Body mass change in breeding Common Terns Sterna hirundo

In 1991–1993 we studied the body mass change of adult Common Terns Sterna hirundo breeding on the German North Sea coast. In each year females' body mass increase prior to laying was very similar (about 5.4 g day^-1). This mass increase resulted in an average body mass of about 175 g on the day of egg-laying. In 1992 and 1993 the females' body mass decreased significantly during incubation, while males' body mass was constant. In 1991, however, neither sex showed a decrease in body mass while incubating. In all years, incubating females and males were heavier than chick-rearing parents. This difference was not caused by higher mass in early incubation, because males and females were still significantly heavier in the last week of incubation than in the first week of chick-rearing. Decreasing mass during incubation and the low body masses during chick-rearing are considerd to be stress induced rather than an adaptive strategy to lower flight costs. The decrease in female body mass during incubation in 1992 and 1993 and the lower body mass of males in these years could be related to less favourable foraging conditions compared to 1991, when both males and females showed the highest average body mass values of the 3 years.     

Intraspecific kleptoparasitism improves chick growth and reproductive output in Common Terns Sterna hirundo

Kleptoparasitism is a well‐known foraging strategy used opportunistically by many seabirds. Here, we investigated the effect of intraspecific kleptoparasitism on chick growth and reproductive output in Common Terns Sterna hirundo. Effects were compared between two groups comprising (1) individuals using kleptoparasitism during the chick‐rearing period (kleptoparasitic group, n = 18), and (2) individuals in pairs that never performed kleptoparasitism throughout the season (‘honest’ group, n = 21). The null models best described variation in mass at day 3 and the pre‐fledging mass, indicating no significant effect of the explanatory variables. However, the best models describing the linear growth rate (days 3–13) and peak mass included the parents' foraging strategy (kleptoparasitic vs. honest parents) as an explanatory variable. These two growth parameters were higher in chicks of kleptoparasitic parents. Kleptoparasitic foraging strategy was also associated with higher pre‐fledging survival, as the reproductive performance (i.e. number of fledglings) was significantly higher in the kleptoparasitic than in the honest group. We suggest that by stealing food (and consequently feeding offspring more frequently with high‐quality prey), kleptoparasitic parents are able to produce higher quality chicks with enhanced survival.     

Post-fledging body mass increase in Common Terns Sterna hirundo: influence of age, sex and year

We studied the inter-year and inter-sex variation of the post-fledging body mass development of Common Terns Sterna hirundo in 2000 and 2001 at the Banter See colony, northern Germany. Here, post-fledglings can be identified and weighed remotely and automatically by a transponder system that makes use of automated balances installed at the colony. Individuals were sexed with PCR amplification methods. After fledging, young generally continued to increase their mass. However, in 2000, the young did not significantly increase their mass during the post-fledging period. In 2001, conditions were more favourable and body mass increased continuously. Further, in 2001, male post-fledglings were significantly heavier than female post-fledglings. Once having left the colony area (on average 18–23 days after fledging in 2000, and 14–16 days after fledging in 2001), post-fledgling body mass had still not reached adult body mass. The longer a juvenile stayed at the colony, the higher was its final body mass, which if acting as a threshold level may control departure time. Neither brood size nor hatching order affected post-fledging mass or period. In the unfavourable year 2000, when many individuals were found dead after fledging, fledging age but not fledging mass was found to be a predictor of post-fledging survival before departure: individuals fledging when older had a lower survival probability. Our results stress the importance of the post-fledging period for body mass increase and survival prior to departure. The variation in post-fledging mass growth between years and between the sexes is discussed with respect to parental effort and a possible selective provisioning of sons over daughters.     

Age‐dependent breeding dispersal and adult survival within a metapopulation of Common Terns Sterna hirundo

Dispersal is increasingly recognized as a process of fundamental importance in population dynamics and other aspects of biology. Concurrently, interest in age‐dependent effects on survival, including actuarial senescence, has increased, especially in studies of long‐lived seabirds. Nevertheless, datasets necessary for studying dispersal and age‐dependent effects are few, as these require simultaneous data collection at two or more sites over many years. We conducted a 22‐year capture‐mark‐recapture study of Common Terns Sterna hirundo at three breeding colonies 10–26 km apart in Buzzards Bay, Massachusetts, USA. All birds in the study were of known age (range 2–28 years, median 7 years, n = 3290) and 77% were of known sex. Estimates of adult recapture, survival and breeding dispersal rates were obtained for all age‐classes from 2 to 20 years. The model that acquired 100% of the QAIC_c (Akaike's Information Criterion adjusted for small sample size and overdispersion) weight in our analysis included age‐specificity in all parameters but no relationship with sex. Our study may be the first to demonstrate age‐specificity in recapture, survival and breeding dispersal rates simultaneously, using a single model. Annual rates of breeding dispersal ranged from <0.01 to 0.27, with a population‐weighted mean of 0.065; they decreased with increasing distance between colony sites and, unexpectedly, increased with age. Breeding dispersal did not increase consistently after years with predation on adults or after an attempt to displace birds from an oiled site. Survival rates did not vary among sites or years. Annual adult survival increased from 0.80 in 2‐year‐old birds to a maximum of approximately 0.88 around age 8 years and then declined to 0.76 at age 20 years, yielding strong evidence for actuarial senescence. The peak annual survival rate of 0.88 is at the low end of other estimates for Common Tern and in the lower part of the range recorded for other terns, but total numbers in the three colonies increased seven‐fold during the study. This was part of a slower increase in the regional population, with net immigration into the study colonies. Our results demonstrate the biological significance of breeding dispersal in local population dynamics and age‐related effects on survival and dispersal from a metapopulation of a long‐lived seabird.     

Is Divorce in Young Common Terns,Sterna hirundo,after Recruitment just a Question of Timing?

The probability of divorce in birds has been linked with age, breeding experience, reproductive output and synchrony in return. Here, we investigate the consequences of first breeding attempts in common terns for mating in the subsequent season. Nearly 20% of all first‐time breeders disappeared or skipped at least one season after recruitment. In 84 pairs, which consisted of at least one recruit and of which both partners returned to the colony, the divorce rate was 45%. We compared reproductive success, arrival dates, and asynchrony in arrival dates of pairs of the first breeding season against the second season, for both reunited and divorced pairs and males and females separately. First, in pairs of which both members came back to the colony, we found an increase of reproductive success most pronounced in males. In the second season reproductive success of divorced compared with reunited pairs was higher, as only divorced pairs significantly improved the number of fledglings, and again this relation was stronger in males. Secondly, females of reunited pairs arrived significantly earlier from the first to the second season and by far more days than their males. However, in divorced pairs former mates did not differ in the number of days they advance their arrivals. Finally, divorced males arrived on average 4 d earlier than their former mates, whereas divorced females arrived 5 d later compared with their former mates of the recruitment season. Contradictory to nearly all other divorce studies in birds so far, we found a clear fitness gain in divorced males. We suggest that the improvement in reproductive success of young males stems from a side‐effect of the birds’ quality and ability to reach the breeding site in appropriate time and earlier as potential competitors. In long‐lived bird species the heterogeneity among young individuals in the timing of arrival at the colony seems to explain why former recruit‐pairs reunite or split. For young males we suggest as best explanation of divorce that they profit from ‘pushing for an empty chair’, while females seem to profit from their choosiness and may actively decide between former and other mates.     

Parental care of the Common Tern Sterna hirundo

Parental care activities of male and female Common Terns Sterna hirundo were recorded over two breeding seasons. Males and females exhibited distinct parental roles throughout a breeding bout. Courtship feeding by males was extensive prior to and during egg-laying, but declined with the onset of incubation. Females performed significantly more incubation behaviour than males although both sexes spent equal time attending at the nest site. During the chick stage, females spent significantly more time on the territory than did males. Chick feeding was largely the responsibility of the male; males fed chicks at a rate approximately three times higher than that of females. In addition, whereas females showed no trend in the size of fish delivered to chicks relative to chick age, the size of fish delivered by males increased with chick age. Courtship feeding activities and extensive chick feeding contributions by male Common Terns appear to outweigh parental contributions by females, contrary to predictions for a monogamous species.     

Costs of incubation in Common Terns Sterna hirundo: replication and extension of a clutch‐enlargement experiment

Manipulation of clutch size during incubation has been used in a number of studies to investigate life‐history costs of incubation. We increased or decreased clutches of Common Terns Sterna hirundo during incubation and measured subsequent chick growth and productivity. Our results provide little support for the hypothesis that costs of incubation in Common Terns are substantially affected by the number of eggs incubated.     

Mate provisioning, nutritional requirements for egg production, and primary reproductive effort of female Common Terns Sterna hirundo

We assessed the nutritional importance of mate provisioning to females during egg production and its effects on clutch parameters (egg size, length of the laying period) in Common Terns Sterna hirundo: (1) we estimated the costs of egg production by modeling the daily protein, lipid, and energy requirements of laying females, and (2) compared these costs to both the amount, and the timing, of the male's contribution via mate provisioning. Net lipid, net protein, and gross energy requirements for a three‐egg clutch were estimated to be 5.4 g, 8.6 g, and 569 kJ respectively. Peak protein and lipid requirements occurred one (day ?1) and two (day ?2) days before laying, respectively. Peak energy requirement occurred on day ?1; a cost of 127% to 157% above maintenance. Variation in male provisioning effort (in terms of energy and nutrients delivered) paralleled variation in predicted female requirements for egg production at the level of individual pairs. Males delivered protein in excess of the female's requirements on all days investigated. Male lipid delivery accounted for 45% of female net requirement on the day when demand was greatest (day ?2), but exceeded requirements on all other days. However, the proportion of the female's total energy budget (egg production, maintenance, and activity costs) that was supplied by her mate rose from an average of 29% on day ?2 to 76% during the interval between second and third eggs. Paradoxically, females that were fed at higher rates during the interval between first and second eggs produced clutches with lower total volumes, smaller last‐laid eggs, and clutches with a greater egg‐size hierarchy than conspecifics receiving less food from their mates. Also, females fed at higher rates during the interval between second and third eggs took longer to produce their clutch. These negative relationships between mate provisioning and clutch parameters contrast with previous studies in this and other species.     

Are Morphometrics Sufficient for Estimating Age of Pre-Fledging Birds in the Field? A Test Using Common Terns (Sterna hirundo)

Age is a key component of fitness, affecting survival and reproductive capacities. Where it is not possible to study known individuals from birth, morphometrics (predominantly patterns of plumage development for birds) are most often used to estimate age. Although criteria for age estimations exist for many species, the degree to which these criteria improve the precision of estimates remains to be tested, restricting their widespread acceptance. We develop a photographic tool for estimating ages of Common Tern (Sterna hirundo) chicks and test it using 100 human observers of varying prior experience across four breeding colonies (three North American sites and one European site) and under controlled laboratory conditions. We followed the design approach of other morphometric tools, expanding it to create a user-friendly guide (divided into six age groupings). The majority (86%) of observers improved in chick-aging accuracy when using the tool by an average of 20.1% (±1.4 SE) and correctly estimated 60.3% (±1.4) of chick ages. This was similar to the intrinsic aging ability of our best field observer (63.3%). Observers with limited experience showed the greatest increases in chick-aging accuracy over experienced observers who likely had established a method for estimating chick ages prior to using the tool. Even the best observers only correctly estimated ages of chicks 62.9% (±2.8) of the time in the field and 84.0% (±2.9) of the time in the lab when using the tool and typically underestimated ages. This indicates that developmental variation between individual chicks can prevent completely reliable age estimates and corroborates the few existing data that suggest that morphometric criteria fail to achieve robust levels of accuracy and may introduce error into studies that rely on them. We conclude that novel approaches for estimating age, not only morphometric criteria, must be pursued.     

Feeding ecology of the Common Tern Sterna hirundo in a wintering area in southern Brazil

The wintering diet of Common Terns Sterna hirundo was studied by using 714 pellets collected on roosting sites at the mouth of the Lagoa dos Patos and on adjacent coastal beaches, in Rio Grande do Sul, southern Brazil, from March 1999 to February 2000. A total of 12 340 individual prey items of 35 different food types was found. Fish was the most important food type in the diet, constituting 32% by number and 93% by mass. Insects contributed 67% by number but only 3% by mass. The main food types were sciaenid fishes Paralonchurus brasiliensis, Micropogonias furnieri, Cynoscion guatucupa and Macrodon ancylodon. Several of these are important commercial species; fisheries potentially impact food availability to the terns, and terns may contribute significantly to the natural mortality of these fishes. Clupeiform fishes, the urophycid fish Urophycis brasiliensis and flying ants (Camponotus sp.) were also important. Species composition of the diet (food types), both by number and by mass, differed significantly between months. Prey sizes ranged in length from 12.7 mm to 217.4 mm. The average estimated total length of fish taken was 77.7 mm, but the mean differed significantly among prey species. The importance of demersal sciaenids to the diet of the Common Tern, a surface predator, may be explained by their association with aquatic predators, especially adult Bluefish Pomatomus saltatrix and Striped Weakfish Cynoscion guatucupa, and the Franciscana Dolphin Pontoporia blainvillei, which drive these fish to the surface. The occurrence of flying ants in the diet was related to offshore winds, which carried these insects out to sea. The occasional high availability of insects possibly changed the cost/benefit relationship of several food types, causing diet changes. The high number of prey species, the temporal variations in the composition of the diet and the wide range of prey sizes are evidence of the high dietary plasticity of the Common Tern, at wintering areas in southern Brazil.     

Colony size and not nest density drives reproductive output in the Common Tern Sterna hirundo

Density is known to be an important factor in population size regulation. Several mechanisms of density limitation have been identified in colonial birds. We studied competition in Common Terns Sterna hirundo to assess whether the factor limiting reproductive output was competition for nest‐sites, which is dependent on local nest density, or density‐dependent competition for food resources, which is dependent on overall colony size using the same foraging area. We found strong associations of both colony size and nest density with reproductive output in five colonies of Common Terns in three different habitats (one marine, two freshwater). Based on detailed long‐term datasets of six separate sub‐colonies of the Banter See colony that differed in nest density, we found that reproductive success was not related to nest density but to overall colony size, possibly a result of resource depletion and food competition. We also found carry‐over effects of colony size during rearing on post‐fledging return rate. These results have important implications for the conservation management plans aimed at recovering declining populations of Common Terns.     

Predation by Kelp Gulls Larus dominicanus at a mixed-species colony of Royal Terns Sterna maxima and Cayenne Terns Sterna eurygnatha in Patagonia

We studied the predator-prey interaction between Kelp Gulls Larus dominicanus and Royal Terns Sterna maxima and Cayenne Terns Sterna eurygnatha at Punta León, Argentina, from 1990 to 1992. Kelp Gulls were the main predator of tern eggs in all 3 years, accounting for 99% of observed predations (n = 454). A total of 99% of predations occurred on peripheral tern nests. Total predation rate on both tern species was highest in 1990 and varied significantly between years, with between 0.01 and 0.03 cases of predation per hour. Predation rates in all years were significantly higher on Cayenne than on Royal Terns probably because the latter were significantly more aggressive towards gulls than were Cayenne Terns. Royal and Cayenne Terns displayed aggressively towards gulls at least once in 64% and 10% of the observation periods, respectively. Defence behaviour, such as attacking and chasing the predator, was used more frequently by Royal than Cayenne Terns (13.2% v 3.6%). Predation on tern eggs was restricted to only a few Kelp Gulls which nested close to the tern colony periphery, and 50% of the eggs that were stolen were taken by only two pairs during 1991 and 47% by three pairs in 1992. This suggests that the current expansion of the Kelp Gull numbers at Punta León might not necessarily result in a proportional increase in predation rate on both tern species. Stealing eggs from the ground was more successful than from the air (77% v 17%). The nesting strategy shown by Royal and Cayenne Terns at Punta León appears to be an important factor which reduced the impact on their breeding success of high potential predation pressure by gulls.     

Offspring sex ratio in the Common Tern Sterna hirundo, a species with negligible sexual size dimorphism

In many vertebrates, male offspring are affected more than female offspring by adverse conditions during growth, resulting in facultative adjustment of offspring sex ratio by parents in response to social and environmental conditions during breeding. The greater vulnerability of male offspring is generally attributed to their higher energy requirements associated with their larger size, although greater sensitivity to adverse conditions could be related to other factors such as negative effects of androgens on male physiology. To control for sexual differences in body size, we examined variation in offspring sex ratio in the Common Tern Sterna hirundo , a species with negligible sexual size dimorphism. In this species, the last-laid egg (termed the c-egg) is smaller than the first two and hatches last, so that the chick obtains relatively little food and hence has a low probability of survival to fledging. This species thus provides a powerful model for examining sex-linked mortality and sex ratio variation under natural conditions. We found that the sex ratio of c-eggs, but not of earlier laid eggs, was significantly biased in favour of females. Chicks hatched from c-eggs (termed c-chicks) had low survival but female c-chicks had significantly higher survival than male c-chicks. These data provide strong evidence that factors other than sexual size dimorphism are responsible for producing greater vulnerability of male offspring to adverse conditions during growth.     

Divorce and asynchronous arrival in common terns, Sterna hirundo

We investigated which of three hypotheses (better option, incompatibility or asynchronous arrival) best explains divorce in the common tern. One partner did not return the next year in 18.5% of 150 pairs. Among the 106 pairs in which both mates returned, the divorce rate was 18.9%. We found no significant differences in: breeding performance or condition in relation to the probability of divorce; quality of previous mates and new mates, mean age in relation to pair bond status; breeding success before and after divorce nor did this differ from breeding success of reunited pairs. Hence the better option and incompatibility hypotheses were not supported. However, divorce was more likely in pairs in which mates arrived asynchronously on the breeding grounds, supporting the asynchronous arrival hypothesis. Median arrival asynchrony for divorced pairs was 7.5 days and for reunited pairs 2 days; mates arriving more than 16 days apart always split up. About 20% of divorced birds lost breeding status in the year of divorce, probably as a consequence of their late arrival. Our results suggest that terns search for a new mate as soon as they arrive on the breeding grounds and that mates remain faithful to each other to avoid the costs of searching for a new partner. Thus, synchrony in arrival facilitates pair bond maintenance rather than asynchrony promoting divorce, since divorce appears to be a side-effect of asynchrony and not an active decision. Copyright 1999 The Association for the Study of Animal Behaviour.     

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.