Chick-rearing Period Research Articles

Blood fatty acids indicate inter- and intra-annual variation in the diet of Adélie penguins: Comparison with stomach content and stable isotope analysis

Adélie penguin ( Pygoscelis adeliae) diet is an important indicator of prevailing environmental conditions and resource availability. In this study, dietary variation within and between years was studied with fatty acid signature analysis (FASA), stomach content analysis (SCA) and stable isotope analysis (SIA). We profiled the fatty acid (FA) composition of whole blood collected from adult penguins throughout the breeding season, and from chicks during the crèche period, in 2001 and 2002. Differences were detected in FA profiles between years, breeding stage and age (adults vs. chicks). These patterns broadly corresponded to those observed from SCA and SIA, with a mix of krill and fish consumed in the early part of the breeding season in both years, krill dominating the diet during the chick-rearing periods in 2001, and fish in 2002. Different metabolic and physiological demands between stages, and ages, may also influence FA profiles but warrants further investigation. In-situ calibrations of adult FA blood profiles were made using corresponding stomach samples to quantify diet composition. Using linear discriminate function analysis, we classified adult FA profiles into 3 meal-types: krill, fish or mixed. A higher proportion of adults had fish-like profiles during the arrival and guard periods. Krill-like profiles dominated during the incubation and crèche periods, although there were a relatively high proportion of fish-like and mixed profiles as well. These patterns corresponded to results from SCA and SIA. This study demonstrates that FASA has the potential to be integrated with other dietary tools to enhance diet monitoring studies, which are currently integral to ecosystem management and conservation measures. The in-situ calibration method used offers a simple and effective alternative to more rigorous calibration techniques developed elsewhere.

Foraging behavior of Adélie penguins during incubation period in Lützow-Holm Bay

The diving behavior of Adelie penguins Pygoscelis adeliae was investigated using time–depth recorders during the incubation period in the fast sea-ice area of Lutzow-Holm Bay, Antarctica. Dive profiles and activity/time allocation suggested that penguins were obligated to walk on the fast-ice for 90–100 km until a polynya, which they used as an access to the pack-ice zone. Dive depth did not differ between males and females, though males’ dive duration was longer than that of females. Dive depth was slightly shallower and dive duration was shorter during the incubation than during the chick-rearing phase. Birds dove throughout the day, although less frequently around midnight, and there was no clear diel change in dive depth. This daily dive pattern during incubation period was similar to that previously observed during the chick-rearing period in a fast sea-ice area, but differed from that observed in sea-ice-free area. Variations in diving behavior resulted from different environmental conditions, such as foraging area with different sea-ice condition, as well as from different life history strategies.

TIME BUDGETS OF COMMON MURRES VARY IN RELATION TO CHANGES IN INSHORE CAPELIN AVAILABILITY

Changes in capelin (Mallotus villosus) biology since the 1990s have directly or indirectly induced variable breeding conditions for many seabirds in the Northwest Atlantic. Time budgets of the same individual Common Murres (Uria aalge) were examined in relation to annual variations in the arrival of inshore spawning capelin during three consecutive chick-rearing periods (1998–2000) on Great Island in the Witless Bay Ecological Reserve, Newfoundland, Canada. Despite high foraging effort (longer provisioning trips, lower co-attendance time, and faster colony departure after a brooding bout), chick-feeding rates were low during early chick rearing in 2000 due to a mismatch between early egg hatching and the later inshore arrival of spawning capelin. Time budgets of murres breeding on Great Island were examined in relation to those of murres in other nearby colonies and to long-term changes in capelin spawning behavior across decades (1982–1985 versus 1998–2000). Recent overall provisioning rates on Great Island were comparable to those of other nearby colonies during the same time period (with the exception of Funk Island) and to those reported during the 1980s. However, murres breeding in recent years in the reserve exhibited higher foraging effort, which is likely linked to recent changes in the availability of capelin as a result of later spawning, changes in capelin distribution, and their overall smaller size. Murres currently may be constrained to timing their egg laying such that hatching coincides with the arrival of inshore capelin rather than peak capelin abundance and, thus, are vulnerable to sudden changes in the ecosystem.

Metabolic adjustments in breeding female kittiwakes (Rissa tridactyla) include changes in kidney metabolic intensity

Black-legged kittiwakes (BLKIs) reduce self-maintenance cost through reductions in mass-specific basal metabolic rate (BMR), body mass and the size of visceral organs during the chick-rearing period. In the present study, we measured kidney in vitro oxygen consumption and plasma 3,3',5-triiodo-L: -thyronine (T3) levels of incubating and chick-rearing female BLKIs, to test whether the decrease in BMR is caused mainly by decreased metabolic intensity or simply by reductions in the size of organs with high metabolic intensity. Body mass and body condition were lower in chick-rearing birds compared with the incubating birds. In contrast to the previous findings, however, the kidney mass did not differ between the two breeding stages. Plasma T3 levels decreased substantially during the breeding season, indicating a reduction in BMR. Over the same period, kidney mass-specific oxygen consumption decreased (by 17.2%) from the incubating to the chick-rearing stage. Thus, the reduction in BMR found in breeding BLKIs seems partly explained by adjustments in metabolic intensity of visceral organs. Lowered metabolic intensity of visceral organs would permit increased allocation of energy to offspring at the expense of their own self-maintenance.

VARIATION IN MARINE DISTRIBUTIONS OF CASSIN'S AUKLETS (PTYCHORAMPHUS ALEUTICUS) BREEDING AT TRIANGLE ISLAND, BRITISH COLUMBIA

Abstract Approximately 55% of the world's population of Cassin's Auklets (Ptychoramphus aleuticus) nests at Triangle Island, British Columbia. To improve our understanding of the biology of these birds during the breeding season, we tracked 112 radiomarked individuals over three years, 1999–2001. We flew high-altitude surveys to describe the at-sea distributions of the marked birds during the chick-rearing period. Using radiotelemetry point locations, we determined that the location of marine use areas, distance from colony, and water depth varied significantly across years. In 1999 and 2000, radiomarked birds were, on average, ∼50 km southwest of Triangle Island in waters approximately 1400–1800 m deep. However, in 2001, radiomarked birds were, on average, ∼80 km northwest of Triangle Island in waters ∼725 m deep. Intra-annually (i.e., between surveys spanning days or weeks), there were no such large-scale directional shifts in marine use area. The size of marine use areas (quantified using kernel home r...

The diet of European Shag Phalacrocorax aristotelis, Black-legged Kittiwake Rissa tridactyla and Common Guillemot Uria aalge on Canna during the chick-rearing period 1981–2007

Chick diet of European Shags Phalacrocorax aristotelis, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge at Canna was investigated over a 27- year period. The diet was mainly composed of Sprats Sprattus sprattus, Lesser Sandeels Ammodytes marinus and members of the Gadidae (a variety of species but mainly Trisopterus spp. and Whiting Merlangius merlangus). Other groups (ten families of fish, crustaceans, cephalopod molluscs and polychaete worms) were of minimal importance. Lesser Sandeels dominated the diet of young Black-legged Kittiwakes and European Shags, Sprats the diet of young Common Guillemots, whereas gadid otoliths were by far the commonest items found in pellets regurgitated by older European Shags. There were few significant temporal changes in species composition or the size of prey taken over the 27 years and the results confirm earlier findings that gadids are a normal and important part of the diet of seabirds at this colony.

Sex-specific differences in body condition indices and seasonal mass loss in Tufted Puffins

Reduced prey availability can affect the growth and survival of nestling seabirds. However, few studies have demonstrated similar effects on indices of adult body condition. We examined body condition and seasonal mass loss of breeding adult male and female Tufted Puffins (Fratercula cirrhata) at Chiniak Bay, Kodiak Island, Alaska, in 2004-2005. We determined sex using genetic analysis, developed a discriminant function to determine sex using morphometric measurements, and examined the body condition of adult males and females relative to the growth rates of their offspring. We found that morphological measurements were only moderately useful for sexing Tufted Puffins, with 74% of adults (N = 176) correctly classified. We also found that the relationship between adult body mass and size differed between sexes and conclude that body condition indices must be calculated separately for each sex to avoid inter- and intrasexual bias. Body condition of male and female Tufted Puffins declined during the chick-rearing period. However, body condition of females did not differ between years, whereas male condition declined to a greater degree during 2004 when the mass of young at fledging was significantly lower. Although these results suggest that adult male Tufted Puffins sacrifice their own body condition in years of diminished nestling growth and females do not, reasons for this apparent intersexual difference in reproductive strategies remain unclear.

Nutritional and feeding ecology in Cory’s shearwater Calonectris diomedea during breeding

In birds, parents adjust their feeding behaviour according to breeding duties, which ulti- mately may lead to seasonal adjustments in nutritional physiology and hematology over the breeding season. Although avian physiology has been widely investigated in captivity, few studies have inte- grated individual changes in feeding and physiological ecology throughout the breeding season in wild birds. To study relationships between feeding ecology and nutritional ecophysiology in Cory's shearwater Calonectris diomedea, we weighed and took blood samples from 28 males and 19 females during the pre-laying, egg-laying, incubation, hatching and chick-rearing periods of the breeding season. In addition, we fitted 6 birds with geolocators to track their foraging movements throughout the reproductive period. Thus, we examined individual changes in (1) nutritional condition (bio- chemistry metabolites); (2) oxygen carrying capacity (hematology); and (3) feeding areas and forag- ing effort (stable isotopes and foraging movements). Geolocators revealed a latitudinal shift in main feeding areas towards more southern and more neritic waters throughout the breeding season, which is consistent with the steady increase in δ 13 C signatures in the blood. Geolocators also showed a decrease in foraging effort from egg-laying to hatching, reflecting the activity decrease associated with incubation duties. Plasma metabolites, body mass and oxygen carrying capacity were associated with temporal changes in nutritional state and foraging effort in relation to recovery after migration, egg formation, fasting shifts during incubation and chick provisioning. This study shows that com- bining physiological and ecological approaches can help us understand the influence of breeding duties on feeding ecology and nutritional physiology in wild birds.

Physiological Condition of Incubating and Brood Rearing Female Great TitsParus majorin Two Contrasting Habitats

Little is known about the physiological causes and consequences of habitat choice decisions in birds. We compared size-corrected body mass, hematocrit, plasma albumin, beta- and gamma-globulin and triglyceride concentrations, as well as the albumin/globulin ratio of female Great Tits breeding in deciduous and coniferous forests in Estonia. Females were sampled during incubation and during the chick-rearing period. Whereas the nest box occupation rate and clutch size were higher in the deciduous habitat, the female nutritional condition during incubation was better in the coniferous habitat. This habitat difference in condition was not explained by the differences in the start of egg-laying or clutch size. Females lost more mass from incubation to the chick-rearing stage in the coniferous than in the deciduous forests. Although the values of most blood parameters changed from incubation to the chick-rearing stage, no habitat-related change was observed. There were no habitat differences in the number and quality of fledglings or in female condition during the chick-rearing period. Our results indicate that incubating female Great Tits are in a worse nutritional state in the preferred deciduous habitat fragments than those in the non-preferred coniferous habitat. However, habitat-related differences in condition during incubation seem to be unrelated to habitat-specific reproductive decisions.

Introduced mammals coexist with seabirds at New Island, Falkland Islands: abundance, habitat preferences, and stable isotope analysis of diet

The largest known colony of Thin-billed prions Pachyptila belcheri has been coexisting with introduced mammals for more than 100 years. Three of the introduced mammals are potential predators of adults, eggs and chicks, namely ship rats Rattus rattus, house mice Mus musculus and feral cats Felis catus. We here determine habitat preferences over three seasons and dietary patterns of the unique set of introduced predators at New Island, Falkland Islands, with emphasis on the ship rats. Our study highlights spatial and temporal differences in the levels of interaction between predators and native seabirds. Rats and mice had a preference for areas providing cover in the form of the native tussac grass Parodiochloa flabellata or introduced gorse Ulex europaeus. Their diet differed markedly between areas, over the season and between age groups in rats. During the incubation period of the prions in November–December, ship rats had mixed diets, composed mainly of plants and mammals, while only 3% of rats had ingested birds. The proportion of ingested birds, including scavenged, increased in the prion chick-rearing period, when 60% of the rats consumed prions. We used δ13C and δ15N to compare the importance of marine-derived food between mammal species and individuals, and found that rats in all but one area took diet of partly marine origin, prions being the most frequently encountered marine food. Most house mice at New Island mainly had terrestrial diet. The stable isotope analysis of tissues with different turnover times indicated that individual rats and mice were consistent in their diet over weeks, but opportunistic in the short term. Some individuals (12% of rats and 7% of mice) were highly specialized in marine-derived food. According to the isotope ratios in a small sample of cat faeces, rodents and rabbits were the chief prey of cats at New Island. Although some individuals of all three predators supplement their terrestrial diet with marine-derived food, the current impact of predation by mammals on the large population of Thin-billed prions at New Island appears small due to a number of factors, including the small size of rodent populations and restriction mainly to small areas providing cover.

Avian predation at a Southern Rockhopper Penguin colony on Staten Island, Argentina

We studied predation risk in relation to nest location and subcolony size in Southern Rockhopper Penguins (Eudyptes chrysocome chrysocome) during the chick-rearing period. Striated Caracaras (Phalcoboenus australis), the main predator, preferentially attacked from tussock grasses which are found in the periphery of all subcolonies (peripheral tussocks) and often scattered within them (central tussocks). The greatest numbers of predation and attempted predation events were observed on nests in the periphery of the subcolony next to peripheral tussocks, and on those nests next to central tussocks. Central tussocks offer Striated Caracaras an additional “edge” area from which to prey, much in the same way as do the peripheral tussocks. Predation rate per individual was not correlated with subcolony size possibly due to the presence of central tussocks which, by creating an extra edge area, change the subcolony shape. There is a suggestion (P = 0.06) of increased probability of nest success with subcolony size.

Feeding, fasting and foraging success during chick rearing in macaroni penguins

Foraging behaviour and energy expenditure were measured continuously throughout the chick-rearing period of free-ranging macaroni penguins Eudyptes chrysolophus. These data were integrated with values obtained from the literature on body mass, assimilation efficiency, body reserve consumption and deposition rates, chick growth and energy expenditure and energy content of food in a new type of algorithm to predict (with 95% confidence limits (CL)) foraging success and daily changes in body mass. A successfully breeding pair of macaroni penguins will capture 111.7 kg (95% CL: 79.4 to 158.0 kg) of prey during the chick-rearing period. The crucial phase of the chick rearing period was around the time that chicks creche, when prey consumption rates more than dou- bled as the male assisted in foraging and recovered from a long fast. Female macaroni penguins extracted 2.28 (1.60 to 3.26) and males extracted 2.84 (2.02 to 3.99) g of prey from their environment for every minute spent submerged during foraging. Only 15.3 (14.7 to 15.6)% of all prey consumed was fed to chicks. While food capture rates increase in the middle of the breeding season, this may be more a function of greater food availability than a response to demands from their chick. Male and female macaroni penguins have differing breeding strategies with the male showing the characteris- tics of a capital then income breeder while the female has a strategy that shows characteristics of both capital and income strategies simultaneously. The high synchronicity and precise timing of the mac- aroni penguin breeding season and timing of the increase in prey capture rates suggest an influx of prey to their foraging area during the middle of the breeding season. A depletion of prey resources in the foraging area used during the breeding season could affect foraging success and have pro- found effects on the body condition and composition of this species and its ability to raise chicks suc- cessfully.

Response of Dovekie to Changes in Food Availability

Dovekie (Alle alle) parents feed their chicks almost exclusively with high energy Arctic zooplankton associated with cold polar waters. In years with a stronger influx of warm Atlantic waters (positive NAO index) in their traditional feeding grounds, they may be forced to forage in suboptimal feeding areas. We compared the Dovekie chick diet in Hornsund (Spitsbergen) in the early, mid and late phases of the chick-rearing period in two seasons with different distributions of cold (Arctic) and warm (Atlantic) water masses. Parents delivered to their chicks food loads of lower wet weight (in the mid and late phase) and lower energetic value (in the late phase) under warm water conditions compared to colder water conditions. This suggests worse foraging conditions for Dovekies and foraging in suboptimal feeding areas during warm water events. However, data from the mid phase showed that Dovekies are able to compensate by increasing the number of daily feedings. In this phase, chicks received the same amount of food (by mass and energy) as in cold years. It is unknown how large the inflow of Atlantic water can be before it has negative implications for adult Dovekie daily time budget, body condition, survival rate, their breeding success and fitness.

Habitat utilisation, feeding tactics and age related feeding efficiency in the Caspian Gull Larus cachinnans

The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success. As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success.

Age-Related Survival and Behavior of Snowy Plover Chicks

AbstractAbstract. Precocial young often experience high mortality prior to achieving flight, especially in the first days after hatching. We quantified relationships between chick age, survival, behavior, and response to natural and anthropogenic danger for the threatened Western Snowy Plover (Charadrius alexandrinus nivosus) in coastal northern California, USA. Plover chicks were most likely to die in the first three days of their 28-day prefledging period, especially on sandy ocean beaches compared to coarser substrates of river habitats. Chick survival in both habitats increased across the ∼120-day chick-rearing period. Improved survival of older chicks coincided with an age-related reduction in brooding and increased distance from the tending parent, which was not related to season. Lower survival of younger chicks was correlated with a tendency to lie motionless when approached by humans; nearly all older chicks responded to human approach by running to evade danger. Chicks of all ages were more likely to lie motionless when potential avian predators flew nearby. Age-dependent survival of Snowy Plover chicks paralleled a trend of increasing development, thermal independence from adults, and capability of evading predators.

Regulation of energy expenditure in free-living animals under extreme conditions.

The Emperor Penguin (Aptenodytes forsteri) reproduces at the borders of the Antarctic continent. Because of the large size of the species, birds require a long period between egg laying and chick independence. Egg laying starts in the middle of the Antarctic winter. Only the males incubate the egg during a period of about 90 days, during which they only can rely on their internal (fat) reserves. After hatching, the females return from the sea to take over the feeding of the small chick. Using stable isotope methodology, measurements were made for the development of body mass, body composition, and energy expenditure. A comparison was made between incubating males while huddling with other males, versus males which were incubating solitarily. The latter males were permanently exposed to the harsh environmental conditions and had to cover high thermoregulatory costs, which resulted in an elevation of the metabolic rate by about 20%. In these birds, the stored fat deposits deminished at a relatively high rate, leading to a high incidence of nest desertion. The Cape Gannet (Morus capensis) is an other seabird species facing extreme and unpredictable feeding conditions. The entire world's population is distributed over six colonies in southern Africa, where they feed on various fish species. The location of the food is rather unpredictable, and related to the level of upwelling of the nutrient-rich Antarctic water. We applied GPS loggers to monitor daily feeding activity, and Doubly Labeled Water to measure energy expenditure in free-ranging birds during the chick-rearing period.

The winter distribution of chinstrap penguins from two breeding sites in the South Shetland Islands of Antarctica

Satellite telemetry was used to determine the winter movements of eight chinstrap penguins from two breeding colonies in the South Shetland Islands, Antarctica during the 2000 and 2004 austral winters. Chinstrap penguins foraged largely inshore, on the shelf, north of the South Shetland Islands during 2000, but foraged mainly offshore in pelagic waters in 2004. Analyses of foraging trip durations during the chick-rearing periods that preceded the 2000 and 2004 winters suggest that prey were more abundant inshore during the summer of 2000 than in 2004. Oceanographic data further revealed the presence of a strong shelf-slope front in 2000 that was absent in 2004. In addition, two of the six chinstrap penguins from the colony in Admiralty Bay migrated from the South Shetland Islands region to the vicinity of the South Orkney and South Sandwich Islands, distances of 800 and 1,300 km, respectively. We postulate that the differences in winter migratory behavior among chinstrap penguins from this colony may reflect individual ties to different ancestral epicenters of chinstrap populations; one older and local in the South Shetland Islands and one relatively recent, arising from the emigration of chinstrap penguins that occurred during the expansion of this species in the mid-1900s.

Parental sex roles of Malaysian plovers during territory acquisition, incubation and chick-rearing

Shorebirds show high variability in parental care strategies among species, populations, and environments. Research on shorebird parental sex roles can help to understand the selective pressures that shape avian breeding strategies. Although several studies have examined parental care strategies in holarctic shorebirds, very little research has been conducted in the tropics. Here we examined parental sex roles during territorial defence, incubation, and chick-rearing in Malaysian plovers Charadrius peronii in the Gulf of Thailand. The costs and gains of particular parental behaviour may vary between the sexes and can be affected differently by environmental factors and chick age. Thus we also examined how temperature, prey availability, chick or embryo age, and time of day affected parental sex roles. Males spent more time defending territories and were further away from chicks whereas females spent more time incubating eggs. Both adults contributed to chick defence during disturbances throughout the entire chick-rearing period. Total nest attendance (sum of both sexes) was affected by the modelled temperature of an unincubated egg. Prey availability, embryo age, and time of day had no effect on total nest attendance. Males adjusted incubation effort in response to temperature only at high temperatures (>36°C) whereas females adjusted nest attendance at high and low temperatures. Chick age had no effect on the proportion of time adults spent defending territories or responding to disturbance. Pairs were more likely to fledge chicks if both the male and female spent more time defending territories. For Malaysian plovers, high cooperation between the sexes during parental care may help to achieve high quality breeding territories, maintain body conditions during hot days, protect offspring from predators and attacking conspecifics, and contribute to high lifetime reproductive success.

Diurnal Time Budget of the Black-necked Crane During the Breeding Season

From 26 March to 2 August 2003, both the time budget and the diurnal rhythm of behavior of breeding Black-necked Cranes (Grus nigricollis) were studied at Ruoergai Wetlands National Natural Reserve, Sichuan Province, China. The breeding season was divided into three phases: Pre-reproduction, Incubation and Chick-rearing periods. Foraging behavior was the most prevalent during the breeding season, accounting for 45% of the diurnal time budget, followed by breeding activity (28%), locomotion (10%), maintenance (9%), alert (7%), resting (1%) and “other behavior” including excretion, calling and antagonism (0.6%). During the three breeding phases, there were significant differences in the time budget of all behavior types except for locomotion and “other behavior”. In terms of frequency of events, alert behavior was the highest, occurring 43 times per hour, followed by foraging (31), maintenance (17), locomotion (11), breeding activity (5), “other behavior” (1) and resting (0.7). During the three breeding phases, there were significant differences in the frequency of various behavior types except for “other behavior”. During the day, the rhythm of different behavior types varied according to time.

Parental behaviour of Spanish Imperial Eagles Aquila adalberti: sexual differences in a moderately dimorphic raptor

Capsule The sexes make significantly different contributions in the tasks of nest-building, incubation and food provisioning to chicks. Aims To determine the division of parental activities during breeding (nest-building, incubation, brooding, food provisioning and feeding). Methods Between 1991 and 1998 focal observations were made at nests using 20–60× telescopes. During the incubation period, 11 pairs were studied involving a total of 2812 observation hours over 269 days. During the chick-rearing period, seven pairs were studied involving a total of 5499 observation hours spread over 503 days. The birds were sexed and identified individually on the basis of size, position during copulation, vocalization and plumage differences. Results Significant differences were found between the sexes in parental behaviour. Males scarcely participated in incubation and contributed significantly more than the females only in food provisioning, both during incubation (food transfers to feed the female) and during chick-rearing (to feed the chicks). Females were responsible for most of the incubation, nest-building (during incubation and chick-rearing), brooding, shading and feeding of chicks. Nest attendance by females decreased over time. In both sexes, food provisioning increased with brood size. Conclusion Intersexual differences are discussed in the context of the reversed sexual size dimorphism and parental investment strategies. We suggest that in Spanish Imperial Eagles reversed sexual size dimorphism is best explained by the prey capture difficulty hypothesis, than by parental role division during reproduction.