Do sex ratio and development differ in sexually size‐dimorphic shiny cowbirds (Molothrus bonariensis) parasitizing smaller and larger hosts?

Two possible patterns of bias in primary sex ratio have been proposed for size‐dimorphic brood parasites that do not evict host chicks: (1) larger males should be laid at greater frequency in hosts larger than the parasite because they compete better (increasing their survival) than females with large host nest‐mates, and (2) more costly males (i.e. the larger sex) should be laid at greater frequency in hosts smaller than the parasite because, in these hosts, parasite nestlings are provisioned at a higher rate and grow faster than in larger hosts. We tested these hypotheses in two hosts of the sexually size‐dimorphic shiny cowbird, Molothrus bonariensis, one smaller (house wren, Troglodytes aedon) and one larger (chalk‐browed mockingbird, Mimus saturninus) than the parasite. We measured: (1) sex ratio at laying; (2) development of sexual differences in body mass during the nestling stage; and (3) chick survival and sex ratio of chicks before fledging. In both hosts, we found sexual differences in body mass of nestlings from 7 days of age onwards, although we did not find a bias in the sex ratio of eggs laid and chicks fledged. The results of the present study do not support the hypothesis that shiny cowbird females benefit from biasing the primary sex ratio depending on the size of the hosts they parasitize. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 442–448.     

Shiny cowbird Molothrus bonariensis egg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size affects chick growth and survival and it is an important component of reproductive success. The shiny cowbird Molothrus bonariensis is an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that differ markedly in body size: the chalk‐browed mockingbird Mimus saturninus (70–75 g), and the house wren Troglodytes aedon (12–13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were five days of age, but there was no association between egg size and growth rate, or asymptotic mass. There were no sexual differences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Differences in egg size between hosts and the effect of egg size on body mass after hatching support the hypothesis that different females are specialized in the use of hosts that differ in body mass.     

Consequences of sex-specific growth on sibling competition in black-headed gulls: a sexually-size dimorphic species with scramble competition

Biased mortality of the larger sex during the early developmental period has been reported for a number of size-dimorphic bird species. This can partly be explained by the fact that growing to larger size renders the larger sex more vulnerable to food shortage. However, since sibling rivalry is often size-dependent, chicks of the larger sex should have a competitive advantage. This raises the question as to why the larger sex does not always benefit from its size in sibling competition. We studied sibling competition in the black-headed gull (Larus ridibundus), a sexually-size dimorphic species with male-biased mortality. We manipulated the natural brood sex ratio and placed one male chick in direct competition with one female chick while concurrently controlling for differences in age, size and laying order. Male chicks outgrew their female siblings by 15% in asymptotic body mass and did not suffer from enhanced mortality. Female chicks tended to be more alert when the parents returned to the nest and were more persistent in gull-typical begging displays. Females were more likely to get the first food item, but they did not get more food, possibly due to a size-mediated dominance over the non-monopolizable regurgitated food. Thus, it is unlikely that sex differences in competitiveness significantly contribute to male-biased mortality in black-headed gulls. The previously reported male-biased mortality is more likely due to a disadvantage of a higher food demand and a higher sensitivity towards low egg quality, as has been shown in previous studies.     

Diversionary feeding and nestling diet of Hen Harriers Circus cyaneus

Capsule: Diversionary feeding reduced Hen Harrier Circus cyaneus nestlings’ natural food intake by half. Red Grouse Lagopus lagopus scotica chicks constituted 0–4% of all nestling food items. Annually, this reduced annual grouse chick production by 0–6%.

Aim: To quantify proportions of diversionary and natural food (including grouse) delivered to Hen Harrier nestlings in relation to brood size, male status and natural prey abundance.

Methods: We recorded diversionary food provisioned to 25 Hen Harrier broods (2008–15) and studied the diet of 15 broods using observations from hides, nest cameras and regurgitated pellet analysis. Variation in nestling diet was analysed using compositional analysis.

Results: Hen Harriers took 76% of diversionary food provided. Depending on assessment method, average nestling diet was 44–53% diversionary food, 39–55% natural prey (including 24–45% passerines, 4–15% small mammals, 0–4% grouse chicks) and 0–9% unknown items. The amount of diversionary food consumed was not influenced by male status, brood size or natural prey abundance. The number of Red Grouse chicks delivered annually was 34–100% lower than expected under unfed conditions, however, the confidence intervals associated with these estimates were large.

Conclusion: Diversionary food influenced Hen Harrier nestling diet and reduced the number of Red Grouse chicks taken relative to modelled predictions. It may help reduce conflict between Hen Harrier conservation and Red Grouse shooting, but only if overall grouse productivity is thereby maintained or increased.     

Effects of radiotagging on the weight gain and survival of Curlew Numenius arquata chicks

CapsuleNo detrimental effects on weight gain or survival to fledging stage were found.

Aims To assess the effects of radiotagging on the weight gain and survival of Curlew chicks.

Methods Small (≤ 1.5 g) radiotags were attached to Curlew chicks at hatching, during studies in four different areas of the UK. Tagged chicks were recaptured regularly and their weight gain and survival compared to non-tagged chicks using within and between-brood comparisons.

Results Radiotagging did not affect weight gain. While there was no evidence of reduced survival of tagged chicks, comparisons were based on small samples.

Conclusions Small radiotags can be invaluable in determining the survival and causes of mortality of the chicks of waders and other precocial birds.     

Reproductive measures and chick provisioning of Cory's Shearwater Calonectris diomedea borealis in the Azores

Capsule Timing of breeding influenced wing-length at fledging, and egg size may be an indicator of fledging weight and the amount of food received by chicks.

Aims To investigate chick growth, temporal patterns of chick food provisioning and the importance of indices of parental condition or quality, egg size and hatching date, to predict nestling body mass and wing-length at fledging, and compare breeding and chick feeding characteristics between colonies in the northeast Atlantic.

Methods A survey of Cory's Shearwater nests was carried out at Vila islet. A sample of 52 chicks, ringed and weighed at hatching, was selected to study chick growth and food provisioning.

Results Hatching success (51%) was much lower than fledging success (87%). Both hatching date and egg size contributed to explain wing-length at fledging, but hatching date, which was negatively correlated with wing-length at fledging, had the most important contribution (22%). There was some indication that egg size may explain variation in fledging weight and the amount of food received by chicks. Food delivery and feeding frequency of chicks varied throughout the chick development stage and three phases were distinguished: (1) 0–29 days, the highest feeding frequency values and a linear increase in food delivery; (2) 30–69 days, an oscillation in food delivery and medium feeding frequencies; (3) 70–90+ days, a sharp decrease in both food delivery and feeding frequency.

Conclusion Variation in food availability did not seem sufficient to override the overall importance of indices of parental quality in determining reproductive measures and chick provisioning. Breeding and feeding characteristics were similar between colonies in the northeast Atlantic, with variability in chick provisioning higher further south.     

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

Capsule: 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.     

Identification and breeding biology of the diving petrels Pelecanoides georgicus and P. urinatrix exsul at South Georgia

Abstract

The biology of two species of diving petrel (Pelecanoides georgicus and P. urinatrix exsul) was studied at Bird Island, South Georgia. Existing criteria (bill shape and morphology, wing length) for distinguishing these species are reviewed, and several new characters are recognised. For adults bill depth, the colour of the posterior part of the tarsus, and vocalisations are distinctive; chicks have down of different colours (pale grey in P. u. exsul, dark grey in P. georgicus). At South Georgia the species breed in different habitats and at different times - P. u. exsul nests in steep tussock slopes in very early summer, P. georgicus in fine scree slopes in midsummer. P. georgicus lays a proportionately larger egg, and has an incubation period of 46 days (c.54 days in P. u. exsul) and a chick fledging period of 46 days. The fledging period of P. u. exsul is 54 days, very similar to recorded values for P. u. urinatrix (53.5 days) and P. u. chathamensis (56 days). Data on feeding frequency and feed size were derived from daily chick weighings and from twice-daily weighing during 30 days preceding fledging. In both species chicks are fed every night, and often by both parents. In P. georgicus true mean chick feed size is c.37 g; in P. u. exsul it may be slightly less. Analysis of chick stomach contents suggests that P. u. exsul feeds extensively on copepods, whereas P. georgicus largely takes krill (Euphausia superba). P. u. exsul breeding adults commence moulting before their chicks have fledged; P. georgicus moults exclusively in the non-breeding season. Ectoparasites were collected, and the feather louse Pelmatocerandra setosa was found to be restricted to P. u. exsul, and Pelmatocerandra enderleini to P. georgicus. P. georgicus, which breeds later and whose chicks fledge faster, is suggested to be better adapted to the climatic and marine environmental conditions than P. u. exsul, which may be the more recent colonist of these high latitudes.     

Providing chicks with extra food lowers male but not female provisioning in the House Sparrow Passer domesticus

We assessed whether adult House Sparrows Passer domesticus adjusted their provisioning in response to an experimental increase in the nutritional condition of their nestlings. When we supplemented chicks directly with additional food, male parents, but not female parents, reduced their provisioning. The results for males, but not females, run contrary to a previous experiment in this species. In addition, female provisioning was positively associated with both brood size and the age of the brood. In contrast, whereas male provisioning was positively associated with brood size, males did not increase provisioning as their chicks grew older. Males, but not females, exhibited repeatability in their provisioning. Food supplementation had a larger positive effect upon nestling survival in smaller broods than in larger broods. Overall, there appear to be fundamental differences between males and females in how decisions regarding the level of parental investment in the current brood are made.     

Testing the relationship between clutch size and brood size in the Coot (Fulica atra)

The time between egg laying and chick fledging is of crucial importance for the survival of young birds. I analyzed breeding output at consecutive phases of growth of young Coots (Fulica atra) relative to the clutch size and laying date. Considering the specific breeding biology of the Coot, I tested whether chick survival reveals clutch size-dependent variability. Clutch size did not affect hatching success; it only affected brood size, and that merely temporarily. During the first 20 days after hatching, i.e. during the time of the highest chick mortality, birds with larger clutches lost chicks at a higher rate. As a result, the number of fledged chicks was independent of the initial number of chicks, and pairs with different clutch sizes had a similar number of fledglings. The laying date had no effect. This pattern of age-related chick survival points to the greater role of the type of chick growth (semi-precocial) and behavior in their survival.     

Sex‐specific growth in chicks of the sexually dimorphic Black‐tailed Godwit

Sexual size dimorphism (SSD) is common in birds and has been linked to various selective forces. Nevertheless, the question of how and when the sexes start to differentiate from each other is poorly studied. This is a critical knowledge gap, as sex differences in growth may cause different responses to similar ecological conditions. In this study, we describe the sex‐specific growth – based on body mass and five morphometric measurements – of 56 captive Black‐tailed Godwit Limosa limosa limosa chicks raised under ad libitum food conditions, and conclude that all six growth curves are sex‐specific. Females are the larger sex in terms of body mass and skeletal body size. To test whether sex‐specific growth leads to sex‐specific susceptibility to environmental conditions, we compared the age‐specific sizes of male and female chicks in the wild with those of Black‐tailed Godwits reared in captivity. We then tested for a relationship between residual growth and relative hatching date, age, sex and habitat type in which the wild chicks were born. Early‐hatched chicks were relatively bigger and in better condition than late‐hatched chicks, but body condition and size were not affected by natal habitat type. Female chicks deviated more negatively from the sex‐specific growth curves than male chicks for body mass and total‐head length. This suggests that the growth of the larger females is more susceptible to limiting environmental conditions. On average, the deviations of wild chicks from the predicted growth curves were negative for all measurements, which suggests that conditions are limiting in the current agricultural landscape. We argue that in estimating growth curves for sexually dimorphic species, it is critical first to make accurate sex and age determinations.     

Golden Plover Pluvialis apricaria breeding success on a moor managed for shooting Red Grouse Lagopus lagopus

Capsule The best estimate of breeding success was a mean of 0.57 fledglings per pair, which when combined with adult survival rates, successfully explained the observed population trend.

Aims To quantify Golden Plover breeding success on a moor managed for shooting Red Grouse Lagopus lagopus.

Methods An intensive study recorded the fate of individual Golden Plover nests and, using radiotelemetry, chicks. The factors associated with mortality were examined, allowing the construction of a model of breeding success. Adult survival was estimated from return rates of colour-ringed birds.

Results Estimated rates of daily nest survival during laying (0.8636) were significantly lower than during incubation (0.9913). The daily survival rate of chicks less than nine days (0.8868) was significantly lower than for older chicks (0.9792). A population model based on these parameters overestimated the rate of nest losses, but accurately described brood survival and fledging success. Although predation rates were low, poor survival of young chicks through starvation or exposure suggest other factors were limiting breeding success at the study site.

Conclusions Predation rates of Golden Plover nests and chicks can be low on moorlands managed for shooting Red Grouse. However, in the absence of predation, other factors may still reduce chick survival and limit breeding success.     

Effects of geolocation archival tags on reproduction and adult body mass of sooty shearwaters (Puffinus griseus)

Abstract

We attached 11 g (1.4% body‐mass equivalent) global location sensing (GLS) archival tag packages to tarsi of 25 breeding sooty shearwaters (Puffinus griseus, titi) on Whenua Hou (Codfish Island), New Zealand during the chick‐rearing period in 2005. Compared with chicks reared by non‐handled adults that did not carry tags, deployment of tags on one or both adult parents ultimately resulted in 35% reduction in chick body mass and significantly reduced chick skeletal size preceding fledging (19 April). However, body mass between chick groups was not significantly different after controlling for skeletal size. Effects on chicks were more pronounced in six pairs where both parents carried tags. Chick mass was negatively related to the duration that adults carried tags. In this study, none of the chicks reared by pairs where both parents were tagged, 54% of chicks reared by pairs where one parent was tagged, and 83% of chicks reared by non‐handled and non‐tagged parents achieved a previously determined pre‐fiedging mass threshold (564 g; Sagar & Horning 1998). Body mass of adults carrying tags and returning from trans‐equatorial migration the following year were 4% lighter on average than non‐tagged birds, but this difference was not statistically significant. Reduced mass among chicks reared by adults carrying tags during the chick‐provisioning period indicated that adults altered “normal” provisioning behaviours to maintain their own body condition at the expense of their chicks. Population‐level information derived from telemetry studies can reveal important habitat‐linked behaviours, unique aspects of sea‐bird foraging behaviours, and migration ecology. Information for some species (e.g., overlap with fisheries) can aid conservation and marine ecosystem management. We advise caution, however, when interpreting certain data related to adult provisioning behaviours (e.g., time spent foraging, provisioning rates, etc.). If effects on individuals are of concern, we suggest shorter‐term deployments, smaller and lighter tags, and alternative attachment techniques, especially when investigating threatened or endangered species.     

Chick starvation in yellow‐eyed penguins: Evidence for poor diet quality and selective provisioning of chicks from conventional diet analysis and stable isotopes

The link between poor reproductive success and diet was investigated in yellow‐eyed penguins Megadyptes antipodes, by assessing diet at two localities separated by about 30 km: the north coast of Stewart Island where breeding success is low (0.38–0.67 chicks per pair in recent years), and Codfish Island where breeding success is higher (0.96–1.51 chicks per pair), and relating this to published data from South Island localities, where average breeding success was 1.1 chicks per pair. Diet composition, meal sizes and energetic content of meals and prey were determined from stomach contents, and stable isotope analyses of chick down, fledgling feathers and adult blood provided information on diet throughout the fledging period. The high proportion of stomachs that were empty or lacked diagnostic remains reduced sample size considerably, and variability between samples reduced the power to detect significant differences in meal size, proportions of empty stomachs and prey diversity of meals. Energetic content of Stewart Island meals was less than Codfish Island meals, and there was a non‐significant trend for smaller meal sizes and reduced prey diversity among Stewart Island samples. Both localities had lower prey diversity and smaller meals than South Island penguins. Blue cod Parapercis colias accounted for 99% of prey biomass in Stewart Island and 70% in Codfish Island stomach samples, where 27% of prey biomass was opalfish Hemerocoetes monopterygius. Isotopic mixing models carried out on larger sample sizes indicated that opalfish comprised a large proportion of the diet at both locations, with adults selectively provisioning chicks with opalfish while feeding mainly on blue cod themselves. We suggest the large blue cod consumed by Codfish Island and Stewart Island penguins, larger than those consumed by South Island penguins, is difficult to transfer to chicks by regurgitation. Oyster dredging around Stewart Island may have reduced the availability and abundance of alternative prey to Stewart Island penguins.     

Parental behaviour of Spanish Imperial Eagles Aquila adalberti: sexual differences in a moderately dimorphic raptor

Capsule The sexes make significantly different contributions in the tasks of nest-building, incubation and food provisioning to chicks.

Aims To determine the division of parental activities during breeding (nest-building, incubation, brooding, food provisioning and feeding).

Methods Between 1991 and 1998 focal observations were made at nests using 20–60× telescopes. During the incubation period, 11 pairs were studied involving a total of 2812 observation hours over 269 days. During the chick-rearing period, seven pairs were studied involving a total of 5499 observation hours spread over 503 days. The birds were sexed and identified individually on the basis of size, position during copulation, vocalization and plumage differences.

Results Significant differences were found between the sexes in parental behaviour. Males scarcely participated in incubation and contributed significantly more than the females only in food provisioning, both during incubation (food transfers to feed the female) and during chick-rearing (to feed the chicks). Females were responsible for most of the incubation, nest-building (during incubation and chick-rearing), brooding, shading and feeding of chicks. Nest attendance by females decreased over time. In both sexes, food provisioning increased with brood size.

Conclusion Intersexual differences are discussed in the context of the reversed sexual size dimorphism and parental investment strategies. We suggest that in Spanish Imperial Eagles reversed sexual size dimorphism is best explained by the prey capture difficulty hypothesis, than by parental role division during reproduction.     

Winter territoriality and its implications for the breeding ecology of White-throated Dippers Cinclus cinclus

Capsule: Pairs of White-throated Dippers Cinclus cinclus which defended winter territories bred earlier than non-territorial individuals, but there was no difference in reproductive success.

Aims: The effect of winter territoriality on breeding ecology has rarely been studied in resident birds. We carried out a preliminary investigation of whether winter territorial behaviour and territory size affect the timing of reproduction, breeding territory size and reproductive success in a riverine bird, the White-throated Dipper.

Methods: We monitored an individually marked population of White-throated Dippers in the UK. Wintering individuals were classified as either territorial or ‘floaters’ according to their patterns of occurrence and behaviour, and their nesting attempts were closely monitored in the subsequent months. Winter and breeding territory sizes were measured by gently ‘pushing’ birds along the river and recording the point at which they turned back.

Results: All birds defending winter territories did so in pairs, but some individuals changed partners before breeding. Territorial pairs that were together throughout the study laid eggs significantly earlier than pairs containing floaters and those comprising territorial birds that changed partners. However, there were no significant differences in clutch size, nestling mass or the number of chicks fledged. There was no relationship between winter territory length and lay date or any measure of reproductive success, although sample sizes were small. Winter territories were found to be significantly shorter than breeding territories.

Conclusion: Winter territoriality may be advantageous because breeding earlier increases the likelihood that pairs will raise a second brood, but further study is needed. Territories are shorter in winter as altitudinal migrants from upland streams increase population density on rivers, but this may also reflect seasonal changes in nutritional and energetic demands.     

Costs and benefits of colony size vary during the breeding cycle in Black-headed Gulls Chroicocephalus ridibundus

We studied the effects of colony size on individual reproductive success in a multi-site population of Black-headed Gulls Chroicocephalus ridibundus where colony size ranged from 10 to 5,000 pairs. By focusing on family size, the number of chicks attended by individually marked parents, and accounting for between-individual variation, we detected a negative colony-size effect during the very first days of life of the chicks that was compensated by a subsequent increase in the proportion of surviving chicks with colony size. We suggest that this result originates in the interplay between overcrowding costs acting on hatching success, and benefits of colonial breeding, most probably more efficient food-searching (foraging enhancement), acting on chick survival. However, the frequency of complete colony failure increased with decreasing colony size. Taking this hazard risk into account yielded a corrected estimate of the effect of colony size on breeding success, and indicated that the largest colonies were the most productive. This pattern is congruent with the previous finding that larger colonies are more attractive to dispersing breeders.     

Environmental correlates of lake occupancy and chick survival of Black-throated Divers Gavia arctica in Scotland

Capsule Lake occupancy was related to the abundance of fish prey for adults (salmonids) and chick survival was related to type of diet, prey abundance and weather.

Aims To identify the factors determining lake occupancy and chick survival of Black-throated Divers breeding in Scotland.

Methods Measures of water chemistry, fish, invertebrates and physical characteristics were made for 90 lakes where occupancy had been monitored in at least five years between 1984 and 1995. Data on chick survival were available over this period for a smaller sample of lakes.

Results High occupancy rates were associated with a high abundance of small salmonids (the birds' principal prey) and complex shorelines. The mean number of chicks fledged per nest that hatched was 0.95 at lakes where Three-spined Sticklebacks Gasterosteus aculeatus and/or Minnows Phoxinus phoxinus were present; the mean number of chicks fledged was 0.64 where they were absent (and chicks are fed invertebrates). Chick survival on lakes with sticklebacks/Minnows was positively and significantly related to June temperature, and the abundance of these fish. At lakes lacking these small fish, chick survival was inversely related to sunshine duration in June. It is suggested that sunny weather in June can accelerate the emergence of aquatic insect larvae (especially Ephemeroptera) and lead to food shortage for small diver chicks.

Conclusions It is likely that there are sufficient suitable lakes to accommodate a moderate expansion of the Scottish Black-throated Diver population. An abundance of suitable fish prey, especially small-sized (<230 mm) salmonids, is of fundamental importance for diver conservation.     

Feeding frequency influences crèching age in the Dalmatian pelican, Pelecanus crispus

Crèching behaviour is common in colonial seabirds; nevertheless, the factors inducing chicks to aggregate remain relatively poorly understood. It has been proposed that brood size, laying date and nest attendance are important factors in the formation of a crèche. Moreover, in most species of pelicans, chicks join crèches following the development of homoeothermy and coincident with the end of the brooding behaviour. We studied effects of feeding rate, nest attendance, brood size, laying date and homoeothermy on the age at which chicks entered the crèche at a colony of Dalmatian pelicans (Pelecanus crispus), in Srebarna, Bulgaria. Single chicks were fed more frequently than chicks from two-chick broods. Unlike American white pelicans (Pelecanus erythrorhynchos), Dalmatian pelicans maintained brooding behaviour a further 9 days after chicks had developed thermoregulation abilities. In contrast to nests with two chicks, nests with only one chick were never left unattended by the parents before the chick reached the crèching stage. Laying date, nest attendance and brood size did not affect the age that the chick entered the crèche. The age the chick entered the crèche was not correlated with the age of homoeothermy acquisition, but chicks significantly joined the crèche at younger ages when the mean number of feeds per chick per day during the rearing period in the nest was higher. This result suggests an implication of growth rate in the crèching age. Joining the crèche earlier can provide benefits that could have strong implications for the chicks’ future reproductive lives.     

Effect of intraspecific interactions on seasonal decline in productivity of Common Terns Sterna hirundo

Capsule Kleptoparasitic activities of older chicks from earlier nests did not contribute to late reproductive declines.

Aims To determine whether intraspecific interactions, such as kleptoparasitism and aggression, were experienced more frequently by birds breeding late in the season as a result of exposure to breeders at a more advanced stage. If so, to investigate whether this was the cause of the observed seasonal decline in reproductive parameters observed at Bird Island, where nesting density is high and interactions are more probable.

Methods Plots were fenced within the colony, exploiting natural variability in distribution of early and peak breeders to create two treatments: plots with only late-laying terns and those with a mixture of early-, peak- and late-layers. Hatching success, productivity and the growth and survival of chicks were measured for all late-laying pairs. Intraspecific interactions, adult attendance and provisioning of chicks were recorded during 9600 minutes of nest observations made within two periods: a few days after hatching and one week later.

Results The frequency of intraspecific interactions was maintained by the kleptoparasitic activities of older chicks within the mixed-laying-date treatment and was significantly lower in plots containing only late breeders with chicks of similar ages (mean 11.0 days). The overall rate was rarely greater than two interactions per nest per hour and there was no corresponding reduction in the growth or survival of chicks from late nests or any change in the provisioning activities of late-breeding adults.

Conclusion Increased frequency of intraspecific interactions experienced by late breeders in the presence of early-breeding conspecifics resulted from the kleptoparasitic activities of older chicks but was not sufficient to contribute to the observed seasonal reproductive decline at this dense breeding colony.