Interactions with Great Skuas Stercorarius skua as a factor in the long‐term decline of an Arctic Skua Stercorarius parasiticus population

Many of the UK’s seabird species have displayed high variation in breeding success since the 1980s, largely due to changes in the availability of Lesser Sandeels Ammodytes marinus, their main prey. During this time, Arctic Skuas Stercorarius parasiticus experienced a rapid decline in the UK and the species has subsequently been placed on the Red List of birds of conservation concern. Although shortage of Lesser Sandeels is likely to be an influential factor, the Arctic Skua’s breeding range overlaps with that of the Great Skua Stercorarius skua, a larger bird with a more varied diet, and interspecific interactions for nesting habitat may exert an additional pressure on Arctic Skua breeding populations. Results from four censuses, spanning 21 years, were used to model habitat use and analyse distributional change in nesting Arctic Skuas at a major colony located on Fetlar, Shetland, Scotland. The decline in Arctic Skuas was not uniform across the island and competition with Great Skuas for nest‐sites appears to have influenced localized breeding distribution. By 2006, Arctic Skuas had been almost entirely excluded from shrub heath, blanket bog and coastal heath habitats, which were identified as preferred habitat in 1986. In 2006, Arctic Skua breeding territories were mainly restricted to one core area of preferred habitat where over 90% nested in high density as this habitat became increasingly occupied by Great Skuas. The more generalist foraging habit of the Great Skua allowed the population to grow rapidly as numbers of the more specialist Arctic Skua decreased during times of low sandeel availability. Our model suggests that both interspecific competition for territories with Great Skuas and food limitation have played important roles in the decline of Arctic Skuas on Fetlar.     

The effect of food availability on the foraging behaviour of breeding Great Skuas Catharacta skua and Arctic Skuas Stercorarius parasiticus

Seabirds' behavioural activity budgets have been proposed as an indicator of changes in marine food supplies. Reduced recruitment and a low biomass of sandeels Ammodytes spp. around Shetland in the late 1980s were associated with a dramatic increase in the foraging effort of breeding skuas. We present data on the territorial attendance patterns of breeding Great Skuas Catharacta skua and Arctic Skuas Stercorarius parasiticus in Shetland in 1987 and examine three aspects of the recorded increase in foraging effort.
Time budget studies indicate that recording territorial attendance by infrequent, daily spot observations provides an accurate measure of the time spent off-territory by both skua species. None of the pairs of Arctic Skua studied ever left their chicks unattended in 1987. The same was true of 10 out of 27 pairs of Great Skuas. Although the remaining pairs of Great Skuas frequently left their chicks unprotected, evidence of pair co-ordination to minimise this suggests that their foraging schedules were still constrained by the need to protect their offspring. During periods of high chick-demand, variation between pairs' foraging efforts largely reflected variation in those of the females. Thus, we suggest that increased chick neglect in years of food shortage arises as females abandon their role of guardian for that of provider. By the end of the breeding season in 1987, both male and female Great Skuas foraged from 65 to 89% of daylight hours. The lack of any further increase in the foraging effort of Great Skuas in subsequent years of poorer reproductive success was almost certainly because time and energy constraints prevented either sex from doing so. We suggest that the comparatively short amount of time spent foraging by Arctic Skuas in 1987 may reflect an advantage conferred by their role as specialist kleptoparasites.     

VARIATION IN THE CAPPED WHEATEAR

Williams, A. J. 1980. Aspects of the breeding biology of the Subantarctic Skua at Marion Island. Ostrich 51:160-167.

The breeding biology of the Subantarctic Skua Catharacta antarctica lonnbergi was studied in two austral summers at Marion Island. Eggs were laid between 23 October and 19 December. The clutch size was one (6%) or two (94%) eggs. The laying interval was two (20%) or three (80%) days and the incubation period was 29 days. Chicks could fly at 50–65 days after hatching. Chicks from first and second hatched eggs in the same brood had similar growth rates. Chicks in one- and two-chick broods had similar growth rates until 50 days after hatching when chicks reared singly were heavier. Egg mortality was 15,8%, chick mortality was 34,8% and the overall breeding success was 50,9%. The results are discussed in comparison with previous studies of Subantarctic and South Polar C. maccormicki Skuas.     

Skuas on the Eurasian tundra; relative occurrence of species, ages and colour phases

The breeding ranges of the three closely related skua species in the genus Stercorarius are highly sympatric on the Arctic tundra. During the Swedish-Russian Tundra Ecology Expedition, 1994, the ages and colour phases of Pomarine Skuas Stercorarius pomarinus , Arctic Skuas Stercorarius parasiticus and Long-tailed Skuas Stercorarius longicaudus were recorded at 15 sites along a transect across 140̀ of longitude from the Kola Peninsula in the west to Wrangel Island in the east. An index of the lemming numbers was also measured. Pomarine Skua comprised 52% of the 1587 skuas seen, with 38% Long-tailed Skua and 10% Arctic Skua. The Arctic Skua occurred at low densities all along the transect. The Pomarine Skua was most common at the northernmost sites in this lemming year. It was absent from all sites with low rodent densities but also from two more southerly sites with high lemming numbers. The Long-tailed Skua showed a distribution sympatric with that of the Pomarine Skua but occurred at lower densities, and it also bred at the more southerly sites. No skuas in second-year plumage were observed. Older subadult skuas were observed in increasing numbers with age. The proportion of subadults was markedly higher in the Pomarine Skua (10%) compared with the two other species. The proportion of dark Pomarine Skuas was about 8% all along the transect. In the Arctic Skua, there was an extremely marked shift from 64% dark birds on the Kola Peninsula to an almost complete dominance by the light phase in the rest of Arctic Russia. I suggest that dark skuas are more efficient kleptoparasites over the sea while the light phase is at an advantage hunting over the tundra.     

THE SIGNIFICANCE OF KLEPTOPARASITISM DURING THE BREEDING SEASON IN A COLONY OF ARCTIC SKUAS STERCORARIUS PARASITICUS IN ICELAND

Breeding and kleptoparasitism of Arctic Skuas was studied at Vik, Iceland, in 1973. Hatching success was 88.9%; fledging success was 0.27 fledglings per pair. The heavy chick mortality occurred mostly in the first week after hatching. In the early part of the breeding season skuas fed by robbing kittiwakes of their food at sea and by eating arthropods at the breeding grounds. At the time of hatching of skua eggs, which coincided with the hatching of Puffin eggs, skuas switched their feeding activities to kleptoparasitism of Puffins and fish so obtained was the principal item of most skuas' diet thereafter. In the first week post-hatching, the energy balance of an average adult skua pirating Puffins was estimated to be negative, but changed to positive a week later, although later, coinciding with an influx of non-breeders it turned back to negative. Arctic Terns which normally breed at Vik, and are exploited by skuas, failed to breed in 1973, and the abnormally heavy chick loss is therefore attributed to this failure of the terns. The success of the skuas kleptoparasitizing Puffins depends on the proximity of the interactions to the cliff or to the ocean, where Puffins seck refuge. Skuas catch fish mostly in the air, especially if it is dropped from high. Fish dropped from low is mostly taken by competing gulls and Ravens, which mostly control the ground and lower airspace. Skuas chasing in groups enhance their success, but the average success per member decreased with group increase. However, the success of the group-member in the ‘best’ position was equal to that of a single skua. Puffins carry 1 (large)-6 (smaller) fish, an approximately equal load irrespective of number of fish. Skuas preferentially chase Puffins carrying ‘large’ fish, thus maximizing their yield per effort. Arctic Skuas responded to changes in the numbers of arriving Puffins with a functional response, but their monitoring of the food supply was far from perfect.     

Long-term dataset reveals declines in breeding success and high fluctuations in the number of breeding pairs in two skua species breeding on King George Island

Marine ecosystems face a variety of threats induced by environmental changes and anthropogenic activities. Seabirds are predators often used as indicator species to monitor the status and health of their communities and the environment. Here, we present the results from a 35-year monitoring time series of Brown Skuas (Catharacta antarctica lonnbergi) and South Polar Skuas (C. maccormicki) breeding sympatrically in the Maritime Antarctic on Fildes Peninsula and Potter Peninsula, King George Island. Our results reveal high annual variability in the number and proportions of breeding pairs across the entire study period. Apart from that, the breeding pair numbers of Brown Skuas were relatively stable. By contrast, the breeding pair number of mixed species and South Polar Skua pairs increased substantially until 2003/2004 and 2010/2011, respectively. Both pair types experienced a decline in the breeding pair numbers within recent years. Despite the strong fluctuations in the number of breeding pairs, the sum of occupied territories has been stable during the last 9 years. The breeding success of all pair types declined significantly, and within recent years, both South Polar Skuas and mixed species pairs completely failed to produce offspring. The ultimate causes driving the breeding success decline remain unclear. The overall increase in the number of skuas might have raised density-dependent factors and resulted in a higher predation rate between conspecifics. The more recent total breeding failures, however, indicate a drastic shortage in local food availability.     

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.     

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.     

Factors affecting chick growth in the South Polar Skua (Catharacta maccormicki): food supply, weather and hatching date

This study investigated chick growth in a pelagic Antarctic seabird, the South Polar Skua (Catharacta maccormicki). The factors food supply, weather and hatching date were analysed in a population of 54 breeding pairs at King George Island/South Shetland Islands. Food supply was manipulated by offering fish corresponding to 20% of daily energy demand of chicks to half of the breeding pairs every second day. Growth of mass, head, wing and tarsus was followed and related to the treatment, weather conditions, hatching date and interactive effects. Food supply did not limit chick growth in the studied season. Parents seemed to try to feed their chicks at a maximum rate and succeeded in the studied season because the general food supply was very good. Low temperatures and strong winds depressed chick growth. A growth advantage of food-supplemented chicks could be observed when the natural conditions for chick growth were sub-optimal. Chick growth rate was strongly negatively associated with hatching date and worsening weather during the reproductive season could be excluded as explanatory variable for this finding.     

Influence of rodent density on nesting associations involving the Bar-tailed Godwit Limosa lapponica

The Bar-tailed Godwit Limosa lapponica rarely defends its nest aggressively against predators, but rather associates with other more aggressive species for protection. I studied the distribution of Bar-tailed Godwit nests relative to nests of the aggressive Longtailed Skua Stercorarius longicaudus, as well as to nests of two slightly less aggressive alternative 'protective umbrella' species, the Whimbrel Numenius phaeopus and the Grey Plover Pluvialis squatarola. Rodents are the main prey of the Long-tailed Skua but, in years when numbers of rodents are low, Long-tailed Skuas also eat eggs and chicks of other birds. Data on rodent densities and nest distribution were collected on Finnmarksvidda, Northern Norway, in 1987, 1989, 1990, 1992 and 1997, and on Taimyr, Siberia, in 1994 and 1995. It is concluded that the Bar-tailed Godwit is able to evaluate the probability of nest predation from Long-tailed Skuas, based on direct or indirect information about the status of local rodent populations. This information is used to choose between Long-tailed Skuas and other potential species for protection against nest predators.     

Intra-seasonal variation in zooplankton availability,chick diet and breeding performance of a high Arctic planktivorous seabird

Different phenological responses to climate changes by species representing preys and predators may lead to mismatch between functionally dependent components of an ecosystem, with important effects on its structure and functioning. Here, we investigate within-season variation in zooplankton availability, chick diet composition and breeding performance of a small planktivorous seabird, the little auk (Alle alle) in two large colonies in Hornsund and Magdalenefjorden, Spitsbergen, differing in synchrony of breeding (11-day vs. 22-day hatching period, respectively). Assuming similar zooplankton phenology and existing differences in duration of the little auk breeding period, we expected lower availability of the preferred food in the less synchronized colony in Magdalenefjorden and in consequence a negative effect on nestling body mass and survival. We found that in both colonies Calanus glacialis (copepodite stage CV) was the most important prey item in the chick diet making up 68–87 % of the biomass and energy of all prey items. The only exception was the end of the chick-rearing period in Magdalenefjorden, when contribution of this prey item was significantly lower (24–26 %). Thus, late breeders in Magdalenefjorden were apparently mismatched regarding C. glacialis CV availability. However, the hatching date did not affect birds fitness (reproductive output and chick pre-fledging mass) significantly. Results of our study indicate that little auks breeding on Spitsbergen can respond to a wide range of environmental conditions and prey availabilities through the plasticity of their foraging behaviour, which may help them to maintain their optimum fitness level in changing and unpredictable environments.     

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.     

Climate fluctuation effects on breeding of blue penguins (Eudyptula minor)

Abstract

El Niño and La Niña climate perturbations alter sea currents and food availability for seabirds in many areas of the world. This changes their breeding success and mortality. Blue penguin (Eudyptula minor) breeding success is dependent upon whether one or two clutches per season are laid, and the hatching and fledging success of these clutches. This study uses six years of data from five blue penguin breeding colonies, three from Taiaroa Head, Otago Peninsula and two from Oamaru, to examine whether annual variation in breeding success correlates with El Niño/La Niña perturbations. When La Niña conditions prevailed, penguins started breeding later, and there was a lower proportion of double breeders than in El Niño and normal years. The probability of a newly hatched chick surviving to fledging was also dependent on whether large‐scale climatic conditions prevailed, whereas hatching success and overall breeding success (number of fledged chicks per breeding pair) showed no correlation with climate perturbations.     

Influence of weather on reproductive success of northern fulmars in the Canadian high Arctic

The northern fulmar (Fulmarus glacialis) is a common seabird of the North Atlantic Ocean, with breeding colonies broadly dispersed between 45°N and 80°N. At higher latitudes, breeding fulmars experience extensive sea-ice and presumably snow and low temperatures which do not affect fulmars in the southern part of the breeding range. We studied the relationship between weather and reproductive success of northern fulmars breeding at two colonies in the Canadian high Arctic. Collectively, hatching success, fledging success, and productivity (chicks fledged per egg laid) were similar between our study and results from colonies located south of the Arctic. However, a larger proportion of fulmars at apparently occupied sites (AOS) in high Arctic colonies appeared to forego egg-laying, resulting in lower proportions of chicks fledged per AOS. Extreme inclement weather was the major factor influencing nesting success, resulting in pulses of egg or chick loss during or immediately following major storms, although the mechanism of effects appeared to differ between the two colonies. For Arctic fulmars, the risks of nest failure due to stochastic, deleterious weather events may be offset by the predictable abundance of food supplies during chick-rearing in Arctic waters.     

Phenological mismatch in Arctic‐breeding shorebirds: Impact of snowmelt and unpredictable weather conditions on food availability and chick growth

The ecological consequences of climate change have been recognized in numerous species, with perhaps phenology being the most well‐documented change. Phenological changes may have negative consequences when organisms within different trophic levels respond to environmental changes at different rates, potentially leading to phenological mismatches between predators and their prey. This may be especially apparent in the Arctic, which has been affected more by climate change than other regions, resulting in earlier, warmer, and longer summers. During a 7‐year study near Utqia?vik (formerly Barrow), Alaska, we estimated phenological mismatch in relation to food availability and chick growth in a community of Arctic‐breeding shorebirds experiencing advancement of environmental conditions (i.e., snowmelt). Our results indicate that Arctic‐breeding shorebirds have experienced increased phenological mismatch with earlier snowmelt conditions. However, the degree of phenological mismatch was not a good predictor of food availability, as weather conditions after snowmelt made invertebrate availability highly unpredictable. As a result, the food available to shorebird chicks that were 2–10 days old was highly variable among years (ranging from 6.2 to 28.8 mg trap^?1 day^?1 among years in eight species), and was often inadequate for average growth (only 20%–54% of Dunlin and Pectoral Sandpiper broods on average had adequate food across a 4‐year period). Although weather conditions vary among years, shorebirds that nested earlier in relation to snowmelt generally had more food available during brood rearing, and thus, greater chick growth rates. Despite the strong selective pressure to nest early, advancement of nesting is likely limited by the amount of plasticity in the start and progression of migration. Therefore, long‐term climatic changes resulting in earlier snowmelt have the potential to greatly affect shorebird populations, especially if shorebirds are unable to advance nest initiation sufficiently to keep pace with seasonal advancement of their invertebrate prey.     

Assessment of territory quality and its effects on breeding success in a migrant passerine, the Wheatear Oenanthe oenanthe

This paper examines how a returning migrant assesses the quality of an area as a breeding territory before the period of peak food demand and how effective the assessment is in terms of breeding success. Male Wheatears Oenanthe oenanthe return from Africa to choose territories in the Breckland of eastern England about March, females arriving shortly after males. The food supply was predictable: prey densities during the breeding period (egg-laying to chick independence) were strongly correlated with prey densities at the same sites during the period of arrival and territory establishment. Prey densities were also related to vegetation structure, averaging highest on short turf. Male arrival date and territory size were not significantly related to prey density but were strongly related to vegetation structure, implying that birds used vegetation as an indirect clue to prey availability. Neither territory size nor nest spacing appeared to affect nest losses caused by predators. The major variations in number of young fledged (other than predation) were caused by the number of nestlings hatched and presence of a second brood. Both early arrival and an early first brood improved first-brood success and were necessary for a second brood. Not all birds which arrived early bred early enough for a second brood. First-brood hatching date was strongly negatively correlated with pre-breeding prey availability but not significantly related to vegetation structure. Thus by using vegetation as a clue to habitat quality, some pairs suffered reduced breeding success. This result implies that birds may not be able adequately to assess prey density directly at the time of territory establishment. The critical period for food availability may not be the period of peak demand (nestling period), when food is relatively abundant, but is probably the pre-breeding period when females must accumulate reserves for eggs and when the food supply is poor. Food supply during this period may determine the timing of breeding and the ability to rear a second brood, and may thus have a greater effect on breeding success.     

PREDATION AND KLEPTOPARASITISM BY SKUAS IN A SHETLAND SEABIRD COLONY

Feeding methods and relations of Great Skuas and Arctic Skuas to prey were studied in a seabird colony at Hermaness, Shetland. Great Skuas obtained food by kleptoparasitism, predation and scavenging. They induced Gannets to regurgitate by interfering with their flight; grasping the Gannet by the wing or tail or pushing it down with the feet on its back. Gannets tried to escape by descending to the surface, and regurgitated during 12% of the chases, most frequently when pursued by several birds. Great Skuas caught Puffins by swooping at flocks in the colony. Puffins flying with fish to their young were also chased, releasing food on one fifth of the attacks, or escaping down to the sea and diving. Great Skuas also took Kittiwake nestlings by hovering and grasping the chick with the bill, killing and eating it on the surface. Adult Kittiwakes from nearby nests took to the air, mobbing the predator. More Kittiwakes were engaged in mobbing at unsuccessful than at successful predation attempts, indicating that colonial breeding may be of selective value under such predation. Two different estimates pointed to a Kittiwake nestling predation of 0–12 and 014 young per pair. Fledging success of Kittiwakes was estimated at 0–87-1-06 young per pair, considerably lower than at English colonies where predators are absent. In spite of the predation, the Kittiwake colony showed no signs of decrease. Agonistic behaviour and other evidence indicate that Great Skuas defend feeding territories at the seabird colony. Skuas, gulls and Fulmars competed for food at carcasses. Fulmars dominated and chased away skuas. Arctic Skuas deprived Puffins of food. They patrolled the cliff, intercepting Puffins arriving with fish, snatching it from their victim's bill, or inducing them to release fish. Puffins continuing their inward flight lost food more often (30%) than birds descending to the sea (15%)—sometimes diving below. This opportunity to escape may explain the lower success of skuas at Hermaness than at a Puffin colony farther inland from the shore (Grant 1971). Other factors being equal, proximity to the sea may thus reduce the risk of kleptoparasitism.     

Contrasting effects of GPS device and harness attachment on adult survival of Lesser Black‐backed Gulls Larus fuscus and Great Skuas Stercorarius skua

Telemetry has become an important method for studying the biology and ecology of animals. However, the impact of tracking devices and their method of attachment on different species across multiple temporal scales has seldom been assessed. We compared the behavioural and demographic responses of two species of seabird, Lesser Black‐backed Gull Larus fuscus and Great Skua Stercorarius skua, to a GPS device attached using a crossover wing harness. We used telemetry information and monitoring of breeding colonies to compare birds equipped with a device and harness, and control birds without an attachment. We assessed whether tagged birds have lower short‐term breeding productivity or lower longer term overwinter return rates (indicative of overwinter survival) than controls. For Great Skua, we also assessed whether territory attendance within the breeding season differed between tagged and control birds. As with previous studies on Lesser Black‐backed Gull, we found no short‐term impacts on breeding productivity or long‐term impacts on overwinter return rates. For Great Skua, there was no evidence for impacts of the device and harness on territory attendance or breeding productivity. However, as found by a previous study of Great Skuas using a different (body) harness design, there was strong evidence of reduced overwinter return rates. Consequently, a device attached using a wing harness was considered suitable for long‐term deployment on Lesser Black‐backed Gulls, but not on Great Skuas. These findings will inform the planning of future tracking studies.     

The effect of habitat quality on foraging patterns, provisioning rate and nestling growth in Corsican Blue Tits Parus caeruleus

Many bird species face seasonal and spatial variation in the availability of the specific food required to rear chicks. Caterpillar availability is often identified as the most important factor determining chick quality and breeding success in forest birds, such as tits Parus spp. It is assumed that parents play an important role in mediating the effect of environment on chick development. A reduction in prey availability should therefore result in increased foraging effort to maintain the amount of food required for optimal chick development. To investigate the capacity of adults to compensate for a reduction in food supply, we compared the foraging behaviour of Blue Tits Parus caeruleus breeding in rich and poor habitats in Corsica. We monitored the foraging effort of adults using radiotelemetry. We also identified and quantified prey items provided to nestlings by using a video camera mounted on the nest. We found that the mean travelling distance of adults was twice as great in the poor habitat as it was in the rich. Despite the marked difference in foraging distance, the proportion of optimal prey (caterpillars) in the diet of the chicks and the total biomass per hour per chick did not differ between the two habitats. We argue that relationships between habitat richness, offspring quality and breeding success cannot be understood adequately without quantifying parental effort.     

PREY SELECTION AND TEMPORAL VARIATION IN THE DIET OF SUBANTARCTIC SKUAS AT INACCESSIBLE ISLAND,TRISTAN DA CUNHA

Ryan, P.G., & Moloney, C.L. 1991. Prey selection and temporal variation in the diet of Subantarctic Skuas at Inaccessible Island, Tristan da Cunha. Ostrich 62:52-58.

More than 2 500 prey items of Subantarctic Skuas Cuthuructa anturcricu were identified from regurgitated pellets collected at a roost of non-breeding skuas during summer 1989–90 at West Point, Inaccessible'Is-land. Most prey items (96,5%) were birds, primarily burrow-nesting procellariiforms. Sixteen bird species were recorfed in the diet, but five secies accounted for 94,6% of identifiable prey remains: Whitebellied Storm Petrel Fieettu grullariu (53,5%), Whitefaced Storm Petrel Pelugodrorna marina 15,1%), Broad-billed Prion Pacaptilu vittutu (14,0%), Great Shearwater Puffinus gruvis (7,3%), and Iommon Diving Petrel Pelecunoids urinutrix (4,7%). Petrel chicks were important in the diet, particularly Great Shearwaters. The main non-avian prey were fish and goose barnacles Lepas sp., but their importance may have been underestimated. The remains of a rat Rattus sp. presumably were derived from the neighbouring island of Tristan da Cunha; rats are not known to occur on Inaccessible Island. Prey size affected seabird availability to skuas, and dietary composition vaned throughout the summer in relation to seabird breeding cycles. Both these factors reduce the value of skua diet as an indicator of the relative abundances of burrow-nesting petrels. There was no correlation between skua diet and estimates of breeding bird abundance, but this may be a consequence of studying non-breeding skuas. Subantarctic Skua diet in 1989–90 was similar to that recorded in previous years, with consistent seasonal trends between years.