Validating growth and development of a seabird as an indicator of food availability: captive‐reared Caspian Tern chicks fed ad libitum and restricted diets

ABSTRACT For seabirds raising young under conditions of limited food availability, reducing chick provisioning and chick growth rates are the primary means available to avoid abandonment of a breeding effort. For most seabirds, however, baseline data characterizing chick growth and development under known feeding conditions are unavailable, so it is difficult to evaluate chick nutritional status as it relates to foraging conditions near breeding colonies. To address this need, we examined the growth and development of young Caspian Terns (Hydroprogne caspia), a cosmopolitan, generalist piscivore, reared in captivity and fed ad libitum and restricted (ca. one‐third lower caloric intake) diets. Ad libitum‐fed chicks grew at similar rates and achieved a similar size at fledging as previously documented for chicks in the wild and had energetic demands that closely matched allometric predictions. We identified three general characteristics of food‐restricted Caspian Tern chicks compared to ad libitum chicks: (1) lower age‐specific body mass, (2) lower age‐specific skeletal and feather size, such as wing chord length, and (3) heightened levels of corticosterone in blood, both for baseline levels and in response to acute stress. Effects of diet restriction on feather growth (10–11% slower growth in diet‐restricted chicks) were less pronounced than effects on structural growth (37–52% slower growth) and body mass (24% lower at fledging age), apparently due to preferential allocation of food resources to maintain plumage growth. Our results suggest that measurements of chick body mass and feather development (e.g., wing chord or primary length) or measurement of corticosterone levels in the blood would allow useful evaluation of the nutritional status of chicks reared in the wild and of food availability in the foraging range of adults. Such evaluations could also inform demography studies (e.g., predict future recruitment) and assist in evaluating designated piscivorous waterbird conservation (colony) sites.     

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.     

Experimental increase of flying costs in a pelagic seabird: effects on foraging strategies,nutritional state and chick condition

A central point in life history theory is that parental investment in current reproduction should be balanced by the costs in terms of residual reproductive value. Long-lived seabirds are considered fixed investors, that is, parents fix a specific level of investment in their current reproduction independent to the breeding requirements. We tested this hypothesis analysing the consequences of an experimental increase in flying costs on the foraging ecology, body condition and chick condition in Cory’s shearwaters Calonectris diomedea. We treated 28 pairs by reducing the wing surface in one partner and compared them with 14 control pairs. We monitored mass changes and incubation shifts and tracked 19 foraging trips per group using geolocators. Furthermore, we took blood samples at laying, hatching and chick-rearing to analyse the nutritional condition, haematology, muscle damage and stable isotopes. Eighty-day-old chicks were measured, blood sampled and challenged with PHA immune assay. In addition, we analysed the effects of handicap on the adults at the subsequent breeding season. During incubation, handicapped birds showed a greater foraging effort than control birds, as indicated by greater foraging distances and longer periods of foraging, covering larger areas. Eighty-day-old chicks reared by treated pairs were smaller and lighter and showed a lower immunity than those reared by control pairs. However, oxygen demands, nutritional condition and stable isotopes did not differ between control and handicapped birds. Although handicapped birds had to increase their foraging effort, they maintained physical condition by reducing parental investment and transferred the experimentally increased costs to their partners and the chick. This result supports the fixed investment hypothesis and is consistent with life history theory.     

Does the foraging strategy of adult Short‐tailed Shearwaters cause obesity in their chicks?

The chick provisioning behaviour of Short-tailed Shearwaters Puffinus tenuirostris breeding at the northern edge of their distribution on Montague Island, New South Wales, was examined in February and March 1997. The duration of individual foraging trips of parents, weight changes of adults and chicks, and meal sizes delivered to chicks were determined. It was found that individual parents mixed a long foraging trip to Antarctic waters (14.4±2.0 days) with one to three short foraging trips (1.36±0.7 days, mode=1 day). Adults gained body mass on long trips and lost weight on short trips. The size of meals fed to the chicks was significantly greater after a long trip (161±21 g) than after a short trip (135±28 g), although short trips increased the overall chick feeding frequency. The variable number of short trips made by adult Short-tailed Shearwaters and the relationship between short trips and adult body condition were consistent with current life-history theory: adults do not sacrifice their own body condition to increase food delivered to their chicks.
Modelling revealed that this dual foraging strategy inevitable leads to chicks enduring long intervals between meals. These long intervals may have led to the evolution of an over-feeding strategy by parents and the nestling obesity reported in this shearwater. The durations of the long trips from Montague Island were significantly greater than those for Short-tailed Shearwaters breeding at the centre of their distribution in Tasmania, although there was no significant difference in the length of short trips. A commitment to feed regularly in Antarctic waters may explain why the breeding distribution of this species does not extend much further north.     

Seasonal changes in the provisioning behaviour and mass of male and female wandering albatrosses in relation to the growth of their chick

Procellariiform seabirds have a number of extreme life-history characteristics in common, in particular low reproductive rates and slow postnatal development, which are generally assumed to reflect the difficulty in acquiring energy in the marine environment. The wandering albatross (Diomedea exulans) is a sexually dimorphic species with the longest postnatal growth found in any bird, suggesting severe constraints on provisioning and possible sex-specific strategies of provisioning. We studied the provisioning behaviour and mass changes of male and female parent wandering albatross throughout the 9-months rearing period to examine how each sex adjusts its foraging effort in relation to the needs of the chicks and the seasonal changes in food availability. The study was carried out on the Crozet Islands, using an automated system recording continuously the attendance pattern of parents between March and December 1994. During the brooding period when energy requirements are highest, parents only perform trips of short duration to sea, and their body condition deteriorates. When the chick is old enough to be left alone, the parents mix short and long foraging trips. The proportion of short trips is very high until July, allowing high rate of food delivery and rapid growth, and at the same time the body condition of adults improves. From August this proportion declines until fledging in December. As a result, the feeding rate decreases from August and adult condition declines, suggesting that feeding conditions at sea are better during the first part of the chick-rearing period, i.e. in autumn and winter. Male parents perform more short trips of shorter duration and provide larger meals than females, delivering an estimated total after brooding of 110 kg of food, compared to 70–80 kg delivered by females. Meal size is inversely related to the body condition of male chicks but not to that of female chicks, suggesting that food delivery is regulated by the adults in response to the condition of the male chick. Male chicks received larger meals and more food every month than female chicks, and overall it was estimated that they receive, after brooding, 195 kg of food compared to 180 kg for the female. As a result, male chicks have a higher growth rate, attain a higher asymptotic mass, and are larger and heavier at fledging than female chicks. However, the differences are relatively small between the chicks of each sex and suggest that energy may be used differently between the sexes to maximise fitness. The results of the study suggest that provisioning effort of wandering albatrosses is adjusted by parents in relation to the availability of food, to the energetic needs of the chick and to the sex of the chick. The adult body mass is likely to play an important role in the long term for the regulation of provisioning, deficits in body mass probably providing the buffer in high power-requirement periods. Accepted: 20 March 2000     

Effects of color banding, radio tagging, and repeated handling on the condition and survival of Lapwing chicks and consequences for estimates of breeding productivity

ABSTRACT.   Color bands and radio tags are widely used to facilitate individual recognition and relocation of precocial chicks in studies of prefledging survival. However, the accuracy of data collected and subsequent estimates of survival rates rely on the assumption that such techniques do not affect the parameters under study. We compared the body condition and survival of color-banded and radio-tagged Lapwing ( Vanellus vanellus ) chicks with noncolor-banded and nonradio-tagged individuals using a 10-year dataset ( N = 3174 chicks, with 205 color banded and 700 radio tagged). Color bands did not adversely affect chicks. However, radio-tagged chicks and their untagged broodmates were handled more frequently because these broods were more readily encountered than those without a tagged member. Chicks disturbed and handled more frequently had lower body condition indices and higher mortality rates. Simulations of the impact of tagging and handling on breeding productivity under two scenarios indicated a 26% reduction in productivity in situations where nest and chick survival rates were low (as in our study), but only a 7% reduction in productivity for a simulated population with the higher levels of nest and chick survival associated with a stable population. The frequent disturbance associated with radio-tracking and recapturing chicks, rather than the attachment of a tag or physical handling of chicks, may affect body condition. Frequent handling and disturbance may affect body condition by reducing foraging time, increasing stress levels, or increasing predation risk. Because our results suggested that the negative impact of handling could last up to a week, we recommend that investigators avoid disturbance of shorebird chicks more frequently than every 8 d.     

The effects of loggers on the foraging effort and chick-rearing ability of parent little auks

We studied the effects of loggers attached to chick-rearing little auks (Alle alle) on their daily time budget (proportion of time spent in the colony and at sea), foraging activity (duration and proportion of long and short foraging flights), chick provisioning rate and their growth and development on Spitsbergen. We found that experimental parent birds performed shorter but more frequent long foraging flights and reduced the frequency of short foraging flights. They spent more time at the colony and reduced chick provisioning rate compared to control birds. Nestlings reared by experimental parents weighed significantly less at their middle, peak and fledging age and departed colony later than chicks of control parents. Little auks depend on energy-rich copepods associated with cold Arctic waters and are expected to face the climate-induced worsening of the foraging conditions, which may have negative impact on their time/energy budget and survival. The study may help to determine the level of extra effort little auks need to invest to breed successfully.     

Parent-offspring interactions in food provisioning of Manx shearwaters: implications for nestling obesity

Procellariiform seabirds such as the Manx shearwater Puffinus puffinus, rear only one chick at a time but may breed many times in their lives; parents should thus limit food delivery to the chick in keeping with the balance between current and future reproductive output. Yet procellariiform chicks accumulate large quantities of lipid, which may provide a buffer against pronounced and unpredictable variation in food provisioning, resulting in part from an inability of parents to regulate food supply to the nest. We switched chicks between nests to examine the roles of parents and offspring in controlling food delivery. The serial autocorrelation in age-specific body masses for unmanipulated chicks decreased from 0.61 (P< 0.01) to 0.35 (NS) over a period of 15 days and remained nonsignificant thereafter. By contrast, the serial autocorrelation for switched chicks increased from 0.64 (P< 0.01) to 0.83 (P< 0.001) and the serial cross-correlation rose from 0.23 (NS) to 0.50 (P< 0.05). These results supported both chick determination and parental determination models of food provisioning, indicating that chicks conveyed information about their nutritional status, which parents acted upon by adjusting their rate of food delivery. We discuss these results in relation to the optimization of nestling lipid reserves and parental foraging effort. We suggest that information conveyed by the chick's begging intensity serves to reduce the provisioning rate to well-fed chicks, but parents cannot or do not increase food provisioning to poorly fed chicks. Such adjustment of food provisioning does not refute the hypothesis that nestling obesity provides a buffer against highly variable food delivery. Copyright 1999 The Association for the Study of Animal Behaviour.     

Foraging and provisioning strategies of black-browed albatrosses in relation to the requirements of the chick: natural variation and experimental study

We studied regulation of the food supply to black-browed albatrosschicks at Kerguelen by simultaneously recording the provisioningrates achieved by individual parents and satellite trackingforaging birds during two seasons, by studying changes in adultmass, and by experimentally manipulating the food requirementof chicks. In 1994 black-browed albatrosses had a higher breedingsuccess and produced heavier chicks that grew faster than in1995. They spent a similar time foraging but brought heaviermeals to their chick in 1994. Satellite tracking indicated thatin both seasons birds foraged in the same oceanographic area,250 km from the colony. Travel times to and from this area remainedunchanged, and similar times were spent foraging there. In ourstudy area, black-browed albatrosses appear to rely on a foodresource that is predictable in location, but whose availabilityvaries from one year to the next. The principal difference betweenyears of differing food availability was that birds broughtlarger meals when food was more abundant Costs of commutingto nearby feeding areas are probably low and allow the deliveryof energy to the chick at a high rate. A study carried out in1991 indicated that there was no relationship between the changesin adult mass from one trip to the next and the duration offoraging trips or feed mass, suggesting that adult body conditionhad little influence on the provisioning strategy of this species.An experiment whereby some chicks were deprived of food andothers received supplementary food showed that parents of underfedchicks spent the same time foraging and brought slightly largeramounts of food to their chicks as control parents. We suggestthat parents are searching for food to the maximum limits oftheir ability and thus cannot reduce further foraging time,but underfed chicks can swallow more food. Parents of overfedchicks delivered less food and increased the time between feeds.The reduction in provisioning frequency is interpreted as thecapacity of parents to modify their foraging behavior accordingto the nutritional status of the chick, but the reduction offeed mass is probably the result of chicks being close to theirmaximum assimilatory capacity. Comparison between Procellariiformspecies indicates extensive differences in the degree to whichparents can regulate the supply of food to their chicks. Neriticspecies like black-browed albatrosses appear to have a reducedability to regulate, and especially to increase provisioningrates, whereas more pelagic species may have a greater regulationability     

The individual counts: within sex differences in foraging strategies are as important as sex‐specific differences in masked boobies Sula dactylatra

Understanding how animals allocate their foraging time is a central question in behavioural ecology. Intrinsic factors, such as body mass and size differences between sexes or species, influence animals’ foraging behaviour, but studies investigating the effects of individual differences in body mass and size within the same sex are scarce. We investigated this in chick‐rearing masked boobies Sula dactylatra, a species with reversed sexual dimorphism, through the simultaneous deployment of GPS and depth‐acceleration loggers to obtain information on foraging movements and activity patterns. Heavier females performed shorter trips closer to the colony than lighter females. During these shorter trips, heavier females spent higher proportions of their flight time flapping and less time resting on the water than lighter females did during longer trips. In contrast, body mass did not affect trip duration of males, however heavier males spent less time flapping and more time resting on the water than lighter males. This may occur as a result of higher flight costs associated with body mass and allow conservation of energy during locomotion. Body size (i.e. wing length) had no effect on any of the foraging parameters. Dive depths and dive rates (dives h^?1) were not affected by body mass, but females dived significantly deeper than males, suggesting that other factors are important. Other studies demonstrated that females are the parent in charge of provisioning the chick, and maintain a flexible investment under regulation of their own body mass. Variation in trip length therefore seems to be triggered by body condition in females, but not in males. Consequently, shorter trips are presumably used to provision the chick, while longer trips are for self‐maintenance. Our findings underline the importance of accounting for the effects of body mass differences within the same sex, if sex‐specific foraging parameters in dimorphic species are being investigated.     

Effect of GPS tagging on behaviour and marine distribution of breeding Arctic Terns Sterna paradisaea

Tracking tags have been used to map the distributions of a wide variety of avian species, but few studies have examined whether the use of these devices has impacts on the study animals that may bias the spatial data obtained. As Global Positioning System (GPS) tags small enough for deployment on terns (family: Laridae) have only recently become available, until now tracking of this group has been conducted by following unmanipulated individuals by boat, which offers a means of comparing distributions obtained from GPS‐tracking. We compared the utilization distributions (UDs) of breeding Arctic Terns Sterna paradisaea obtained by GPS‐tracking 10 individuals over 2 weeks, with UDs derived from contemporaneous visual boat tracks from 81 individuals. The 50% and 95% UDs of both methods had high similarity scores, indicating good agreement in the density distributions derived from the two methods. The footprints of the UDs of tagged birds were ~ 75–80% larger, which may reflect an effect of tagging on foraging range or the occasional inability to follow by boat individuals which roamed further from the colony. We also compared the nest attendance and chick provisioning rates of adults that were (1) fitted with a GPS tag and leg‐flag, (2) handled and marked with a leg‐flag but not tagged and (3) fitted with a leg‐flag in a previous year but unhandled in the year of the study. There was some evidence that birds fitted with both a GPS tag and leg‐flag spent slightly less time at the nest compared with unhandled birds and those fitted with a leg‐flag only. Both treatments where birds were fitted with a leg‐flag in the year of the study had similarly lower provisioning rates to those of unhandled control birds > 48 h after handling, suggesting that negative effects on provisioning are due to capture and handling or leg‐flag attachment rather than to GPS tag attachment/loading per se. Overall brood‐provisioning rate was compensated for by the increased effort by the unhandled partner. Our study suggests that despite slight effects of GPS‐tagging on behaviour, the estimates of marine density distribution obtained were very similar to those of unmanipulated birds.     

Parental provisioning behaviour in a flock-living passerine, the Vinous-throated Parrotbill Paradoxornis webbianus

The amount of food delivered by parents to their chicks is affected by various life history traits as well as environmental and social factors, and this investment ultimately determines the current and future fitness of parents and their offspring. We studied parental provisioning behaviour in the Vinous-throated Parrotbill Paradoxornis webbianus, a species with an unusual social system that is characterised by flock-living, weak territoriality and variable nesting dispersion. Parental provisioning rate had a positive influence on chick mass gain, suggesting that provisioning rate is an effective measure of parental investment in this species. Males and females fed nestlings at approximately the same rate, and no other carers were observed at nests. Parents coordinated provisioning rates so that they mostly fed chicks synchronously. However, the extent to which parents coordinated provisioning was associated with their social environment, synchrony being positively related to local breeding density and negatively to nearest neighbour distance. The rate at which parents provisioned nestlings showed the same relationships with social measures, being greatest at higher density and when neighbours were closer. Visit rate was also related to chick age, but not to brood size, brood sex ratio, extra pair paternity, laying date, temperature, parents’ body characters, time of day or year. We conclude that a breeding pairs’ social environment plays an important role in determining parental investment, probably through its effects on the opportunities that parents have for foraging with conspecifics.     

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.     

Effects of satellite transmitters fitted to breeding Cory's shearwaters

We studied the effects of a new technique to fix satellite transmitters on the feeding behavior and chick body growth of the Cory's shearwater (Calonectris diomedea). Transmitters did not significantly affect the breeding bird's body mass or diving performance. However, on average, pairs with ≥1 adult equipped with a transmitter brought a lighter meal (54.8 ± 18.9 g) to nestlings than did control birds or the same pair with the transmitter removed (77.4 ± 8.3 g) leading to a decreased chick body growth. However at fledging, chicks in control and treatment groups had similar body mass whether parents were equipped with a PTT or not. Our method of transmitter attachment could therefore be an appropriate alternative for studying the ecology of procellariiforms weighing <1 kg given that birds are fitted with transmitters for <2 weeks and restricted to 1 parent only. © 2011 The Wildlife Society.     

Relationships between brood size and parental provisioning performance in chinstrap penguins during the chick guard phase

 Chinstrap penguins (Pygoscelis antarctica) normally lay two eggs, but brood size is often reduced by mortality during incubation or after hatching. We hypothesized that this variation in brood size would affect the parents’ foraging behavior and their chick provisioning performance. We studied patterns of adult foraging trip duration and frequency, food load delivery, and chick growth rates in relation to brood size during the guard phase in four breeding seasons (1991–1994) on Seal Island, Antarctica. Within a given year, parents with two chicks made more frequent foraging trips to sea and may have transported larger food loads to the nest; however, the duration of foraging trips was unrelated to brood size. Overall, parents with two chicks spent ∼15% more time at sea than parents with only one chick. Both the frequency and duration of foraging trips varied between years. Foraging trip duration may partly reflect the birds’ foraging radius, which probably varies with time in response to shifts in krill distribution. Chick growth rate varied betwen years, but was related to brood size only in 1992, when chicks from two-chick broods grew significantly more slowly than chicks from one-chick broods. Food loads transported to chicks, as well as chick growth rates, were highest in 1994, when concurrent hydroacoustic studies indicated that regional krill biomass was severely depressed. This apparent anomaly suggests that the spatial scale of the krill survey may have been too coarse to detect some high-density krill aggregations within the penguins’ foraging range. Received: 26 September 1995 / Accepted: 12 May 1996     

Body mass changes in Brünnich's guillemots Uria lomvia with age and breeding stage

Body mass of Brünnich's guillemots Uria lomvia breeding at Coats Island, Canada, was measured during incubation and chick‐rearing in 1988–2001. In most years, mass increased during incubation and fell after hatching, leveling off by the time chicks were 18 d old, close to the age at which chicks departed. Mass during incubation increased with age up to about 12 yr, but the mass of birds brooding chicks was not related to age. The trend towards increasing mass during incubation was mainly a consequence of mass increases of young breeders as older birds maintained a constant mass. The variation in adult mass with age during incubation seems likely to reflect age‐related variation in foraging ability, but the loss of mass after hatching, being greater for older birds, appears best explained as a response to the demands of provisioning chicks, with older birds transferring their accumulated reserves to their chicks via higher provisioning rates.     

Sex‐specific mass loss in chick‐rearing South Polar Skuas Stercorarius maccormicki– stress induced or adaptive?

Mass loss of chick‐rearing birds can be the direct consequence of physiological stress (reproductive stress hypothesis) or an adaptive mass adjustment in response to the increased demands on flight efficiency during the flight‐intensive chick‐rearing period (adaptive mass loss hypothesis). To test which of these hypotheses best explains mass loss in South Polar Skuas Stercorarius maccormicki rearing chicks, a food supplementation experiment was carried out in the austral summer 2000/01 at King George Island, Antarctica. Half of the breeding pairs were fed about 20% of the chick's daily energy demand every second day and chick growth and adult nest attendance were recorded. Parents were caught at the start and the end of chick‐rearing to calculate adult mass loss. Male parents of food‐supplemented pairs attended their nest territories more than control males but females kept their attendance constant. Chick growth was only minimally affected and the treatment probably had no fitness consequences. Male Skuas in control pairs had a higher deviation from the body size–mass regression at the end of chick‐rearing compared with the start, supporting the stress hypothesis, whereas female deviation remained unchanged. Males of food‐supplemented pairs were heavier than unsupplemented males at the end of the breeding cycle but not significantly so. Food‐supplemented females were lighter at the end, supporting the adaptive mass loss hypothesis. Adult mass loss is thus best explained by the reproductive stress hypothesis in males but by the adaptive mass loss hypothesis in females. However, the two hypotheses are not mutually exclusive and the results do not exclude the possibility that mass loss in females is stress‐induced but the amount of mass lost is an adaptive adjustment to the reliability of the food supply. The finding that members of a breeding pair may follow different strategies of mass adjustment has implications for the use of mass loss as an index of parental effort. Without knowing which strategy each sex has adopted it is of little use to compare mass loss between parents.     

Regulation of food provisioning in the Antarctic petrel; the importance of parental body condition and chick body mass

1. Two hypotheses may explain how long-lived seabirds regulate the food provisioning to their chick. The fixed level of investment hypothesis states that the parents provide food for their chick according to an intrinsic rhythm, independent of their chick's need. The flexible investment hypothesis states that the parents adjust their food provisioning both according to their chick's and their own need.
2. We tested how the Antarctic petrels adjust the food-provisioning according to their own body condition or to their chick's need. First, we selected parents in poor and good body condition. Then we gave all parents randomly a chick of different body mass, but of the same age. We then measured the chicks daily until they were fed for the first time after swapping.
3. Parents in good body condition at hatching were more likely to produce a chick that was still alive 9 days after hatching than parents in poor body condition. Chick body mass at day 9 and at the end of the guarding period was positively related to the mean body condition of the parents at hatching.
4. The meal size provided by parents in good body condition was larger than that provided by parents in poor body condition. Parents in good body condition delivered more food to small than to large chicks, whereas no such relationship was found among parents in poor body condition.
5. Our results suggest that the Antarctic petrel parents adjust the amount of food delivered to their chick according to both the chick's need and their own body condition, and that the ability to respond to the chick's need is dependent upon their own body condition.     

Adult Brünnich's Guillemots Uria lomvia balance body condition and investment in chick growth

To investigate the covariation of adult body condition and nestling growth, we weighed adult Brünnich's Guillemots Uria lomvia rearing chicks at Coats Island, Nunavut, Canada, each year between 1988 and 2002. We estimated chick mass at 14 days for a sample of chicks reared in the same years. Adult mass and chick mass at 14 days were highly correlated, suggesting that, as feeding conditions deteriorate, adults compromise by reducing their own body reserves, while at the same time delivering less food to their offspring. We compared the prediction of the least-squares regression for the Coats Island data with observations made at Digges Island, a much larger colony about 300 km away, where birds are similar in linear body measurements to those at Coats Island and have a similar body mass while incubating. Adult mass at Digges Island averaged 11% less during chick-rearing than during incubation, compared with only a 5% difference at Coats Island. Mean chick mass at 14 days at Digges Island was lower in all years than was observed for chicks at Coats Island in any year. The observed 14-day chick masses at Digges Island in two years were close to values predicted by adult mass and somewhat lower in two other years (those when chick growth was slowest). At Digges Island, the distribution of mass for brooding adults was right skewed and suggested a lower threshold at 800–850 g, below which Brünnich's Guillemots terminate breeding. We conclude that the correlation between adult and chick mass represents a dynamic equilibrium in which adults simultaneously adjust their own energy reserves and their delivery rate to the chick. This compromise must be based on behavioural choices made by individual birds and is unlikely to be a passive consequence of fluctuating conditions.     

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.