White-chinned Petrel Procellaria aequinoctialis (Huin 1994), have shown that these seabirds can reach a depth of several meters, giving a new insight on the foraging techniques of albatrosses and petrels, which are generally regarded as surface-seizers (Harper 1987, Prince and Morgan 1987). The Blue Petrel Halobaena caerulea and the Thinbilled Prion Pachyptila belcheri are small burrow-nesting petrels (190 and 150 g, respectively). At Kerguelen Islands these two seabirds breed in very large numbers (up to 1 million pairs, Weimerskirch et al. 1989); the prions particularly are believed to have a significant impact on subantarctic resources (Ridoux 1994). During the chick-rearing period (50 days, Weimerskirch et al. 1994), both species alternate long and short foraging trips over pelagic and neritic waters (Weimerskirch et al. 1994) and prey mainly on small crustaceans (Harper 1972, Prince 1980, Ridoux 1994), principally by surface-seizing (Harper 1987, Prince and Morgan 1987). This paper reports the first study on maximum dive depths attained by Blue Petrels and Thin-billed Prions, during the chick-rearing period at Kerguelen Islands.

Male blue petrels reveal their body mass when calling

The blue petrel Halobaena caerulea is a nocturnal burrow-dwelling seabird: probably for this reason, sexual signalling in this species is based primarily on vocalizations. In this paper, we examine the possibility that structural parameters of the male call can provide information to females about male qualities. Several temporal parameters (durations and rhythm) are significantly correlated with body mass or condition of the males. These results are discussed in relation to the fact that body mass may be a good predictor of breeding success in some species of seabirds.

Incubation routine, body mass regulation and egg neglect in the Blue Petrel Halobaena caerulea

The incubation routine and mass changes of male and female Blue Petrel Halobaena caerulea were studied at the Kerguelen Islands to investigate factors influencing the durations of incubation stints and foraging trips at sea and the factors determining nest desertion and return to the nest.The body mass at the start of an incubation shift and also when the bird was relieved varied throughout the incubation period, whereas the mass when birds deserted the nest was stable. Birds deserted the nest when their mass decreased to threshold, independent of the duration of the fast. Temporary egg neglect was observed in successful as well as in unsuccessful breeding attempts, but it increased the risk of breeding failure. The net and daily massgained at sea during the second part of the incubation period were higher than during the first part, suggesting an increase in food availability. During the first part, the mass gained at sea and time spent foraging were inversely related to the mass of the bird before it left the burrow, whereas a similar relationship did not occur thereafter.The results suggest the occurrence of a fixed mass threshold when birds decide to leave the nest if not relieved by their partner. The mass when a bird left its nest inffuenced the time spent foraging or mass gained when food was scarce. Although decision rules to leave the nest or return from the sea are related to body condition. the possibility of neglecting the eggs temporarily enables Blue Petrels to regulate the trade‐off between risks of breeding failure and risks of an increase in adult mortality. A model for behavioural decision to stop incubating or stop feeding, based on a variable set point, is proposed.