Breeding biology of the thin-billed prion<Emphasis Type="Italic"> Pachyptila belcheri</Emphasis> at New Island,Falkland Islands: egg desertion,breeding success and chick provisioning in the poor season 2002/2003

We present data on breeding success, chick growth and chick feeding in thin-billed prions, Pachyptila belcheri, at New Island, Falkland Islands, in the breeding season 2002/2003. As in many populations of seabirds in the region, the overall breeding success was very low. This was mainly caused by low rate of observed burrow occupancy (60%) and hatching (57%) of thin-billed prions, while chick survival was closer to normal. Sixty-eight percent of the chicks survived to fledging. In total, a chick was successfully reared in 23% of the nests or from 38% of recorded eggs. The failed eggs were found to be incubated for 30 days, on average. The time of egg desertion coincided with the time of desertion of other seabirds at New Island, with a period of high sea-surface temperatures and low catch rates by the commercial fisheries. We describe chick growth and use repeated weighings, corrected for metabolic loss, to estimate meal sizes. Chicks were fed a mean 39.2 g in 77% of the nights. We discuss possible reasons for the observed extremely low hatching success, and compare with the breeding success of other seabirds at the Falkland Islands.     

Plasticity of Noddy Parents and Offspring to Sea-Surface Temperature Anomalies

Behavioral and/or developmental plasticity is crucial for resisting the impacts of environmental stressors. We investigated the plasticity of adult foraging behavior and chick development in an offshore foraging seabird, the black noddy (Anous minutus), during two breeding seasons. The first season had anomalously high sea-surface temperatures and ‘low’ prey availability, while the second was a season of below average sea-surface temperatures and ‘normal’ food availability. During the second season, supplementary feeding of chicks was used to manipulate offspring nutritional status in order to mimic conditions of high prey availability. When sea-surface temperatures were hotter than average, provisioning rates were significantly and negatively impacted at the day-to-day scale. Adults fed chicks during this low-food season smaller meals but at the same rate as chicks in the unfed treatment the following season. Supplementary feeding of chicks during the second season also resulted in delivery of smaller meals by adults, but did not influence feeding rate. Chick begging and parental responses to cessation of food supplementation suggested smaller meals fed to artificially supplemented chicks resulted from a decrease in chick demands associated with satiation, rather than adult behavioral responses to chick condition. During periods of low prey abundance, chicks maintained structural growth while sacrificing body condition and were unable to take advantage of periods of high prey abundance by increasing growth rates. These results suggest that this species expresses limited plasticity in provisioning behavior and offspring development. Consequently, responses to future changes in sea-surface temperature and other environmental variation may be limited.     

Fledging success of little auks in the high Arctic: do provisioning rates and the quality of foraging grounds matter?

Long-lived birds often face a dilemma between self-maintenance and reproduction. In order to maximize fitness, some seabird parents alternate short trips to collect food for offspring with long trips for self-feeding (bimodal foraging strategy). In this study, we examined whether temporal and spatial variation in the quality of foraging grounds affect provisioning and fledging success of a long-lived, bimodal forager, the little auk (Alle alle), the most abundant seabird species in the Arctic ecosystem. We predicted that an increase in sea surface temperature (SST), with an associated decrease in the preferred Arctic zooplankton prey, would increase foraging trip durations, decrease chick provisioning rates and decrease chick fledging success. Chick provisioning and survival were observed during three consecutive years (2008–2010) at two colonies with variable foraging conditions in Spitsbergen: Isfjorden and Magdalenefjorden. We found that a change in SST (range 1.6–5.4 °C) did not influence trip durations or provisioning rates. SST was, however, negatively correlated with the number of prey items delivered to a chick. Furthermore, provisioning rates did not influence chick’s probability to fledge; instead, SST was also negatively correlated with fledging probability. This was likely related to the prey availability and quality in the little auk’s foraging grounds. Our findings suggest that predicted warmer climate in the Arctic will negatively influence the ability of parents to provide their chicks, and consequently, the fledging prospects of little auk chicks.     

Reproductive measures and chick provisioning of Cory's Shearwater Calonectris diomedea borealis in the Azores

Capsule Timing of breeding influenced wing-length at fledging, and egg size may be an indicator of fledging weight and the amount of food received by chicks.

Aims To investigate chick growth, temporal patterns of chick food provisioning and the importance of indices of parental condition or quality, egg size and hatching date, to predict nestling body mass and wing-length at fledging, and compare breeding and chick feeding characteristics between colonies in the northeast Atlantic.

Methods A survey of Cory's Shearwater nests was carried out at Vila islet. A sample of 52 chicks, ringed and weighed at hatching, was selected to study chick growth and food provisioning.

Results Hatching success (51%) was much lower than fledging success (87%). Both hatching date and egg size contributed to explain wing-length at fledging, but hatching date, which was negatively correlated with wing-length at fledging, had the most important contribution (22%). There was some indication that egg size may explain variation in fledging weight and the amount of food received by chicks. Food delivery and feeding frequency of chicks varied throughout the chick development stage and three phases were distinguished: (1) 0–29 days, the highest feeding frequency values and a linear increase in food delivery; (2) 30–69 days, an oscillation in food delivery and medium feeding frequencies; (3) 70–90+ days, a sharp decrease in both food delivery and feeding frequency.

Conclusion Variation in food availability did not seem sufficient to override the overall importance of indices of parental quality in determining reproductive measures and chick provisioning. Breeding and feeding characteristics were similar between colonies in the northeast Atlantic, with variability in chick provisioning higher further south.     

Influence of environmental variability on breeding effort in a long-lived seabird, the yellow-nosed albatross

The provisioning parameters, breeding success, adult mass, andsurvival of yellow-nosed albatrosses were studied over 7 successiveyears at Amsterdam Island, southern Indian Ocean. We examinedthe ability of this long-lived seabird to adjust its breedingeffort under different environmental conditions and the fitnessconsequences in terms of survival and quality of offspring produced. Provisioning rate and adult mass varied extensivelybetween years, and the lowest and highest values were associatedwith sea surface temperature anomalies. When waters aroundthe island were colder, adults were in good condition and broughtlarge meals at short intervals, whereas warmer waters resultedin lower provisioning rates, lower adult mass, and lighter chicksat fledging. Adult survival and fledging success were not affectedby sea surface temperature anomalies. Yellow-nosed albatrossesappear to be unable to adjust their breeding effort every season,and their differential breeding investment probably primarilyreflects different levels of food availability. Yellow-nosedalbatrosses are able to regulate their provisioning behavior according to the nutritional status of their chick only whenconditions are favorable. Birds appear to invest primarilyin their own future maintenance rather than in provisioning.They have a wide safety margin in body mass that limits mortalityrisks during good years as well as during poor years. However,during unfavorable seasons adults continue to provision chicksthat have a poor prospect of survival to breeding, withoutadditional survival costs for the parents. Favorable seasonstherefore have a high value in terms of fitness because ofthe high quality of the chick produced. We suggest that understandinghow long-lived animals optimize their provisioning behaviorand lifetime reproduction can only be achieved through studiesencompassing several contrasted seasons.     

Buffering against food availability? The physical environment has little influence on breeding performance of fairy prions (Pachyptila turtur)

The breeding performance of seabirds is constrained by the availability of marine‐based prey, which may be influenced by competition with other seabirds, and environmental conditions. Fairy prion (Pachyptila turtur) populations have declined substantially in New Zealand since the introduction of mammalian predators; remaining small populations provide an opportunity to examine the effect of environmental factors on reproductive success in the absence of competition. Using 11 years of nest monitoring data and eight years of chick measurements we investigated (i) the link between physical environmental factors, breeding success rates and chick fledging size, and (ii) the relationship between chick fledging size and likelihood of natal‐colony recapture. Despite annual variations in the Southern Oscillation Index (SOI) and sea surface temperatures (SST), breeding success and chick fledging weights remained similar throughout the entire period, possibly due to the use of behavioural buffering mechanisms by parents. Fledgling wing lengths differed among years, and were predicted by early winter and early spring values of the SOI and late spring and summer SSTs. In years of high SOI and high SST, fairy prions fledged with longer wings, possibly due to increased availability of the euphausiid Nyctiphanes australis, their main prey. River flows and rainfall were unrelated to breeding success or chick measurements: the influence of freshwater on the marine system at this locality acts at smaller timescales than those studied here. Using the 2004 and 2005 cohorts, there was no link between fledging size and likelihood of recapture, except for the 2005 cohort which had unusually low 6‐year recapture rates, and shorter‐than‐average wing lengths. A combination of low sea temperatures and an El Niño event in 2005 may have led to a reduction in euphausiid availability, ultimately affecting post‐fledging survival.     

Sensitivity of breeding parameters to food supply in Black-legged Kittiwakes Rissa tridactyla

We fed Herring Clupea pallasi to pairs of Black-legged Kittiwakes Rissa tridactyla throughout the breeding season in two years at a colony in the northern Gulf of Alaska. We measured responses to supplemental feeding in a wide array of breeding parameters to gauge their relative sensitivity to food supply, and thus their potential as indicators of natural foraging conditions. Conventional measures of success (hatching, fledging and overall productivity) were more effective as indicators of food supply than behavioural attributes such as courtship feeding, chick provisioning rates and sibling aggression. However, behaviour such as nest relief during incubation and adult attendance with older chicks were also highly responsive to supplemental food and may be useful for monitoring environmental conditions in studies of shorter duration. On average, the chick-rearing stage contained more sensitive indicators of food availability than prelaying or incubation stages. Overall, rates of hatching and fledging success, and the mean duration of incubation shifts were the most food-sensitive parameters studied.     

Male and female contributions to provisioning rates of thin-billed prions, Pachyptila belcheri , in the South Atlantic

We recorded individual provisioning patterns of male and female thin-billed prions in the largest known colony on New Island (Falkland Islands, South Atlantic Ocean), to determine male and female contributions to provisioning rates and to compare them with data from their other main breeding site, on Kerguelen, in the Southern Indian Ocean. Both male and female birds from New Island undertook trips of 1–8 days’ duration (median 3 days), similar to those on Kerguelen, although we did not observe a clear distinction between long and short trips. Meal sizes increased with longer trips but did not differ between male birds and female birds. Thus, as on Kerguelen, male and female thin-billed prions on New Island contributed equally to chick provisioning. The proportion of short trips was comparable to those observed in a more favourable year, and was higher than those in a poor year, on Kerguelen. In order to avoid potential disturbance to the birds in the colony by frequent capture and handling, we used miniature radio-tags to record the presence of individuals in the colony and to evaluate the influence of transmitter load on the birds. Despite generally deteriorating conditions towards the end of the breeding season, chicks of tagged adults were fed normally, indicating that the method is adequate.     

Relationships between sea ice concentration, sea surface temperature and demographic traits of thin-billed prions

Understanding the effects of environmental variations on ecosystems is a major topic in ecology. In this study, we estimated demographic parameters of a seabird population, the thin-billed prion (Pachyptila belcheri) at Kerguelen Islands, and then tested for relationships with inter-annual variations of climatic indices, using long-term capture–recapture data. The annual adult survival probability was 0.825±0.009 and the breeding success was 0.519±0.090. Sea surface temperature anomalies were negatively related with breeding success. By contrast, winter sea ice concentration in the Antarctic seasonal ice zone seemed to negatively influence adult survival. This suggests a connection between sub-Antarctic and Antarctic ecosystems. The actual context of large climatic changes in the Austral Ocean seems to explain a large part of the decreasing trends observed for both the breeding success and the adult survival. Thus, a decrease of the population size of thin-billed prions at Kerguelen could be strongly suspected in the near future.     

Foraging and breeding performance of Japanese cormorants in relation to prey type

Seabirds are high trophic predators in marine ecosystems and are sensitive to change in food supply and thus seabirds can be used as monitors of the marine environment. In order to study the foraging responses of Japanese cormorants Phalacrocorax filamentosus breeding at Teuri Island, Hokkaido to changes in fish availability, the diet was assessed from the regurgitations of parents and chicks, and diving behavior was measured by using time-depth recorders. Breeding performance (brood size, chick growth, breeding success) was monitored using conventional methods to study their breeding responses. Japanese cormorants changed the diet and foraging behavior over four summers. The birds fed mainly on epipelagic schooling fish when they were available and on demersal fish when pelagic fish availability was low. They tended to dive deeper and longer in a year when they fed mainly on demersal fish than the other years, reflecting the change in the depth distribution of prey fish. Chick growth rate did not differ among years, but fledging success was lower in the years of demersal fish as their meal delivery rate was low. When epipelagic schooling fish were considered scare, parents maintained chick growth by reducing brood size. High variability and unpredictability in pelagic fish abundance are key factors affecting the foraging and breeding performance of Japanese cormorants, which could potentially be used to monitor fish resources.     

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.     

Interannual variation in the breeding biology of the Antarctic prion Pachyptila desolata at Bird Island, South Georgia

Interannual variation in aspects of the breeding biology of Antarctic prions was studied for three summers (1989–1992) at Bird island, South Georgia. Egg size, mass and incubation period remained constant. Laying, hatching and fledging were significantly delayed and less synchronous in 1991/92 (range of laying dates 51 days compared to 10–15 days in the two other seasons). This was due to an unusually cold and protracted winter, with ice blocking burrows into the spring, restricting availability of nest sites. Brooding lasted longer in 1991/92 but the overall fledging period was unchanged. Skeletal growth rates did not vary amongst years; growth in mass was slower in 1989/90 but fledging mass was similar in all three years. In 1989/90 and 1991/ 92 later hatched chicks grew (in mass) faster. The survival of chicks from hatching to fledging did not vary amongst years or with hatching date. Feeding frequency was similar between years, once allowance had been made for starlit nights. Thus late and asynchronous breeding in 1991/92 did not result in reduced breeding success either through predation or starvation.
Crustaceans formed 98–99% of the mass of the identifiable portion of regurgitated food samples. Significant annual variation was found within these crustaceans with the presence of krill (least in 1990/91) being inversely related to that of amphipods and copepods. There was no relationship between diet composition and chick growth or survival. Other seabird species, lacking the morphological specialization for feeding on copepods and amphipods, had very low breeding success in 1990/91, when krill was scarce.     

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.     

Effects of habitat fragmentation on provisioning rates, diet and breeding success in two species of tit (great tit and blue tit)

The aim of this study was to examine the effects of forest fragmentation on the ability of parent birds to provide their young with an adequate food supply. To examine whether prey population densities of the great tit (Parus major L.) and the blue tit (P. caeruleus L.) vary between study areas in different forest size classes we compared provisioning rates and chick diet and related these parameters to breeding success. We filmed 217 nests over two breeding seasons and collected data on frass fall as a general estimate of caterpillar availability. Nests which were attended by none or one parent only during filming (n = 46) were excluded from the analyses. In both years and for both species feeding rates were highest in the smallest fragments and lowest in the large forest. There was also a suggestion that differences in feeding rates between areas vary between years. We found no consistent tendency for prey size to change with forest size, although both species brought slightly smaller prey items to the nest in the smallest forest fragments and feeding rates correlated negatively with prey size. Caterpillars were the main item fed to nestlings, in both species. We found no evidence to suggest that either frass fall or the proportion of caterpillars in the diet varied with forest size. There was also no correlation between mean frass fall and the total number of caterpillars brought to the nests, in either species. Breeding success, as measured by clutch size, brood size, fledging weight and fledging success, did not differ between the small fragments and the large forest, in either species. There was also no relationship between provisioning rate (as concerns volume of prey fed to nestlings and the quality of chick diet) and breeding success parameters. In conclusion, this study does not suggest suboptimal foraging or breeding conditions in small fragments compared to a nearby large forest, for either species. Received: 23 June 1997 / Accepted: 29 December 1997     

Chick provisioning rates and growth in Blacklbrowed Albatross Diomedea melanophris and Grey-headed Albatross D. chrysostoma at Bird Island, South Georgia

We compared the parental division of labour and the pattern and rate of parental provisioning by two sympatric species of albatross of similar mass and breeding timetable but differing in diet and in the duration of chick‐rearing. Using electronic weighing platforms inside artificial nests, we recorded chick mass of Black‐browed Albatross and Grey‐headed Albatross at Bird Island, South Georgia every 10 minutes for both species in 1993 and 1994 and for each species in two other years between 1990 and 1996. The chick mass data (nearly one million weighings) were used to calculate meal mass (over 5000 meals) and intervals between meals. Adult birds were fitted with radio‐transmitters which allowed each meal to be allocated to the appropriate parent. The combination of meal mass and foraging trip duration were used to calculate provisioning rates for chicks and individual adults. Overall, Black‐browed Albatrosses delivered significantly lighter meals (569 g) than Grey‐headed Albatrosses (616 g) but more frequently (every 2.07 days and 2.50 days respectively). Thus combining foraging trip data for both parents, Black‐browed Albatross chicks received a meal every 1.22 days compared with 1.26 days for Greyheaded Albatross. These rates did not differ significantly. The contribution of each sex of each species in chick provisioning fluctuated between years, being similar in some years or biased towards males in others. Chicks of both species that failed to fledge received smaller, less frequent meals than successful chicks. In 1990 and 1994, Black‐browed Albatross chick provisioning rates were lower than in 1992 and 1993. In 1990, both meal mass and trip duration were affected, but only in 1994 was trip duration longer. Greyheaded Albatross chick provisioning rate was lower in 1994 than in other years but trip duration was longer. In each species, significant changes in meal mass and trip duration occurred within the chick‐rearing period. Chick provisioning rates invariably declined before chicks attained their peak mass. For both species, chick growth rates and peak and fledging mass, but not fledging age, were affected by differences in provisioning rate.     

Sex differences in Little Auk Alle alle parental care: transition from biparental to paternal-only care

Understanding differences in male and female care in biparental care systems can help interpret the selective pressures that shape parental strategies. We examined Little Auk Alle alle parental care at a breeding colony during the chick-rearing and fledging periods by conducting observations on marked, known-sex pairs, and by examining the sex ratio of birds carrying food to the colony. Little Auks transitioned from biparental to mostly paternal-only care during late chick-rearing. Males delivered more meals and spent more time at the colony than females during late chick-rearing. Very few females were present at the colony by the end of chick-rearing and through the fledging period, and all marked parents observed accompanying their chick to sea were male. Chick mass loss prior to fledging was associated with the lack of provisioning by the female parent, rather than a reduction in feeding frequency by both parents. The occurrence of paternal-only care during and after fledging is discussed in relation to physiological, ecological and phylogenetic constraints.     

Effects of individual life-history traits and weather on reproductive output of Black-headed Gulls Larus ridibundus breeding in the Wadden Sea, 1991–97

Capsule Annual breeding success was relatively constant and is shown to be related to clutch size and growth rate and to be positively affected by rain during egg-laying and advanced chick-rearing phase.

Aims To provide the first long-term data on breeding success of Black-headed Gulls in the Wadden Sea, to analyse its intrinsic and environmental co-variates, and to re-assess the significance of the severe winter 1995/96 on reproduction.

MethodsIn a relatively small colony at the Wadden Sea coast, clutches were selected randomly and enclosed to determine clutch size, egg biometrics, hatching and breeding success, and chick development in 1991 and from 1994 to 1997. Weather data (temperature, rain, wind) were related to life-history traits (clutch size, egg volume, chick development) and reproductive success.

Results Mean annual breeding success was 0.7 fledglings per pair. In 1996, breeding success tended to be lower and chick growth rate was significantly lower. Hatching success was lower in small clutches, chick mortality increased with decreasing chick growth rate. Rain during the egg-laying phase increased clutch size and decreased clutch predation. Rain during the early post-natal phase impaired chick growth. Rain during the phase of linear growth affected chick growth and fledging success positively and brood predation negatively.

Conclusions Breeding success of Black-headed Gulls breeding in the Wadden Sea is relatively constant between years, probably due to the use of terrestrial and marine feeding habitats. Rain may increase the availability of intertidal and terrestrial prey and thus may affect time budgets and food provisioning of parents positively. Lower breeding success in 1996 might have been caused by a relatively dry breeding season and possibly by the preceding severe winter.     

Predicting reproductive success from hormone concentrations in the common tern (<Emphasis Type="Italic">Sterna hirundo</Emphasis>) while considering food abundance

In birds, reproductive success is mainly a function of skill or environmental conditions, but it can also be linked to hormone concentrations due to their effect on behavior and individual decisions made during reproduction. For example, a high prolactin concentration is required to express parental behaviors such as incubation or guarding and feeding the young. Corticosterone level, on the other hand, is related to energy allocation or stress and foraging or provisioning effort. In this study, we measured individual baseline prolactin and corticosterone between 2006 and 2012 in breeding common terns (Sterna hirundo) using blood-sucking bugs. Reproductive parameters as well as prey abundance on a local and a wider scale were also determined during this period. Baseline prolactin and corticosterone varied significantly between years, as did breeding success. At the individual level, prolactin was positively and corticosterone was negatively linked to herring and sprat abundance. At the population level, we also found a negative link between corticosterone and prey abundance, probably reflecting overall foraging conditions. High prolactin during incubation was mainly predictive of increased hatching success, potentially by supporting more constant incubation and nest-guarding behavior. It was also positively linked to a lesser extent with fledging success, which could indicate a high feeding rate of young. Corticosterone concentration was positively related to high breeding success, which may be due to increased foraging activity and feeding of young. In general, our study shows that baseline prolactin and corticosterone levels during incubation can predict reproductive success, despite the presence of an interval between sampling and hatching or fledging of young.     

Drivers of Time-Activity Budget Variability during Breeding in a Pelagic Seabird

During breeding, animal behaviour is particularly sensitive to environmental and food resource availability. Additionally, factors such as sex, body condition, and offspring developmental stage can influence behaviour. Amongst seabirds, behaviour is generally predictably affected by local foraging conditions and has therefore been suggested as a potentially useful proxy to indicate prey state. However, besides prey availability and distribution, a range of other variables also influence seabird behavior, and these need to be accounted for to increase the signal-to-noise ratio when assessing specific characteristics of the environment based on behavioural attributes. The aim of this study was to use continuous, fine-scale time-activity budget data from a pelagic seabird (Cape gannet, Morus capensis) to determine the influence of intrinsic (sex and body condition) and extrinsic (offspring and time) variables on parent behaviour during breeding. Foraging trip duration and chick provisioning rates were clearly sex-specific and associated with chick developmental stage. Females made fewer, longer foraging trips and spent less time at the nest during chick provisioning. These sex-specific differences became increasingly apparent with chick development. Additionally, parents in better body condition spent longer periods at their nests and those which returned later in the day had longer overall nest attendance bouts. Using recent technological advances, this study provides new insights into the foraging behaviour of breeding seabirds, particularly during the post-guarding phase. The biparental strategy of chick provisioning revealed in this study appears to be an example where the costs of egg development to the female are balanced by paternal-dominated chick provisioning particularly as the chick nears fledging.     

Common Guillemot Uria aalge parents adjust provisioning rates to compensate for low food quality

The quantity and quality of food available within the foraging area set important constraints for chick‐rearing birds, but responses to low quality are not well understood. This study explored the potential for parent birds to adjust quantity (feeding rate) and quality (energy content) in chick provisioning, by studying Common Guillemots Uria aalge on Stora Karlsö, Baltic Sea, predominantly utilizing Sprat Sprattus sprattus, during conditions of high food quantity but reduced food quality. Quality is central to reproductive success in this single‐prey loader. From the chick's perspective, provisioning rates should be increased to compensate for low food quality and to fulfil its growing needs with increasing age. However, the high energy cost of flying in Guillemots makes it important for parent birds to minimize commutes to feeding areas. Provisioning parameters were recorded during three dawn‐to‐dusk watches each breeding season from 2005 to 2013, when clupeids, presumably Sprat, constituted 98% of chick diet. Generalized additive mixed models showed that both feeding rate and size of clupeids (a proxy for energy content) varied between years and changed non‐linearly with chick age, but that there was no change within breeding seasons. Chick age and year explained 36% of the variation in feeding rate but only 2% of the variation in the size of clupeids in chick diets. We conclude that parent birds tried to adjust both feeding rate and prey size, but were less successful with the latter. A strong negative correlation was found between annual feeding rates and size of clupeids, evaluated as the differences relative to the baseline year, and adjusted for the effects of chick age. Although the differences between years were small, the relationship indicates a compensation mechanism that does not seem to impact adult survival, and by which increased feeding rates can partly counteract reduced chick energy intake when food quality is low.