Philopatry and colony fidelity of Shags Phalacrocorax aristotelis on the east coast of Britain

Shag Phalacrocorax aristotelis colonies from the Firth of Forth south to the Fame Islands (SE Scotland and NE England) were surveyed in 1982 and 1983 for nesting Shags ringed as chicks or adults in the previous 20 years. On average, 5% of 863 Shags ringed as chicks were breeding away from their natal colony, and only 1% of 401 marked adults had left their breeding colony. The proportion p of chicks that settled at or beyond distance D from the natal colony followed the relationship p = 0.263D-^0.771, implying that more Shags moved 10 km or beyond to breed than expected from diffusion models or a constant-rate dispersal model; adjusted for birds that settled outside the study area, the relationship was p = 0.314D^-0771. There was no effect of colony size on emigration rate, but females showed a greater tendency to settle away from the natal colony than did males. At the natal colony, more males (69%) than females (47%) nested within 300 m of their birthplace. Two mechanisms appear to determine where a Shag settles to breed: a navigational one governing the return to the natal colony, plus the competitive process of nest-site establishment.     

Diving depths of Shags Phalacrocorax aristotelis breeding on the Isle of May

Information on maximum dive depth and the time spent at various depths was obtained from 49 Shags Phalacrocorax aristotelis. On average, the maximum dive depth was 33–35 m; the overall maximum was 43 m. Shags dived repeatedly to the same depth and spent c. 55% of the time between 25 and 34 m which indicated that they were foraging close to the seabed. About 46% of the time underwater was spent foraging and 54% travelling. Average underwater swimming speed was 1.7-1.9 m per second.     

Foraging range and feeding locations of Shags Phalacrocorax aristotelis during chick rearing

We used radio-tracking techniques to determine the foraging range and feeding locations used by individual Shags Phalacrocorax aristotelis during chick rearing on the Isle of May, Scotland. The mean (±s.d.) foraging range was 7.o±1.9 km (maximum 17 km) and there were no significant sex or year differences. All feeding sites were within 7 km of land. Most (> 90%) were either within 2 km of the colony or in two discrete areas 5–13 km to the north and west. Use of the areas varied between years with both males and females making more use of the north area in 1987 than either 1988 or 1989. Birds used > 11% of the area of sea within the maximum recorded feeding range. This restricted distribution appeared to be related to water-depth and bottom sediment type. Shags fed most frequently in water 21–40 m deep, with a bottom of either gravel and sand, or rock with thin patchy sediment cover.     

Sexual ornament size and breeding performance in female and male European Shags Phalacrocorax aristotelis

Research into sexual selection and animal ornamentation has been largely directed at males in sexually dimorphic species. In contrast, there is relatively little empirical information on female ornamentation. Using the European Shag Phalacrocorax aristotelis , a species in which both sexes have an ornamental crest during pair formation, we investigated the evidence for (1) a correlation between crest size and breeding performance and (2) assortative pairing on crest size. Crest size appeared unrelated to age in either sex, though our sample of males did not include first-time breeders. The year of study was one in which overall breeding performance at the colony was poor. Females that did not subsequently produce a clutch had significantly smaller crests during the courtship period than those that did, suggesting a link between ornament size and current condition. This relationship was also significant in males. Among those birds that did produce eggs, there was a significant relationship between ornament size and laying date, again significant in both sexes, with early breeders having larger crests. Early breeding is generally associated with relatively high breeding performance in European Shags. Given that both males and females contribute substantially to parental care and that there will be intrasexual differences in quality, choosiness is likely to be favoured in both sexes, and individuals were found to be paired assortatively with respect to crest size. Our results suggest that ornament size is related to state in European Shags, and thus may be involved in mate choice in both sexes.     

Factors influencing food-load sizes brought in by Shags Phalacrocorax aristotelis during chick rearing

Weights of food loads brought back to the colony by 26 Shags Phalacrocorax aristotelis rearing chicks were determined using the water-offloading procedure. Loads consisted almost entirely of lesser sandeels Ammodytes marinus. Load size was extremely variable, ranging from 8 to 208 g with a mean load weight of 106 g. Data on foraging behaviour collected concurrently demonstrated that Shags brought back heavier loads when they were feeding farther away from the colony and when brood biomass was larger. These two variables together explained 70.3% of the variation in load size. We postulate that adults had already digested their own food requirements by the time they arrived back at the nest, and the contents of the loads were therefore primarily for the young. On 67% of trips, Shags caught, on average, more than one fish per dive. This estimate ignores the food requirements of the adult, and therefore true prey-capture rates must have been higher.     

Age-related effects of a nonbreeding event and a winter wreck on the survival of Shags Phalacrocorax aristotelis

During 1993, about half the adult Shags Phalacrocorax aristotelis alive on the Isle of May, southeastern Scotland, failed to breed. The proportion not breeding was highest (66%) for individuals aged 14–21 years and lowest (12%) for those aged 3–6 years. In February 1994, a prolonged spell of onshore winds resulted in a wreck of these adults such that overwinter survival was reduced from the normal 88% to 14.7%. The age-specific pattern of survival over this winter differed from previous years, with birds aged less than 7 years and more than 16 years surviving less well and middle-aged individuals surviving better than expected. Whether an individual had bred or not in 1993 had little effect on the chances of survival of the youngest and oldest individuals, as most died. However, among Shags aged 12–15 years, those which had bred survived markedly less well than those which had not, indicating that there had been some cost involved to birds in breeding in 1993.     

Dietary changes of Mediterranean Shags Phalacrocorax aristotelis desmarestii between the breeding and post-breeding seasons in the upper Adriatic Sea

Capsule?Shags move between breeding and non-breeding areas and this is associated with a significant change in diet.

Aims?To determine whether the diet of Shags nesting on islets off the Croatian coast is the same as their diet after the post-breeding move to the Gulf of Trieste.

Methods?Diet was determined by the analysis of 611 regurgitated food pellets.

Results?A total of 23 988 prey items were identified in the sample of pellets. Post-breeding Shags in the Gulf of Trieste focused on demersal and relatively immobile Gobiidae (81.5% by number, 87.1% by biomass). The most frequent prey species was Gobius niger (70.8% by number). In the breeding season at Oruda island, Croatia, the diet was more varied. Breeding Shags fed on bentho-pelagic, mobile prey such as Atherina boyeri (28.4% in frequency), Serranus hepatus (16.1%) and Crenilabrus tinca (12.0%), while Gobiidae had a dietary frequency of only 18.1%. With respect to biomass the most important prey were Crenilabrus tinca (19.0%) and Serranus hepatus (18.4%).

Conclusion?We suggest that the movement of Shags within the Adriatic Sea is driven by dietary requirements. Most previous studies of Shag diet have shown that Shags tend to have a more specialized diet during the breeding season, concentrating upon demersal prey species. However, we have found that birds breeding at the Croatian study colony show dietary diversity. We suggest that lack of dietary specialization is a facultative response to local prey abundance, and is probably the result of over-fishing of demersal species in the areas around the breeding locations in which the birds find suitable sites and are little disturbed by human activity. Shags may move immediately after breeding to the Gulf of Trieste because demersal species are likely to be more abundant there. As a consequence, the diet becomes more specialized and is then more similar to the diet of other populations of Shags.     

Estimating dispersal distributions at multiple scales: within‐colony and among‐colony dispersal rates,distances and directions in European Shags Phalacrocorax aristotelis

Knowledge of the rate, distance and direction of dispersal within and among breeding areas is required to understand and predict demographic and genetic connectivity and resulting population and evolutionary dynamics. However dispersal rates, and the full distributions of dispersal distances and directions, are rarely comprehensively estimated across all spatial scales relevant to wild populations. We used re‐sightings of European Shags Phalacrocorax aristotelis colour‐ringed as chicks on the Isle of May (IoM), UK, to quantify rates, distances and directions of dispersal from natal to subsequent breeding sites both within IoM (within‐colony dispersal) and across 27 other breeding colonies covering 1045 km of coastline (among‐colony dispersal). Additionally, we used non‐breeding season surveys covering 895 km of coastline to estimate breeding season detection probability and hence potential bias in estimated dispersal parameters. Within IoM, 99.6% of individuals dispersed between their natal and observed breeding nest‐site. The distribution of within‐colony dispersal distances was right‐skewed; mean distance was shorter than expected given random settlement within IoM, yet some individuals dispersed long distances within the colony. The distribution of within‐colony dispersal directions was non‐uniform but did not differ from expectation given the spatial arrangement of nest‐sites. However, 10% of all 460 colour‐ringed adults that were located breeding had dispersed to a different colony. The maximum observed dispersal distance (170 km) was much smaller than the maximum distance surveyed (690 km). The distribution of among‐colony dispersal distances was again right‐skewed. Among‐colony dispersal was directional, and differed from random expectation and from the distribution of within‐colony dispersal directions. Non‐breeding season surveys suggested that the probability of detecting a colour‐ringed adult at its breeding location was high in the northeastern UK (98%). Estimated dispersal rates and distributions were therefore robust to incomplete detection. Overall, these data demonstrate skewed and directionally divergent dispersal distributions across small (within‐colony) and large (among‐colony) scales, indicating that dispersal could create genetic and demographic connectivity within the study area.     

On a new species of tetrabothriid cestode from the shag Phalacrocorax aristotelis aristotelis (L.)

A new species of tetrabothriid tapeworm parasitic in the gut of the shag is recorded and described.     

Strong temporal consistency in the individual foraging behaviour of Imperial Shags Phalacrocorax atriceps

Individual consistency in foraging behaviour can generate behavioural variability within populations and may, ultimately, lead to species diversification. However, individual‐based long‐term behavioural studies are particularly scarce in seabird species. Between 2008 and 2011, breeding Imperial Shags Phalacrocorax atriceps at the Punta León colony, Argentina, were tracked with GPS devices to evaluate behavioural consistency during their foraging trips. Within a breeding season, individuals were highly consistent in the maximum distances they reached from the shore and the colony, as well as in the time invested in flight and diving across consecutive days during early chick rearing. In addition, each individual had its specific foraging area distinct from the foraging area of other individuals. Comparing between early and late chick rearing in the same season, individuals were consistent, to a lesser degree, in the maximum distance they reached from the colony and the shore, increasing in consistency later on in the season. Within the season, females were more consistent than males in the maximum distance they moved from the colony and the shore, the sexes segregated in their foraging areas and individual females were segregated from one another. Twenty‐eight individuals tracked in different breeding seasons were marginally consistent in their trip durations and maximum distance reached from shore across seasons. Among seasons, foraging locations differed between sexes and among individual females. Individuals from this colony exhibited consistency over time in several aspects of foraging behaviour, which may be due to a combination of individual characteristics such as learning abilities, breeding experience or health, as well as targeted prey type and stability of the environment at this location.     

Foraging behaviour and feeding locations of Rock Shags Phalacrocorax magellanicus from a colony in Patagonia, Argentina

During 1996 and 1997, foraging Rock Shags Phalacrocorax magellanicus were studied at Punta Loma, Argentina using radio-transmitters deployed on ten adult shags during the chick-rearing period. Rock Shags undertook 2.6 ± 0.6 sd trips per day. The mean duration of a feeding trip was 2.6 ± 0.7 hours. A bird spent 36% of daylight hours away from the colony on feeding trips, diving for 92% of the foraging trip, and made a mean of 106 dives per foraging trip. Foraging trip duration was strongly correlated with the total number of dives made in one foraging trip. Rock Shags fed mainly in water less than 10m deep with a gravelly sand bottom and within 5 km of shore. Mean foraging range was 3.8 ± 2.6 km and 2.6 ± 2.3 km for 1996 and 1997, respectively. These results suggest a high foraging effort (diving time per foraging trip) for Rock Shags, presumably associated with poor food conditions close to the colony. Comparison is made with other Phalacrocorax species.     

The metabolism of organochlorine compound by microsomal enzymes of the shag (Phalacrocorax aristotelis).

1. The activities of microsomal enzymes of adult male shags (Phalacrocorax aristotelis) towards the organochlorine substrates HHDN, HCE and Heom were compred with those of microsomal enzymes of the adult male Wistar rat. 2. Liver homogenates showed similar epoxide hydrase activity to kidney homogenates in the shag, but in the rat liver preparations was much more active than the kidney preparation. 3. Liver microsomes of the shag showed smaller than 8% of the epoxide hydrase activity and smaller than 14% of the hydroxylating capacity of liver microsomes from the rat. 4. The relatively low activity of these enzymes is probably the main reason why the shag has been found to contain relatively high levels of dieldrin in ecological studies.     

Influence of body composition on the metabolic rate of nestling European shags (Phalacrocorax aristotelis)

During the early development of avian nestlings, their mass-specific resting metabolic rate (RMR) changes in a biphasic pattern with the peak value often being much higher than that expected for an adult bird of similar body mass. In the present study we examined the possible influence of variations in the size of internal organs in “setting” the high RMR and peak metabolic rate (PMR) during development in a large altricial species, the European shag (Phalacrocorax aristotelis). Thermoneutral RMR and cold-exposure induced PMR were measured in nestlings 15 days old, the age at which the highest RMR occurred during development. Body mass averaged 414 g. Mean values of RMR and PMR were 5.75 W and 9.08 W, respectively; the RMR value corresponds to approximately 250% of the expected value for an adult non-passerine bird of similar body mass. The masses of all the organs measured (breast and leg muscles, heart, liver, intestine, and kidney) varied isometrically with total body mass. However, large chicks had a significantly lower fractional water content than small chicks, suggesting that the former had achieved a higher level of functional maturity. In contrast to what has been suggested for adult birds in general, the heart and kidney masses of shag nestlings were not significantly correlated with the metabolic rates. The intestine length, in contrast, was highly and positively correlated with both the RMR and the PMR, i.e. intestine length was a better predictor of RMR and PMR than was total body mass. In addition, liver mass was positively correlated with RMR. The results of the present study suggest that the liver in particular may play a key role in establishing the high, mass-specific RMR which is attained during development in bird chicks. Our results also support previous suggestions that early in their development, altricial chicks mainly allocate energy to the growth of `energy-processing' organs (such as the intestine and liver) rather than to `energy-consuming' organs. Accepted: 3 March 1999     

Immediate and delayed effects of a gale in late spring on the breeding of the Shag Phalacrocorax aristotelis

At the beginning of May 1982, a westerly gale destroyed or damaged 49% of 761 Shag Phalacrocorax aristotelis nests on the exposed west side of the Isle of May, southeast Scotland; 518 nests on the sheltered east side acted as controls against which to assess subsequent effects of the gale on Shag reproduction and recruitment.
On sites affected by the gale, 90% of pairs rebuilt their nest and laid a second clutch, on average 18 days after the loss of the first one. First-time breeders nested closer to the sea, suffered greater damage to their nests, and deserted their damaged nests more readily after the gale than did experienced ones; a quarter of potential recruits to the west side were deterred from breeding in that year.
By the end of the season, the number of chicks fledged per pair was 31% lower on the west side than on the east. The difference was caused partly by nest desertion, partly by greater hatching failure of full clutches in apparently undamaged nests and especially by lower fledging success of pairs that renested. Irrespective of breeding experience, delayed early breeders produced more chicks than late breeders nesting at the same time, showing that late breeders were of poorer intrinsic quality than early breeders. This was one reason for a seasonal decline in Shag productivity, although environmental factors also played a role.
During April-July, gales like that of May 1982 occur on average once every 5 years on the Isle of May. Despite the disruption that they may cause to nesting Shags, the impact on the long-term growth of the colony is small.     

Site fidelity in predictable and unpredictable habitats

Summary Site fidelity, the tendency to return to a previously occupied location, has been observed in numerous species belonging to at least three phyla. In this paper I develop a general model using dynamic programming to investigate conditions under which fidelity to a previously occupied territory will be advantageous. The results predict that site fidelity should be inversely related to heterogeneity in territory quality and the animal's lifespan and positively related to the cost of changing territories, age and probability of mortality in the habitat. The predictability of reproductive outcome (defined as the probability that next period's outcome will be the same as this period's outcome) also affects site fidelity. In predictable habitats, changing territories may be favoured after a bad previous outcome. In contrast, settlement should be independent of the previous outcome in unpredictable habitats. Individuals should also be site-faithful in unpredictable habitats, as long as the mean territory quality is equal among available territories. I also investigate the success of two potential decision rules (always stay and win-stay: lose-switch) relative to the optimal settlement strategy. The results show that these rules may perform as well as the optimal strategy under certain conditions. The always stay strategy does well in unpredictable habitats, when the mean quality within a territory is equal among territories. In contrast, the win-stay: lose-switch strategy performs best in predictable habitats.     

Perceived mate fidelity and paternal resemblance predict men's investment in children

Here, we investigate whether variation in male parental investment can be explained in terms of (1) men's perception of the degree of resemblance between themselves and their offspring and (2) men's perception of their mates' fidelity. In a sample of men from London's Heathrow airport, both variables were found to predict reported investment. We also examined whether the predictors of investment varied when men were no longer in a relationship with the mother of their children and are therefore no longer investing in mating effort with them. Among men no longer in a relationship with the mother of their children, resemblance became a stronger predictor of investment, while fidelity was no longer a significant predictor. Overall, men provided less investment to their children if they were no longer in a relationship with the mother of their children.     

Survival, site and mate fidelity in South Polar Skuas Catharacta maccormicki at Anvers Island, Antarctica

In 1974–1975, 34 adult South Polar Skuas Catharacta maccormicki were colour-ringed on 18 nest territories at Bonaparte Point, Anvers Island, near Palmer Station along the Antarctic Peninsula. Subsequently, the area was searched for these birds during the austral summers of 1975–1976 to 1984–1985 and in 1987–1988 and 1989–1990. Fifty-three percent were seen in 1984–1985, 32% in 1987–1988 and 21% in 1989–1990. Annual survival rate averaged 95% from 1974–1975 to 1984–1985; no sexual differences were detected (n = 28 of known sex). Strong territory and mate fidelity were apparent: 34 skuas averaged 1.1 nest territories and 1.7 mates each in 16 years. Only 4 of 34 individuals (all females) were known to change territories, and each territory change involved a change of mates. Although males showed higher territory fidelity than females (P < 0.01), most females (four of five) retained their territories when previous mates failed to return. Seventeen of 34 birds changed mates a total of 24 times; at least 20 mate changes followed the death or disappearance of the former mate. Males showed slightly higher mate fidelity than females (P < 0.04). Female South Polar and Brown Skuas Catharacta lonnbergi did not differ in territory or mate fidelity. From 1974–1975 to 1984–1985, 120 South Polar Skua chicks were ringed on 18 nest territories on Bonaparte Point: 17 were resighted in the Palmer area when they were 3–10 years old. All 15 returnecs were found within 3 km of their natal nest sites, and four of them occupied nest territories on Bonaparte Point.