Common Terns Research Articles

Genetic identification for prey birds of the Endangered peregrine falcon (Falco peregrinus)

To reveal the dietary ecology of the peregrine falcon (Falco peregrinus), we conducted a molecular analysis for mitochondrial genes (COI and CYTB) to identify the prey species collected from a feeding place found on Sasu Island, South Korea. The results from noninvasive genetic analysis showed that the sequences obtained came from nine species of birds (Cuculus canorus, Eurystomus orientalis, Limosa limosa, Microscelis amaurotis, Oriolus chinensis, Phasianus colchicus, Sterna hirundo, Streptopelia orientalis, and Turdus pallidus). Five of the species (C. canorus, M. amaurotis, S. hirundo, S. orientalis, and T. pallidus) had previously been observed on this island, but the other four species (E. orientalis, L. limosa, O. chinensis, and P. colchicus) were newly identified as present. No mtDNA sequences of land animals such as amphibians, reptiles, and mammals were found in the dietary remains, suggesting that the peregrine falcon preys mostly on other birds rather than on other animals inhabiting Sasu Island. This island has rich avian diversity and abundant animal populations and therefore supplies sufficient dietary resources for the peregrine falcon. Our findings suggested that a DNA-based molecular method may be useful to identify the prey species of these birds and may be valuable in future studies of the Endangered peregrine falcon.

Synapsis and recombination of autosomes and sex chromosomes in two terns (Sternidae, Charadriiformes, Aves)

The frequency of recombination and the patterns of crossover site distribution along the chromosomes vary considerably among animal species, including closely related species. Several hypotheses concerning the adaptive value and evolution of these variations were proposed. It was supposed that the recombination patterns of the species’ genomes are influenced by their phylogenetic history and ecology. However, most original data were obtained from mammals. The mammals show high karyological variability, which strongly influences the recombination patterns. Therefore it is important to study recombination rate and distribution in more karyologically stable taxa, such as reptiles and birds. We used immunolocalization of SYCP3, the protein of the lateral element of the synaptonemal complex (SC), centromere proteins and the mismatch-repair protein MLH1, which is associated with the recombination nodules, at the synaptonemal complex spreads of prophase oocytes of two tern species, black tern (Chlidonias niger) and common tern (Sterna hirundo). We first described the karyotype of Ch. niger (2n = 74, FN= 94) and identified suggestive rearrangements by which its karyotype differs from that of S. hirundo (2n = 68, FN = 90). We found that these species significantly differed by the numbers of the MLH1 foci per cell (Ch. niger: 53.0±4.2; S. hirundo: 44.1±5.0). We showed that the difference in the crossover numbers per cell was determined by the difference in the SC length (total and of individual bivalents) and by chromosomal rearrangements, which also influenced the distributions of crossover sites along the chromosomes. The difference in recombination patterns was higher between the rearranged homeologues than between the non-rearranged ones. We investigated the synaptic patterns of the heteromor phic Z and W chromosomes, localized the pseudoautosomal regions and estimated their lengths. In spite of several autosomal rearrangements, which differentiate these species, the structure and synaptic patterns of the sex chromosomes have not changed over 9 MY, which have passed since the moment of divergence between the genera Sterna and Chlidonias.

Postbreeding dispersal and staging of Common and Arctic Terns throughout the western North Atlantic

Postbreeding dispersal and staging of Common and Arctic Terns throughout the western North Atlantic

New Microsatellite Markers for the Common Tern (Sterna hirundo) Developed with 454 Shot-Gun Pyrosequencing

Long term studies, focusing on population- and socio-biology research, require the unequivocal identification of individuals. DNA studies with Short Tandem Repeats (STR loci) became a widespread tool in population genetics. We used the next-generation sequencing (NGS) approach with 454 shot-gun pyrosequencing to identify 13 new polymorphic STR loci for the Common Tern,Sterna hirundo. To enlarge the marker set we added two more loci originally developed for Black-legged Kittiwake (Rissa tridactyla) and Red-billed Gull (Chroicocephalus scopulinus) and arranged these 15 loci into three multiplex PCR panels for high throughput genotyping. Loci characterization demonstrated that our marker set is of high quality. A PIC value of about 0.67 and a power of exclusion value of 0.99 were reached. Deviation from Hardy-Weinberg expectations of some loci and low frequencies for null alleles are interpreted as a result of inbreeding and founder effect in the investigated tern colony. We used a test data set of this well-studied breeding colony of Common Tern at Banter Lake, Wilhelmshaven, Germany, to perform a parentage test. Parent-chick relationships, known from the social pedigree of that colony, were compared with genetically calculated ones. In order to test our markers and the used parentage program COLONY, we conducted six competing data sets with varying completeness of included parental genotypes. By including fully sampled parent pairs of known family assignment, results were correct for nest mates, single parents and parent pairs. Our marker set provides a powerful tool to investigate life-time reproductive success and other issues of population and socio-biology for Common Terns,e.g. in the aforementioned colony monitored for decades.

Nest and Colony Site Selection in Common Terns (Sterna hirundo) in Mongolia

Abstract. The biology of the Common Tern (Sterna hirundo) has been studied on several continents, but information from central Asia is limited. Colony characteristics and nest site selection of Common Terns were studied in the desert and steppe of Mongolia to determine factors that influence their choices. Common Terns nested in marshy areas around small ponds, on sand bars adjacent to a lake in the Gobi, and on small islands in a water treatment plant in the steppe of Ulaanbaatar. In the Gobi, they nested with Pied Avocets (Recurvirostra avosetta) that defended the colony site, while the terns left. Common Terns selected colony sites surrounded by water and muck/ mud, and nest sites with good visibility. Their choice of colony and nest sites seemed to be a compromise between avoiding flooding, avoiding nest trampling by livestock, and finding any safe nesting place to avoid terrestrial predators.

Quantitative ornithology with a commercial marine radar: standard‐target calibration, target detection and tracking, and measurement of echoes from individuals and flocks

Summary Marine surveillance radars are commonly used for radar ornithology, but they are rarely calibrated. This prevents them from measuring the radar cross‐sections (RCS) of the birds under study. Furthermore, if the birds are aggregated too closely for the radar to resolve them individually, the bulk volume reflectivity cannot be translated into a numerical density. We calibrated a commercial off‐the‐shelf marine radar, using a standard spherical target of known RCS. Once calibrated, the radar was used to measure the RCS of common and roseate terns (Sterna hirundo L. and Sterna dougallii Montagu) tracked from a land‐based installation at their breeding colony on Great Gull Island, NY, USA. We also integrated echoes from flocks of terns, comparing these total flock cross‐sections with visual counts from photos taken at the same time as the radar measurements. The radar's calibration parameters were determined with 1% error. RCS measurements made after calibration were expected to be accurate within ±2 dB. Mean tern RCS was estimated at −28 dB relative to one square meter (dBsm), agreeing in magnitude with a simple theoretical model. RCS was 3–4 dB higher when birds’ aspect angles were broadside to the radar beam compared with head‐ or tail‐on. Integrated flock cross‐section was linearly related to the number of birds. The slope of this line, an independent estimate of RCS, was −32 dBsm, within an order of magnitude of the estimate from individual birds, and near the middle of the frequency distribution of RCS values. These results indicate that a calibrated marine radar can count the birds in an aggregation via echo integration. Field calibration of marine radars is practical, enables useful measurements, and should be done more often.

The sea level rise impact on four seashore breeding birds: the key study of Sečovlje Salina Nature Park

Climate change is expected to result in an acceleration of current rates of sea level rise, inundating many low-lying coastal and intertidal landscapes. This could have important implications for many coastal habitat types and related organisms that depend on these habitats, including shorebirds that rely on them for feeding, overwintering and breeding. Potential change in the availability of suitable breeding area according to linear and model-based sea level rise scenarios was modeled for four breeding birds (Kentish Plover, Little Tern, Common Tern and Black-winged Stilt) in Secovlje Salina Nature Park, based on precise mapping of nests over a period of 10 years and on present environmental predictors. Different breeding niches for the studied bird species in SSNP were identified, which indirectly indicates different responses to environmental change, in this case triggered by climate change induced sea level rise. Future breeding suitability maps indicate that the Little Tern and the Common Tern could potentially face a drastic decrease of adequate breeding grounds in SSNP later than the Kentish Plover and the Black-winged Stilt. However, these individual species responses to sea level rise, as a climate change driver, is a step forward for conservation biologists and landscape planners in protected areas, as they prepare cost-effective plans for mitigating negative impact on these ecosystems. This study also illustrates an important general point about the likely effects of climate change on ecological resources, which is that climate change does not happen in a vacuum; its impact will interact with pre-existing stress factors.

Telomere length reflects reproductive effort indicated by corticosterone levels in a long-lived seabird.

Telomere length (TL) is a candidate biomarker of ageing and phenotypic quality, but little is known of the (physiological) causes of TL variation. We previously showed that individual common terns Sterna hirundo with high reproductive success had short telomeres independent of age, and this pattern was particularly strong in the longer telomeres of the within-individual TL distribution. To test whether this relation can be attributed to effects of reproductive effort, we investigated baseline corticosterone in relation to reproductive success (number of fledglings) and TL. In this context, we assume that variation in baseline corticosterone can be interpreted as index of energy expenditure and allostatic load. Males with higher corticosterone levels during incubation, compared between and within individuals, achieved higher reproductive success and had shorter telomeres. The effect on telomeres was more pronounced in corticosterone measured later in incubation and in the longer telomeres of the within-individual TL distribution. Female corticosterone level during incubation was neither related to reproductive success nor to TL. That we observed these effects only in males mirrors different parental roles during reproduction in the common tern, where males do most of the chick provisioning. The negative association between reproductive success and TL suggests individual differences in reproductive effort as reflected in, or mediated by, baseline corticosterone. We see this result as a promising step towards unravelling the physiological causes of variation in TL and the costs of reproduction.

Ecology of tern flight in relation to wind, topography and aerodynamic theory.

Flight is an economical mode of locomotion, because it is both fast and relatively cheap per unit of distance, enabling birds to migrate long distances and obtain food over large areas. The power required to fly follows a U-shaped function in relation to airspeed, from which context dependent 'optimal' flight speeds can be derived. Crosswinds will displace birds away from their intended track unless they make compensatory adjustments of heading and airspeed. We report on flight track measurements in five geometrically similar tern species ranging one magnitude in body mass, from both migration and the breeding season at the island of Öland in the Baltic Sea. When leaving the southern point of Öland, migrating Arctic and common terns made a 60° shift in track direction, probably guided by a distant landmark. Terns adjusted both airspeed and heading in relation to tail and side wind, where coastlines facilitated compensation. Airspeed also depended on ecological context (searching versus not searching for food), and it increased with flock size. Species-specific maximum range speed agreed with predicted speeds from a new aerodynamic theory. Our study shows that the selection of airspeed is a behavioural trait that depended on a complex blend of internal and external factors.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Integrating Monitoring and Genetic Methods To Infer Historical Risks of PCBs and DDE to Common and Roseate Terns Nesting Near the New Bedford Harbor Superfund Site (Massachusetts, USA).

Common and roseate terns are migratory piscivorous seabirds with major breeding colonies within feeding range of the polychlorinated biphenyl (PCB)-contaminated New Bedford Harbor (NBH, MA, USA) Superfund site. Our longitudinal study shows that before PCB discharges into NBH ceased (late 1970s), tern eggs had very high but variable PCB concentrations. However, egg concentrations of PCBs as well as DDE (1,1-bis(p-chlorophenyl)-2,2-dichloroethene), the degradation product of the ubiquitous global contaminant DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane), have since declined. Rate constants for temporal decline of PCB congeners in tern eggs varied inversely with log10KOW (n-octanol-water partition coefficient), shifting egg congener patterns away from those characterizing NBH sediment. To estimate the toxic effects on tern eggs of PCB dioxin-like congener (DLC) exposures, we extrapolated published laboratory data on common terns to roseate terns by characterizing genetic and functional similarities in species aryl hydrocarbon receptors (AHRs), which mediate DLC sensitivity. Our assessment of contaminant risks suggests that terns breeding near NBH were exposed historically to toxic levels of PCBs and DDE; however, acute effects on tern egg development have become less likely since the 1970s. Our approach demonstrates how comparative genetics at target loci can effectively increase the range of inference for chemical risk assessments from tested to untested and untestable species.

Long-term trends (1989-2013) in the seabird community breeding in the lagoon of Venice (Italy)

The community of seabirds breeding in the lagoon of Venice, one of the largest around the Mediterranean, was monitored over the 1989-2013 period. Six species have been recorded (<em>Larus michahellis, Chroicocephalus ridibundus, Larus melanocephalus, Sternula albifrons, Sterna hirundo, Sterna sandvicensis</em>). Over the study period, 346 colonies were surveyed; they were located on saltmarsh islets, dredge islands and other artificial sites, in descending order of importance. Beaches were used only at the beginning of the survey. About 4,900 pairs bred in the 2009-2013 years. The community has been always dominated by <em>L</em>. <em>michahellis</em>; in the 2009-2013, this species and <em>S. sandvicensis</em> comprised about 70% of the total. The longterm trends show moderate increase in the size of the whole community. Contrasting trends were observed in different species, with <em>L. michahellis</em>,<em> S. albifrons</em> and <em>S. sandvicensis</em> stable or increasing, whereas <em>C. ridibundus</em> and <em>S. hirundo</em> were declining. The number of pairs nesting on artificial sites increased over the last 15 years, while those on saltmarsh islets decreased correspondingly. This was possibly a response to the increase of mean sea level and storminess observed in the last twenty-five years. <em>S. albifrons</em> and <em>S. sandvicensis</em> populations reached national importance; the population of the latter species is noteworthy at a Mediterranean level.

Male-biased sex allocation in ageing parents; a longitudinal study in a long-lived seabird.

Optimal sex allocation is frequency-dependent, but senescence may cause behaviour at old age to be suboptimal. We investigated whether sex allocation changes with parental age, using 16 years of data comprising more than 2500 molecularly sexed offspring of more than 600 known-age parents in common terns (Sterna hirundo), slightly sexually size-dimorphic seabirds. We decomposed parental age effects into within-individual change and sex allocation-associated selective (dis)appearance. Individual parents did not differ consistently in sex allocation, but offspring sex ratios at fledging changed from female- to male-biased as parents aged. Sex ratios at hatching were not related to parental age, suggesting sons to outperform daughters after hatching in broods of old parents. Our results call for the integration of sex allocation theory with theory on ageing and demography, as a change in sex allocation with age per se will cause the age structure of a population to affect the frequency-dependent benefits and the age-specific strength of selection on sex allocation.

A helping hand: artificial nest site provisioning increases breeding success of a tropical seabird

Common white terns Gygis alba lay a single egg balanced on rocks or branches and consequently are at risk of low nesting success. A novel technique of hollowing out coconut husks and providing artificial nest sites was developed on Cousine Island, Seychelles. Our study aimed to critically assess whether common white tern nesting success differed between artificial and natural nest sites. For natural nest sites, nesting success and the stage of nest failure were also compared between seasons, nest heights, and tree species. Finally, we compared results from our study to those of a study conducted 10 years ago on the same island. Nesting success differed significantly between artificial nests and natural nests, with the success of artificial nests nearly double that of natural nests. Hatching success was similar for artificial and natural nests (65% and 52% respectively), suggesting coconut husks do not reduce egg losses, but rather provide secure perches for young chicks. Most natural nest failures occurred during the egg phase (70%), followed by the youngest chick phase (21%). Of the eggs that hatched in natural nests, 61% fledged successfully. Tree species and season significantly influenced nesting success for natural nests and nest height varied significantly between tree species. Higher nesting success was observed during the mixed seasonal period (50%) followed by the cooler, windier and drier South-East Monsoon season (33%). The hotter and more humid North-West Monsoon period had the lowest nesting success (20%). Comparisons of nesting success at natural and artificial nest sites with the earlier study did not differ significantly, despite a smaller population of common white terns being present on the island at that stage. Our results support the use of artificial nest sites to improve nesting success for this species and serve as a model for similar species currently under threat.

Premature feather loss among common tern chicks in Ontario: the return of an enigmatic developmental anomaly.

In July 2014, we observed premature feather loss (PFL) among non-sibling, common tern Sterna hirundo chicks between two and four weeks of age at Gull Island in northern Lake Ontario, Canada. Rarely observed in wild birds, to our knowledge PFL has not been recorded in terns since 1974, despite the subsequent banding of hundreds of thousands of tern chicks across North America alone. The prevalence, 5% of chicks (9/167), and extent of feather loss we report is more extreme than in previous reports for common terns but was not accompanied by other aberrant developmental or physical deformities. Complete feather loss from all body areas (wing, tail, head and body) occurred over a period of a few days but all affected chicks appeared vigorous and quickly began to grow replacement feathers. All but one chick (recovered dead and submitted for post-mortem) most likely fledged 10–20 days after normal fledging age. We found no evidence of feather dystrophy or concurrent developmental abnormalities unusual among affected chicks. Thus, the PFL we observed among common terns in 2014 was largely of unknown origin. There was striking temporal association between the onset of PFL and persistent strong southwesterly winds that caused extensive mixing of near-shore surface water with cool, deep lake waters. One hypothesis is that PFL may have been caused by unidentified pathogens or toxins welling up from these deep waters along the shoreline but current data are insufficient to test this. PFL was not observed among common terns at Gull Island in 2015, although we did observe similar feather loss in a herring gull Larus argentatus chick in that year. Comparison with sporadic records of PFL in other seabirds suggests that PFL may be a rare, but non-specific, response to a range of potential stressors. PFL is now known for gulls, penguins and terns.

Influence of diet and foraging strategy on reproductive success in two morphologically similar sympatric seabirds

ABSTRACTCapsule: Morphologically similar sympatrically breeding species differ in diet and foraging strategies, which could explain interspecific variation in breeding success and reproductive output.Aims: To compare provisioning rate and diet of nestling Arctic (Sterna paradisaea) and Common Terns (Sterna hirundo), and examine how variation in provisioning behaviour influences reproductive parameters.Methods: Arctic and Common Tern nests were observed from June to July 2011 on Coquet Island, northeast England. We compared chick provisioning rate, diet and energy delivery rate between species, and examined whether these parameters were associated with interspecific differences in chick growth and survival, and total number of chicks fledged.Results: Arctic Terns delivered higher percentages of smaller Sandeels (Ammodytes marinus) and juvenile fish to chicks than did Common Terns, which delivered comparatively high percentages of larger Sprats (Sprattus sprattus). Although chick growth rate and survival were not significantly different between species, Common Terns fledged more chicks than Arctic Terns due to a tendency for larger clutch sizes.Conclusion: Our study suggests that interspecific differences in diet and foraging strategies can support total reproductive output in morphologically similar species.

Common Terns on the East Atlantic Flyway: temporal–spatial distribution during the non-breeding period

We studied the temporal–spatial distribution of Common Terns Sterna hirundo along the East Atlantic Flyway. In 2009 and 2010 experienced adults from a colony on the German North Sea coast were tagged with geolocators recording light intensity and saltwater contact. Main objectives were the inter-individual temporal–spatial variation of migration routes and wintering areas, wintering site fidelity, and time spent at sea across the annual cycle. Geolocators had no effects on various traits of breeders, but their reproductive output suffered from egg breakage. This can be avoided by artificially incubating the eggs. Twelve routes of nine individuals were tracked. Transponder readings at the breeding site showed that birds left the colony 4 weeks before starting autumn migration. In spring and autumn, Common Terns stopped over around the Canary Islands. Main wintering distribution was the upwelling seas alongside the West African coast and similar between years, but different among individuals. Three females wintered further north and more offshore than six males. Pair mates wintered at different locations. Spring migration was longer (56 ± 8 days) than autumn migration (37 ± 17 days). During both migration and wintering the terns spent more time on salt water than during breeding and post-breeding. In most individuals saltwater contact was higher during the day than at night, reduced at sunrise and sunset likely due to foraging, and peaked about noon possibly related to resting or thermoregulation. Detailed ecological and behavioral studies of common terns during wintering are needed to clarify the results based on geolocators.

Population trends of colonial waterbirds nesting in Hamilton Harbour in relation to changes in habitat and management

Since 1975, the diversity and abundance of colonial waterbirds breeding in Hamilton Harbour have increased, making it an important nesting site on the Great Lakes. An adaptive management approach has been employed to control hyper-abundant species and guide conservation efforts for vulnerable species, with the goal of maintaining a diverse waterbird community. Four species exhibited increasing or stable population trends (1975–2013): Double-crested Cormorants (Phalacrocorax auritus; from 0 to 4747 nests); Black-crowned Night-Herons (Nycticorax nycticorax; ranged from 6 to 259 nests); Herring Gulls (Larus argentatus; from 0 to 244 nests); and Caspian Terns (Hydroprogne caspia; from 0 to 496 nests). Cormorants are currently above (2,500 nests), while Caspian Terns (400–600 nests), Night-Herons (100–200 nests) and Herring Gulls (200–300 nests) are within population targets set out in the Hamilton Harbour Remedial Action Plan. Despite conservation efforts, Common Terns (Sterna hirundo) declined from a peak of 1,028 nests (1990) to 333 nests (2013), although currently within the population target (300–600 nests). Ring-billed Gulls (L. delawarensis), through long-term management and habitat restrictions, were reduced from a peak of 39,621 nests (1990) to 11,133 nests (2013), but still exceed the target (<10,000 nests). Changes in the amount of available habitat have affected waterbird distributions: the loss of 42 ha (peak in 1999) of former nesting areas to development has been partially offset by the creation or securement of 1.9 ha of dedicated breeding habitat. Continued management, assessed and refined annually, is required to maintain species diversity in the area. Current management techniques focus on preventing Ring-billed Gulls from nesting on private lands and dedicated Tern nesting habitat, excluding Cormorants from nesting at specific sites, and reducing inter-specific competition with Night-Herons and Herring Gulls. Recommendations and considerations regarding future management and conservation efforts to reach Remedial Action Plan targets in the harbor are outlined.

Distribution, Abundance and Trends of Gulls and Terns Breeding on Sable Island, Nova Scotia, Canada

Sable Island is the most isolated seabird colony site in eastern Canada and the United States, offering a unique opportunity to study the population dynamics of terns and gulls in an area removed from human activities. Sable Island likely supported tens of thousands of terns prior to 1900, but the population declined during the first half of the 20th century, coinciding with colonization by breeding gulls. An island-wide census of terns and gulls was conducted in 2012 and 2013, and those results were compared with surveys conducted over the previous 45 years to assess changes in population abundance and distribution. The current island-wide population of Common (Sterna hirundo) and Arctic (S. paradisaea) terns was approximately 6,500 and 4,200 combined breeding pairs in 2012 and 2013, respectively, down from about 9,000 pairs estimated in 2008/2009, but higher than all estimates between 1970 and 2006. Population growth of these tern species has been concentrated at two large colonies, each with over 2,000 breeding pairs, whereas the number of small colonies on the island has been in decline since 1998. Fewer than six pairs of Roseate Terns (S. dougallii) have nested on Sable Island since 1993, down from counts of more than 100 individuals in the 1970s. Estimates of breeding pairs ranged from 744 to 951 Herring Gulls (Larus argentatus) and 398 to 472 Great Black-backed Gulls (L. marinus) during 2012 and 2013. A survey of gulls documented 33 to 50% fewer Herring Gulls and 23% fewer Great Black-backed Gulls than in 1970. Sable Island's tern and gull populations have fluctuated asynchronously over the past 100 years, and may be returning to a previous ecosystem state when gulls were absent and terns were abundant, but mechanisms underlying these trends are unknown.

Energetic constraint of non‐monotonic mass change during offspring growth: a general hypothesis and application of a new tool

Post-natal growth is an important life-history trait and can be a sensitive indicator of ecological stress. For over 50 years, monotonic (never-decreasing) growth has been viewed as the predominant trajectory of post-natal mass change in most animal species, notably among birds. However, prevailing analytical approaches and energetic constraints may limit detection of non-monotonic (or multiphasic), determinate growth patterns, such as mass recession in birds (weight loss prior to fledging, preceded by overshooting adult mass), which is currently believed to be restricted to few taxa. Energetic surplus and shortfall are widespread conditions that can directly influence the degree of mass overshooting and recession. Thus, we hypothesize that in many species, prevailing energetic constraints force mass change away from a fundamental non-monotonic trajectory to instead follow a monotonic curve. We observed highly non-monotonic, mass change trajectories (overshooting adult mass by up to almost 20%) among common tern Sterna hirundo chicks, a well-studied species long-established as growing monotonically. We quantified the prevalence and magnitude of non-monotonic mass change prior to fledging for 313 common tern chicks that successfully fledged from two discrete populations in multiple years. We used a new approach for analysing non-monotonic curves to examine differences in mass change trajectories between populations under contrasting abiotic (freshwater vs. saltwater) and biotic stresses (low rates of food provisioning). Some degree of mass recession occurred in 73% of all study chicks. Overshooting adult mass followed by extensive mass recession was most prevalent at our freshwater colony, being detected among 34-38% of chicks annually. Non-monotonic trajectories were less marked in populations experiencing ecological stress and among lower quality individuals. Chicks that were provisioned at higher rates were more likely to both overshoot adult mass and experience subsequent mass recession. Our results in common terns provide strong support for the hypothesis that non-monotonic trajectories are the fundamental pattern of mass change but are constrained to be monotonic under energetic shortfall. This justifies future tests of the generality of this hypothesis across a broad range of taxa. We also demonstrate a recent analytical tool that prevents routine fitting of monotonic curves without prior investigation of non-monotonic trends.

Common Tern Sterna hirundo and Arctic Tern S. paradisaea hybridization produces fertile offspring

This study provides the first molecular evidence for hybridization between Arctic Terns Sterna paradisaea and Common Terns S. hirundo. We studied a mixed pair (P1; male Arctic Tern and female Common Tern) and its offspring on Penikese Island, Massachusetts, USA in 2007–2015. The pair maintained a long-term pair bond (8 years); its reproductive performance was comparable to that of Common Terns and higher than that of Arctic Terns at the site. Molecular analyses confirmed that all young raised by the pair (at least 5 males and 4 females) were biological offspring. We describe hybrid young and the adult hybrid in detail to facilitate field identification. Although F1 hybrid young were intermediate between the parent species in certain characteristics, dark feathers extending below the eye, in particular, gave them a distinctly Arctic Tern-like appearance; however, all had secondary feathers that were darker than the wing-coverts, like Common Terns. We detected one male F1 hybrid that returned to breed; it retained some features intermediate between the parent species, but we qualitatively judged it to be more Common Tern-like. It mated with a Common Tern and produced three confirmed backcross hybrid young that closely resembled Common Terns. We speculate that rarity of Arctic Terns, especially females, at and near the study site and age/inexperience of the Common Tern parent were proximate mechanisms for the formation of the P1 pair. Further, because mixed pairs are occasionally observed and hybrids are cryptic, we suggest that Arctic/Common Tern hybridization may occur more commonly than is currently realized.