Sandwich Terns Research Articles

Effect of brood size and hatching sequence on prefledging mortality of Sandwich terns: why lay two eggs?

The mortality of Sandwich tern Sterna sandvicensis chicks held in enclosures was studied in colonies on Griend, in the Dutch Wadden Sea, from 1992 to 1999, and on Hirsholm, in the Danish Katt- egat, in 1997. Survival of chicks until fledging was 73% for chicks hatching from first-laid eggs or single- egg clutches and 59-64% for partially hatched two-egg clutches, whereas 6% of second hatchlings survived until fledging. Less than 2% of all two-chick broods actually fledged two chicks. Because 18% of the two-egg clutches only hatched one egg, 7% of fledglings of two-egg clutches originated from a sec- ond-laid egg. In nests where both eggs hatched, the number of chicks was usually reduced soon after hatching. Within five days of hatching more than 50% of the second hatchlings died of starvation or were preyed upon. It seems that overproduction commonly occurs in Sandwich terns and that invest- ment in a surplus egg mainly serves as an insurance mechanism. On Griend and Hirsholm, chick pro- ductivity of two-egg clutches was somewhat higher than for one-egg clutches. Undernourishment was an important cause of death, either directly by starva- tion or by selective predation of chicks in poor condition. This, in combination with earlier, studies suggests that Sandwich tern parents on Griend are exposed to severe food stress.

Kleptoparasitism and Host Responses in a Sandwich Tern Colony of Eastern Spain

We report on the incidence of kleptoparasites and the host avoidance responses in a colony of the Sandwich Tern (Sterna sandvicensis) at l’Albufera de Valencia, eastern Spain, in 2004. Apart from conspecifics, five species were kleptoparasites of breeding Sandwich Terns, particularly Black-headed Gull (Larus ridibundus) and Slender-billed Gull (L. genei). Avian kleptoparasites attacked 24% of the Sandwich Terns returning to the colony with fish (N = 659) and robbed 5.3% of them. Most terns (85%) delivered their prey to a mate or chick. Fish carried by terns were mainly Clupeiformes species (96%). Prey size declined significantly as the season progressed. The probability of attack increased significantly with fish size and declined significantly as the season progressed. Most attacks were on terns as they landed to deliver their prey. Overall success rate of attacks was 12.5% and were mostly performed by single birds. The time that the terns spent flying before landing to deliver prey increased with the size of carried fish.

Scaling possible adverse effects of marine wind farms on seabirds: developing and applying a vulnerability index

Summary Marine wind farms have attracted substantial public interest. The construction of wind facilities offshore may become Europe's most extensive technical development in marine habitats. Due to political pressure to complete construction soon, assessments of possible wind farm locations, for example in the German sectors of the North Sea and Baltic Sea, have to be based on existing knowledge. In this study, we developed a wind farm sensitivity index (WSI) for seabirds. We applied this index to the Exclusive Economic Zone and the national waters of Germany in the North Sea. We chose nine factors, derived from species’ attributes, to be included in the WSI: flight manoeuvrability; flight altitude; percentage of time flying; nocturnal flight activity; sensitivity towards disturbance by ship and helicopter traffic; flexibility in habitat use; biogeographical population size; adult survival rate; and European threat and conservation status. Each factor was scored on a 5‐point scale from 1 (low vulnerability of seabirds) to 5 (high vulnerability of seabirds). Five of these factors could be dealt with by real data but four could only be assessed by subjective considerations based on at‐sea experience; in the latter cases, suggestions of the first author were independently modulated by experts. Species differed greatly in their sensitivity index (SSI). Black‐throated diver Gavia arctica and red‐throated diver Gavia stellata ranked highest (= most sensitive), followed by velvet scoter Melanitta fusca, sandwich tern Sterna sandvicensis and great cormorant Phalacrocorax carbo. The lowest values were recorded for black‐legged kittiwake Rissa tridactyla, black‐headed gull Larus ridibundus and northern fulmar Fulmarus glacialis. A WSI score for areas of the North Sea and Baltic Sea was calculated from the species‐specific sensitivity index values. Coastal waters in the south‐eastern North Sea had values indicating greater vulnerability than waters further offshore throughout the whole year. Derived from the frequency distribution of the WSI, we suggest a ‘level of concern’ and a ‘level of major concern’ that are visualized spatially and could act as a basis for the selection of marine wind farm locations. Synthesis and applications. The wind farm sensitivity index might be useful in strategic environmental impact assessments (EIA). Results of small‐scale EIA from wind installations should be considered within a more global perspective, provided, for example, by large mapping projects and detailed behavioural studies. This is difficult in normal EIA, particularly in highly dynamic coastal/marine habitats, and the results of this study fill an important gap by providing information on the potential sensitivity of seabirds and the importance of locations of wind installations.

Attendance of scavenging seabirds at trawler discards off Galicia, Spain

The occurrence of scavenger seabirds at fishing trawlers was studied off Galicia, Spain. A total of 9,368 seabirds of 23 species were recorded during 92 fishing operations in 1998 and 1999. The most common species were the yellow-legged and lesser black-backed gull ( Larus cachinnans and L. fuscus ), Sabine´s gull ( L. sabini ), the northern gannet ( Morus bassanus ), the great shearwater ( Puffinus gravis ), sooty shearwater (P. griseus), the Manx and Balearic shearwater ( P. puffinus and P. mauretanicus ), the great skua ( Catharacta skua ) and terns (mainly Sterna hirundo and S. paradisaea ). Other species occurred in small numbers: Leach´s petrel ( Oceanodroma leucorhoa ), the storm petrel ( Hydrobates pelagicus ), the little shearwater ( Puffinus assimilis ), Cory´s shearwater ( Calonectris diomedea ), the parasitic skua ( Stercorarius parasiticus ), the pomarine skua ( S. pomarinus ), the black-headed gull ( Larus ridibundus ), the glaucous gull ( L. hyperboreus ), the kittiwake ( Rissa tridactyla ), the sandwich tern (Sterna sandvicensis), the black tern (Chlidonias niger), the guillemot ( Uria aalge ) and the little auk ( Alle alle ). The maximum number of seabirds recorded at a haul was 320. The maximum number of a particular species ranged from 120 great shearwaters to 250 yellow-legged/lesser black-backed gulls during a single haul. The differences in ship-follower species abundance are related to migratory movements but fisheries could also have a strong influence at a smaller scale on the distribution of seabirds off Galicia. The degree to which seabirds rely on fishery discards as food was not quantified, but may be important for several species.

Feeding ecology of wintering terns in Guinea‐Bissau

We studied the feeding ecology of Little Terns Sterna albifrons, Sandwich Terns S. sandvicensis and Royal Terns S. maxima in the Archipélago dos Bijagós (11°40′N, 15°45′W) in Guinea‐Bissau (West Africa) during the winter of 1992/1993. More than 95% of all prey taken by these terns were roundfish, ranging in weight from 0.3 to 40 g. Birds usually fed alone, but sometimes they were observed feeding in mixed‐species flocks consisting of 15–200 individuals. Capture rate (n fish per hour foraging) in these flocks was higher than that of solitary birds. However, smaller fish were caught by birds foraging in flocks, so food intake rate (g/h) did not differ between solitary and flock‐feeding birds. The relationships between foraging behaviour of the three tern species and abiotic factors, such as time, tide and water clarity, have been investigated. Capture rate of Royal Terns increased with water clarity. For Little Terns and Sandwich Terns, food intake rate was lower in the most turbid waters compared to clearer waters. There was very little foraging activity during high tide. For Little Terns and Royal Terns, food intake rate was about twice as high during receding and low tides as during an incoming tide. Food intake rate averaged 8 g/h in Little Terns, 60 g/h in Sandwich Terns and 45 g/h in Royal Terns. With a rough model, we estimate the maximum rate of daily energy expenditure of terns wintering in the tropics at 3 × BMR (defined as energy expenditure of inactive bird at thermoneutrality in a post‐absorptive state during the resting phase of the daily cycle). From an energetic viewpoint, wintering Sandwich Terns in Guinea‐Bissau seem to have an easy living.

Variation in growth in Sandwich Tern chicksSterna sandvicensisand the consequences for pre‐ and post‐fledging mortality

Fitness consequences of variation in body mass growth and body condition were studied in a Sandwich TernSterna sandvicensiscolony on Griend, Dutch Wadden Sea, during 1990–2000. Body mass increment during the linear growth phase predicted nestling survival probabilities accurately. Chicks growing less than 8 g per day had low survival probabilities until fledging, but within a range of 8–11 g per day growth only small effects on chick survival were observed. Effects of slow growth on survival became obvious after about 10 days after hatching. Slow growing chicks reached a much lower fledging mass, whereas slow growth had only small effects on structural size at fledging. Body condition of the chicks was highly variable and had strong effects on survival until fledging. However, body condition during the nestling stage did not influence post‐fledging survival. Body condition at fledging had no effects on post‐fledging survival and did not affect final mass or body size. It is argued that low fledging mass can be overcome soon after fledging, as parents take their fledglings closer to the foraging areas, thereby avoiding high rates of kleptoparasitism by Black‐headed GullsLarus ridibundus.

Foraging Decisions of Sandwich Terns in the Presence of Kleptoparasitising Gulls

AbstractWe studied prey selection of Sandwich Terns (Sterna sandvicensis) in the presence of kleptoparasitising Black-headed Gulls (Larus ridibundus) on Griend, Dutch Wadden Sea, from 1992 to 1998. Most often, gulls robbed larger sandeels rather than herring from terns. Provided that one parent attends the chick, energy intake would be too low for the chicks' survival if exclusively fed on sandeel. By provisioning an increasing proportion of herring, parent terns could theoretically overcome a potential food shortage when the chicks grow older. However, the proportion of herring did not increase with age of chicks in most years. Instead, parents increased their foraging effort with the growth of their offspring. In years with high proportions of food loss, parents left their chicks unattended at the nest more often. In that way, parents were largely able to counterbalance food shortage of offspring. In 1992 and 1996, when diet composition was unfavorable and food losses to gulls were high, parents left their chicks unattended at the nest for ∼80% of the daylight period. It is argued that the present working level of parents might be higher than in the 1960s.

Foraging Decisions of Sandwich Terns in the Presence of Kleptoparasitising Gulls

Foraging Decisions of Sandwich Terns in the Presence of Kleptoparasitising Gulls

FORAGING DECISIONS OF SANDWICH TERNS IN THE PRESENCE OF KLEPTOPARASITISING GULLS

We studied prey selection of Sandwich Terns (Sterna sandvicensis) in the presence of kleptoparasitising Black-headed Gulls (Larus ridibundus) on Griend, Dutch Wadden Sea, from 1992 to 1998. Most often, gulls robbed larger sandeels rather than herring from terns. Provided that one parent attends the chick, energy intake would be too low for the chicks' survival if exclusively fed on sandeel. By provisioning an increasing proportion of herring, parent terns could theoretically overcome a potential food shortage when the chicks grow older. However, the proportion of herring did not increase with age of chicks in most years. Instead, parents increased their foraging effort with the growth of their offspring. In years with high proportions of food loss, parents left their chicks unattended at the nest more often. In that way, parents were largely able to counterbalance food shortage of offspring. In 1992 and 1996, when diet composition was unfavorable and food losses to gulls were high, parents left their chicks unattended at the nest for ∼80% of the daylight period. It is argued that the present working level of parents might be higher than in the 1960s.

Food and Feeding Biology of the Black Skimmer in Argentina: Evidence Supporting Offshore Feeding in Nonbreeding Areas

-We investigated the diet of the Black Skimmer (Rynchops niger) in three consecutive years (autumn 1998, autumn 1999 and summer 2000) by analyzing 642 pellets collected at Punta Rasa, Samborombon Bay, Buenos Aires, Argentina. Fish was the main prey in the diet (occurrence = 100%), while insects were present in only 2.7% of samples. Among the 14 fish species identified, Argentine Anchovies (Engraulis anchoita) (occurrence = 38.7%, N = 31.5%), Silversides (Odontesthes spp.) (occurrence = 38.3%, N = 26.2%) and White Croakers (Micropogoniasfurnieri) (occurrence = 11.6%, N = 20.5%) were the most important prey. Fish prey averaged 77 mm in length (?34 mm, range 15-169 mm, N = 123), and had an average mass of 6.9 g (?7.3 g, range 0.1-31.6 g, N = 123). Marked differences were observed in the frequency of occurrence and numeric importance of the fish prey species found in the three sampling seasons; while typical marine fishes, such as the Argentine Anchovy, constituted the bulk of the diet during 1998, estuarine juvenile fish prevailed in 1999 and 2000. The comparison of size and mass of prey between seasons also revealed large differences. Fishes caught in 1998 were significantly larger and heavier than those caught in other seasons. The diet of skimmers overlapped with that of South American Terns (Sterna hirundinacea), Royal Terns (S. maximus) and Sandwich Terns (S. sandvicensis) in the area during 1998, when prey was mostly marine fish. Received 17 April 2001, accepted 25June 2001.

Living with Gulls: The Consequences for Sandwich Terns of Breeding in Association with Black-Headed Gulls

We studied the feeding ecology of Sandwich Terns (Sterna sandvicensis) in the presence of kleptoparasitising Blackheaded Gulls (Larus, ridbundus) on the isle of Griend, The Netherlands, between 1992 and 1998. About 30% of all of the food the patents transported to the colony was lost, mainly through intervention by Blackheaded Gulls. The gulls mainly took the larger fish, but showed no preference for the energetically more profitable herring. Apparently, the gulls selected for prey length visibility of the prey, rather than for energy content of the fish. The proportion of food robbed by the gulls increased with the age of the tern chicks and was in parallel to the increase in pre!; length Sandwich Tern parents brought to the colon). Kleptoparasitism showed a clear pattern with time of the day, tide and wind speed. During the first few hours of the day almost no kleptoparasitism occurred, while robbery was high around 09.00h and at dusk. This bimodal pattern in kleptoparasitism might be related to the foraging activities of the gulls. A tidal effect on food loss was less pronounced. although kleptoparasitism was significantly higher during high tide, when foraging activity of gulls for other foods was low and the number of potential pirates in the tern colony was high. Wind strength had a significant negative effect on the amount of. food transported to the colony, while kleptoparasitism increased. Therefore, wind speed severely affected energy intake of th; tem chicks and had strong negative effects on chick growth. During the first two weeks post-hatching, kleptoparasitism was relatively low and had only small effects on chick growth, even under. unfavourable weather conditions. From then on, the negative effects of kleptoparasitism on growth because considerable. Check growth was severely affected by strong winds. Sandwich Terns show several behavioral strategies in order to reduce the rate of food loss by the gulls and to minimize the effects on chick growth and survival.

The Foraging Ecology of Royal and Sandwich Terns in North Carolina, USA

-We studied the foraging ecology of Royal Terns (Sterna maxima) and Sandwich Terns (S. sandvicensis) in North Carolina during the breeding seasons in 1999 and 2000. Foraging habitats were surveyed using 95 fixedpoint observations (30-45 min each) and over 100 h of inshore and offshore transects. Stable isotopes ('3C:'2C, 15N:14N, and s0:1160) in eggshells were analyzed for evidence of differences in foraging habitats. Water masses were delineated using satellite imagery of sea surface temperatures. We determined diet for each species with observations of chick provisioning and fecal analyses. Although Royal and Sandwich Terns nested together in the same colonies, their prey and foraging habitats differed. Sandwich Terns fed their chicks 48% anchovies (Anchoa sp.), 39% herring (Clupeidae)/jacks (Carangidae)/mackerels (Scombridae),and 9% drums (Sciaenidae)/porgies (Sparidae)/ mullets (Mugil sp.), and foraged primarily in the marine coastal environment. Conversely, Royal Terns provided their chicks with 9% anchovies, 30% herring/jacks/mackerels, and 41 % drums/porgies/mullets, and fed more frequently in the estuary. The species' foraging ranges overlapped at inlets and the river mouth. Stable isotope ratios also suggested that foraging differences occurred between colonies. The results of this investigation indicate that Royal and Sandwich Terns in North Carolina partition food resources by habitat and prey choice. Received 2 December 2000, accepted 8 February 2001.

Pursuit plunging by northern gannets (Sula bassana) feeding on capelin (Mallotus villosus).

Northern gannets (Sula bassana) are considered to obtain prey usually by rapid, vertical, shallow plunge dives. In order to test this contention and investigate underwater foraging behaviour, we attached two types of data-logging systems to 11 parental northern gannets at Funk Island in the North-Wiest Atlantic. We documented, for the first time to the authors' knowledge, gannets performing long, flat-bottomed, U-shaped dives that involved underwater wing propulsion as well as rapid, shallow, V-shaped dives. The median and maximum dive depths and durations were 4.6 and 22.0 m and 8 and 38 s, respectively. Short, shallow dives were usually V-shaped and dives deeper than 8 m and longer than 10 s were usually U-shaped, including a period at constant depth (varying between 4 and 28s with median 8s). Diving occurred throughout the daylight period and deepest dives were performed during late morning. On the basis of motion sensors in the loggers and food collections from telemetered birds, we concluded that extended, deep dives were directed at deep schools of capelin, a small pelagic fish, and we hypothesized that V-shaped dives were aimed at larger, pelagic fishes and squids. Furthermore, these V-shaped dives allowed the birds to surprise their pelagic prey and this may be critical because the maximum swimming speeds of the prey species may exceed the maximum dive speeds of the birds.

Migration patterns and moult of Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis using Teesmouth in late summer

Count, recovery and biometric data were analysed to assess the conservation status and use of Teesmouth by Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis in late summer. The recognition of the international importance of Teesmouth to migratory Sandwich Terns was endorsed. Peak late summer counts for 1990–1997 averaged 1,835, over 1% of the international population. Analysis of information from both Common and Sandwich Terns ringed or recovered in late summer at Teesmouth identified the existence of a trans‐Pennine migratory pathway of Common Terns between Teesmouth and Merseyside. Trans‐Pennine migration of Sandwich Terns is also suspected from visual observations of departures from Teesmouth. 55% of adult Common Terns using Teesmouth have yet to initiate primary moult. This, and the recovery patterns, are consistent with the involvement of a substantial Baltic contingent of non‐moulting adults amongst the late summer Teesmouth population. Of those adults in active wing moult, primary moult had progressed less far than for birds in the Wadden Sea at the same time of year. This suggests a cline through northern European breeding populations in the timing of moult. Visible migration, retrap data, arrival dates of different populations and the progression of primary moult, all identify rapid turnover amongst the late summer Common Tern population at Teesmouth, suggesting that over 1% of the UK breeding population, with their young, migrate through Teesmouth

Trends in the use of a small coastal lagoon by waterbirds: Muni Lagoon (Ghana)

This paper presents data from a twelve-year (1986–98) monitoring of waterbird populations at Muni Lagoon, and discusses the ornithological importance of the lagoon and trends in the use of the site by waterbirds. The small brackish lagoon forms part of the Muni-Pomadze Ramsar site situated on the Ghana coast. A total of 48 species of waterbirds were recorded at the site comprising of 29 species of waders, eight species of terns, two species of gulls, seven species of herons and egrets, one species each of duck and cormorant. The most abundant waterbirds species recorded on the site were the Curlew Sandpiper, Ringed Plover, Greenshank, Black-winged Stilt, Common Tern, Black Tern, Royal Tern and the Sandwich Tern. Three species of waders, Black-winged Stilt, Pratincole and Little Tern were observed to breed at the site. The highest numbers of waterbirds occurred on the site in the period September to March. The site remains an internationally important wetland site by virtue of supporting internationally important numbers of four species of terns: Royal Tern, Sandwich Tern, Common Tern and Black Tern. The value of Muni lies more in its use as a feeding/staging site for migrating waders that spend the non-breeding season in Ghana or pass through to winter further south, and as a roosting site for terns which feed mainly at sea. The use of the site by waterbirds has increased by some 400% between 1986 and 1998. This is attributed to improved prey availability resulting from the opening of the lagoon into the sea during the 1994 floods, and reduced disturbance of feeding and roosting birds. This would imply that the value of the site as waterbird habitat could improve considerably with appropriate habitat management interventions and continued protection.

The role of the Ghana coast in the annual cycle of migratory terns

Ntiamoa-Baidu, Y. & Nuoh, A. A. 2000. The role of the Ghana coast in the annual cycle of migratory terns. Ostrich 71 (1 & 2): 183. Data are presented on counts of migratory terns on the Ghana coast over a ten-year period, 1986–1996. Fourteen species of terns were recorded during the period. Influx of terns begins in late August, peaking in September/October, with the highest likelihood of tern occurrence on the Ghana coast recorded in August-November and April-May. Terns fed mostly offshore and used the coastal lagoons as roosting sites, although a few species e.g. Black Tern Chlidonias niger and Little Tern Sterna albifrons, foraged also on the lagoons. Two sites, Songor Lagoon and Densu Delta, regularly held over 50 % of the total count recorded for all sites. A comparison of the day-time counts with dusk roost counts indicate that the day time counts under-represent the total numbers of terns using the Ghana coast. Trends in the populations of the four most abundant tern species: Black Tern, Common Tern Sterna hirundo, Sandwich Tern S. sandvicensis and Royal Tern S. maxima, are analysed; and the implications for conservation of the species are discussed.

Keep the chicks moving: how Sandwich terns can minimize kleptoparasitism by black-headed gulls

Sandwich terns,Sterna sandvicensis, often nest in association with black-headed gulls,Larus ridibundus. The gulls provide protection against predators, but can also adversely affect the terns’ reproductive success through predation and piracy of fish. To test whether leading the chicks away from the nest site is an evasive strategy used by the parents to reduce the incidence of robbery by the gulls, we kept one group of Sandwich tern chicks at their original breeding site, while, with a wire-netting enclosure, we moved another group away from the gulls. The rate of kleptoparasitism was greatly reduced when the tern chicks were moved away from the original nest site, resulting in faster growth and earlier fledging. The rate of food parasitism and chick condition were affected only during the first 5 days of the experiment. After that, the rate of kleptoparasitism no longer differed between chicks that we moved away and those remaining in the colony. A second shift of the chicks again led to less kleptoparasitism and better chick condition. In line with these findings, the condition of free-living chicks that were lured away from their nesting site by their parents also improved. In particular, chicks initially in poor condition, which apparently suffered from high rates of kleptoparasitism, left the colony site. Free-living chicks are often lured away from the robbing gulls. However, not all subcolonies provided suitable escape routes and subsequently chicks in such subcolonies suffered from high mortality rates.

Ecological effects of dumping of dredged sediments; options for management

Dredging and dumping of dredged sediments in estuarine and coastal waters may lead to increased turbidity and enhanced sediment deposition at dump sites. This mainly affects primary production by phytoplankton, performance of visual predators (e.g. fish, birds), and growth and survival of benthic organisms. This paper combines a compilation of literature information and results of additional experimental studies on the effect of enhanced concentrations of suspended matter (SPM) on growth of bivalve molluscs, and on survival of macro- and meiozoobenthos after dumping of dredged sediments. Furthermore, it focuses on non-toxic dredged sediments only. Release of nutrients from dredged sediments did, so far, hardly influence estuarine phytoplankton production. Increased turbidity may affect dab as well as prey location by sandwich terns. Enhanced SPM-concentrations are unfavourable for young herring and smelt. Growth of filter-feeding bivalves may be impaired, especially at SPM-concentrations >250 mg/l. Estuarine nematodes can survive burial by 10 cm of dumped dredged sediment provided that its physical characteristics are similar to those of the original sediment. Sessile benthos organisms such as mussels and oysters can cope with sediment deposition of only 1–2 cm. Other macrozoobenthos can survive sediment deposition of 20–30 cm. Recovery of benthos at a dump site will occur if the interval between successive dumpings is sufficiently long. Options for management of dumping of dredged sediments are described, relating to different locations of dump sites in estuarine and coastal waters, to different seasons, and to the actual use (area and frequency) of dump sites.

Competition for Nest Sites between Kelp Gulls (Larus dominicanus) and Terns (Sterna maxima and S. eurygnatha) in Patagonia

Seabirds frequently nest in mixed-species colonies (Burger 1985). Several studies have shown differences in habitat and nest-site use among seabirds (Buckley and Buckley 1980, Croxall and Prince 1980, Fasola and Canova 1992). However, some degree of overlap in habitat and nest-site requirements may exist between species, resulting in interspecific competition that often leads to nest-site displacement or even the replacement of one species by another (Buckley and Buckley 1980, Burger 1985). Spatial interactions between seabirds can be more intense in areas where habitat is limited or when the population of one of the species is expanding (Burger and Shisler 1978, Trivelpiece and Volkman 1979, Duffy 1983, Ainley 1990). At many mixed colonies of larids, larger species often displace smaller species from their territories, occasionally forcing them to abandon the breeding area (Burger and Shisler 1978, Burger 1985). In addition, larger larids generally arrive at breeding sites earlier (Morris and Hunter 1976, Burger 1985), which gives them a competitive advantage over smaller species. Gulls (Larus spp.) frequently have been involved in the displacement of other species from breeding habitats (Thomas 1972, Burger 1979, Bradley and Monaghan 1986, Furness and Monaghan 1987). Terns appear to be one of the most affected seabirds, and some tern populations have decreased as a result of population expansion by gulls (Thomas 1972, Nisbet 1973, Greenhalgh 1974, Courtney and Blokpoel 1983, Parnell et al. 1988, Ainley and Hunt 1991, Blokpoel and Scharf 1991). At Punta Le6n, Argentina, Royal Terns (Sterna maxima) and Cayenne Terns (S. eurygnatha) breed intermingled in a dense cluster of nests within a Kelp Gull (Larus dominicanus) colony. The Kelp Gull colony increased in size from 3,200 pairs in 1982 to 6,500 in 1995, and gulls currently occupy almost all of the available nesting space (Yorio et al. 1994). Given the overlap in nest-site use and the current expansion of Kelp Gulls, we expected that competition for nesting space could result in negative effects on both species of terns. In this paper, we document spatial interactions between Kelp Gulls and Royal and Cayenne terns at Punta Le6n, Patagonia. We

The price of eggs: increased investment in egg production reduces the offspring rearing capacity of parents

Understanding the selective pressures shaping the number of offspring per breeding event is a key area in the study of life-history strategies. However, in species with parental care, costs incurred in offspring production, rather than rearing, have been largely ignored in both theoretical and empirical studies until relatively recently. Furthermore, the few experimental studies that have manipulated production costs have not yet teased apart effects that operate via the parental phenotype from effects on the quality of the resulting young. To examine whether increased egg production influences parental brood rearing capacity independently of effects operating via egg quality, we experimentally increased egg production in gulls and then examined their capacity to rear a control clutch. We found that the capacity of parents to rear the control brood was substantially reduced solely as a consequence of having themselves produced one extra egg. The paradox that, in many species, parents apparently aim for fewer young per breeding event than the experimentally and theoretically demonstrated optimum, has partly arisen from the failure to take into account the constraints imposed by production costs.