Energetic consequences of contrasting winter migratory strategies in a sympatric Arctic seabird duet

At the onset of winter, warm‐blooded animals inhabiting seasonal environments may remain resident and face poorer climatic conditions, or migrate towards more favourable habitats. While the origins and evolution of migratory choices have been extensively studied, their consequences on avian energy balance and winter survival are poorly understood, especially in species difficult to observe such as seabirds. Using miniaturized geolocators, time‐depth recorders and a mechanistic model, we investigated the migratory strategies, the activity levels and the energy expenditure of the closely‐related, sympatrically breeding Brünnich's guillemots Uria lomvia and common guillemots Uria aalge from Bjørnøya, Svalbard. The two guillemot species from this region present contrasting migratory strategies and wintering quarters: Brünnich's guillemots migrate across the North Atlantic to overwinter off southeast Greenland and Faroe Islands, while common guillemots remain resident in the Barents, the Norwegian and the White Seas. Results show that both species display a marked behavioural plasticity to respond to environmental constraint, notably modulating their foraging effort and diving behaviour. Nevertheless, we provide evidence that the migratory strategy adopted by guillemots can have important consequences for their energy balance. Overall energy expenditure estimated for the non‐breeding season is relatively similar between both species, suggesting that both southward migration and high‐arctic winter residency are energetically equivalent and suitable strategies. However, we also demonstrate that the migratory strategy adopted by Brünnich's guillemots allows them to have reduced daily energy expenditures during the challenging winter period. We therefore speculate that ‘resident’ common guillemots are more vulnerable than ‘migrating’ Brünnich's guillemots to harsh winter environmental conditions.     

Comprehensive and Comparative Analysis of the Helminth Fauna of the Dominant Colonial Seabird Species on the Murman Coast

This work reports the results of the long-term study of the helminth fauna of the dominant colonial seabird species (common guillemots, Brünnich’s guillemots, kittiwakes, great black-backed gulls, and herring gulls) in the western, central, and eastern parts of the Murman coast. The presence of 52 parasitic worm species (18 trematode species, 21 cestode species, 11 nematode species, and 2 acanthocephalan species) was demonstrated. In the western, central, and eastern regions, the respective numbers of 32, 43, and 34 helminth species were registered. Twenty species of seabird parasites were found to be ubiquitous along the Murman coastal zone. Local foci of the trematode Maritrema arenaria and cestode Laricanthus lateralis infestation along the western part of the Murmansk coast were revealed. The highest helminth species diversity (38 species) was demonstrated for the herring gull; the lowest (7 species), for the common guillemot. Four parasite species were common for the helminth fauna of all bird species. A significant number of helminth infections were mixed ones (complexes of two or three bird species with the common helminth species in different combinations). The highest similarity of the helminth fauna was revealed in the pairs of the common guillemot and the Brünnich’s guillemot and of the great black-backed gull and the herring gull. A few parasitic worm species detected only in a single bird species were registered. The particularities and the possible causes of the geographical distribution and specificity of helminthes were analyzed, and a comprehensive assessment of the parasitological situation in the Murman coastal zone was performed.     

Population differentiation and evolution in the common guillemot Uria aalge

Common (Uria aalge) and Brünnich's guillemots (U. Iomvia) are colonial seabirds that nest in temperate to arctic oceans throughout the Northern hemisphere. They are very similar in the characteristics of ecology, demography and life history that are thought to determine the extent of differentiation among populations, yet geographic variation in morphology is notably greater in common guillemots. Despite evidence of strong natal philopatry, previous analyses of allozymes and the mitochondrial cytochrome b gene revealed little genetic differentiation among North Atlantic colonies of Brünnich's guillemots. To determine if the more extensive morphological variability in common guillemots reflects greater genetic variability, we sequenced part of the cytochrome b gene for 160 common guillemots from 10 colonies distributed throughout the Northern hemisphere. Genotype frequencies and phylogenetic relationships among genotypes both indicated that Atlantic and Pacific populations are genetically distinct. Genetic divergence among genotypes suggested that differentiation of these populations has resulted from separation by Pleistocene glaciers and the Bering Landbridge, as well as by currently unsuitable breeding habitat in the Arctic Ocean. Cytochrome b genotype frequencies also differed among Atlantic colonies, and appeared to define a cline similar to that described for morphological characters. Analyses of sequence variation suggested that this variation probably results from secondary contact between two refugial populations from the Pleistocene glaciations, rather than from isolation by distance or selection. In contrast, the Atlantic population of Brünnich's guillemots appears to have arisen through recent expansion of a single homogeneous refugial population.     

The importance of prey aggregations to the distribution of Brünnich's guillemots in Storfjorden,Svalbard

 We studied the influence of the distribution of prey and hydrographic fronts on the spatial distribution of foraging Brünnich’s guillemots (Uria lomvia) in Storfjorden, southeastern Svalbard in late July 1992. Two large breeding colonies, comprising a total of 540,000 individuals, were located adjacent to the study area, and large numbers of Brünnich’s gullemots from these colonies foraged within the area, as well as to the south, outside of Storfjorden. Within the study area, most guillemots foraged on the west side of the fjord, coincident with a weak subsurface front between warm Atlantic water, which penetrated Storfjorden from the south, and cold Arctic water. Food samples from the guillemots collected in the study area contained primarily crustaceans (Parathemisto spp. and Thysanoessa inermis) and polar cod Boreogadus saida. Acoustic observations of prey were differentiated into two classes of signals, which we interpreted as originating from aggregated and dispersed organisms. The numbers of foraging guillemots were strongly correlated with the strength of echoes of the aggregated type, whereas correlations with dispersed echoes were consistently weaker. The distribution of foraging guillemots showed no significant correlations with either horizontal or vertical gradients of physical properties of the water column. Our finding that guillemots respond differently to aggregated and dispersed prey has important implications both for the interpretation of past work on the foraging ecology of marine birds, and for the management of fisheries. Received: 9 June 1995/Accepted: 5 January 1996     

An important late summer aggregation of fin whales Balaenoptera physalus,little auks Alle alle and Brünnich’s guillemots Uria lomvia in the eastern Greenland Sea and Fram Strait: influence of hydrographic structures

The distribution at sea of upper trophic levels—seabirds and marine mammals—is depending on their food availability: high concentrations reflect high prey abundance and thus high biological production. Polar marine ecosystems are characterized by low biodiversity and high biological patchiness. The distribution of predators, as a consequence, shows a similar patchiness. During two expeditions of icebreaking RV Polarstern in June–July 2011, biodiversity in the arctic marine zone north of 70°N was very low, with low numbers of species: 20 seabirds, eight cetaceans, five pinnipeds and polar bear. Moreover, a few species accounted for the majority in numbers: four bird species for 95 % of the total of 23,000 seabirds recorded during 700 transect counts: fulmar Fulmarus glacialis, kittiwake Rissa tridactyla, Brünnich’s guillemot Uria lomvia and little auk Alle alle. Among the marine mammals, 250 fin whales Balaenoptera physalus accounted for 80 % of the identified large cetaceans, 270 white-beaked dolphin Lagenorhynchus albirostris for 100 % of the small cetaceans and 180 harp seals Pagophilus groenlandica for 80 % of the identified pinnipeds. Their quantitative distribution was depending on water masses and oceanic fronts, large cetaceans—mainly fin whales—showing an important aggregation on the shelf slope off western Spitsbergen, as well as little auks and Brünnich’s guillemots. So that this zone, shelf slope and front of mixed Arctic/Atlantic Waters, showed unusually high seabird and cetacean concentrations. Seasonal factors possibly influencing their distribution are addressed.     

Arctic seabird food chains explored by fatty acid composition and stable isotopes in Kongsfjorden, Svalbard

Marine birds are important predators in the marine ecosystem, and dietary studies can give useful information about their feeding ecology, food webs and oceanographic variability. The aim of this study was to increase our understanding of the diet and trophic level of the seabirds breeding in Kongsfjorden, Svalbard. We have used fatty acids and stable isotopes, both of which integrate diet information over space and time, to determine trophic relationships in marine food webs. Fatty acid compositions of muscle from Little auk (Alle alle), Brünnich’s guillemot (Uria lomvia), Black-legged kittiwake (Rissa tridactyla), Northern fulmar (Fulmarus glacialis) and Glaucous gull (Larus hyperboreus) were determined and compared with their prey species. Canonical analysis (CA) showed that fatty acid composition differed among the five seabird species. Little auk, Black-legged kittiwake and Northern fulmar had high levels of the Calanus markers 20:1n9 and 22:1, indicating that these seabirds are a part of the Calanus food chain. Brünnich’s guillemot differed from the other species with much lower levels of 20:1n9 and 22:1. Brünnich’s guillemot is a pursuit diver feeding on fish and amphipods deeper in the water column, below 30 m. Glaucous gull also differed from the other seabird species, with a larger variation in the fatty acid composition indicating a more diverse diet. Trophic level analysis placed Little auk at the lowest trophic level, Brünnich’s guillemot and Black-legged kittiwake at intermediate levels and Glaucous gull and Northern fulmar at the highest trophic level.     

Climate-Driven Ichthyoplankton Drift Model Predicts Growth of Top Predator Young

Climate variability influences seabird population dynamics in several ways including access to prey near colonies during the critical chick-rearing period. This study addresses breeding success in a Barents Sea colony of common guillemots Uria aalge where trophic conditions vary according to changes in the northward transport of warm Atlantic Water. A drift model was used to simulate interannual variations in transport of cod Gadus morhua larvae along the Norwegian coast towards their nursery grounds in the Barents Sea. The results showed that the arrival of cod larvae from southern spawning grounds had a major effect on the size of common guillemot chicks at fledging. Furthermore, the fraction of larvae from the south was positively correlated to the inflow of Atlantic Water into the Barents Sea thus clearly demonstrating the mechanisms by which climate-driven bottom-up processes influence interannual variations in reproductive success in a marine top predator.     

Genetic diversity and population history of two related seabird species based on mitochondrial DNA control region sequences

Geographical variation in two related seabird species, the razorbill (Alca torda) and common guillemot (Uria aalge), was investigated using sequence analysis of mitochondrial DNA (mtDNA) control regions. We determined the nucleotide sequence of the variable 5' segment of the control region in razorbills and common guillemots from breeding colonies across the Atlantic Ocean. The ecology and life history characteristics of razorbill and common guillemot are in many respects similar. They are both considered highly philopatric and have largely overlapping distributions in temperate and subarctic regions of the North Atlantic, yet the species were found to differ widely in the extent and spatial distribution of mtDNA variation. Moreover, the differences in genetic differentiation and diversity were in the opposite direction to that expected from a consideration of traditional classifications and current population sizes. Indices of genetic diversity were highest in razorbill and varied among colonies, as did genotype frequencies, suggestive of restrictions to gene flow. The distribution of genetic variation suggests that razorbills originated from a refugial population in the south-western Atlantic Ocean through sequential founder events and subsequent expansion in the east and north. In common guillemots, genetic diversity was low and there was a lack of geographical structure, consistent with a recent population bottleneck, expansion and gene flow. We suggest that the reduced level of genetic diversity and differentiation in the common guillemot is caused by an inherent propensity for repeated population bottlenecks and concomitantly unstable population structure related to their specialized feeding ecology.     

Seabird species associations and affinities to areas covered with sea ice in the northern Greenland and Barents Seas

Species associations and affinity to sea ice among arctic marine birds were studied during ship transects in the northern Greenland and Barents Seas in the period 1980–1984. Associations were investigated at the scale of visual contact, and the sampling units were 10-min periods, corresponding to a transect length of 1.5–3 km. In the Greenland Sea, three or four of the most abundant species, fulmar (Fulmarus glacialis), little auk (Alle alle), Brünnich's guillemot (Uria lomvia) and kittiwake Rissa tridactyla, composed a recurrent group in all years. In the Barents Sea, fulmars, Brünnich's guillemots and kittiwakes were most often clustered. A positive association with sea ice was found in more than one cruise in three seabird species: black guillemots, ivory gulls and little auks, whereas seven other species showed negative association with ice in more than one cruise. The observed species associations and affinities to sea ice reflect similarities in diet among the species involved. Received: 23 February 1994 / Accepted: 12 January 1997     

Seabirds and marine mammals in the eastern Barents Sea: late summer at-sea distribution and calculated food intake

The at-sea distribution of seabirds and marine mammals in the eastern Barents Sea was determined using standardized transect counts during three cruises of RV“Dalnie Zelentsy” (Murmansk) in late summer 1991, 1992 and 1993. Totals of 32,268 seabirds, 485 pinnipeds, 25 cetaceans and 4 polar bears were counted during 554 half-hour counts. Numbers were converted into densities, total biomass and calculated daily food intake. Mean total food intake in kg fresh weight/km².day was 3.1 for the entire zone and all years; fish eaters dominated the whole region, with an intake of 1.3 (mainly Brünnich’s guillemot, Uria lomvia, and harp seal, Phoca groenlandica), followed by zooplankton eaters (0.85, mainly fulmar, Fulmarus glacialis) and mixed zooplankton and fish feeders (0.75, mainly minke whale, Balaenoptera acutorostrata, and kittiwake, Rissa tridactyla). Year-to-year variations were of little importance, while geographic differences were obvious between Norwegian coastal, Atlantic and Barents Sea water masses, both quantitatively and qualitatively (relative importance of main diets). Within each zone, a strong geographic heterogeneity was noted, with high local concentrations at fronts between water masses and at ice edges.     

Breeding synchrony and predator specialization: A test of the predator swamping hypothesis in seabirds

Reproductive synchrony is a widespread phenomenon that is predicted to be adaptive for prey with specialist predators but not for those with generalist ones. I tested this prediction in three polar seabird species characterized by different levels of predator specialization. In the Antarctic petrel, for which the only predator was highly specialized, hatching dates were highly synchronous and chicks that hatched close to the mean hatching date had a higher survival. In black‐legged kittiwakes and Brünnich's guillemots, whose predators were generalists, breeding was less synchronous and there was no fitness advantage in hatching close to the mean. This study emphasizes the potential importance of the relative timing of reproduction for individual fitness and supports the hypothesis that the adaptive value of breeding synchrony depends on the predator functional response.     

Investigating the effects of age‐related spatial structuring on the transmission of a tick‐borne virus in a colonially breeding host

Higher pathogen and parasite transmission is considered a universal cost of colonial breeding due to the physical proximity of colony members. However, this has rarely been tested in natural colonies, which are structured entities, whose members interact with a subset of individuals and differ in their infection histories. We use a population of common guillemots, Uria aalge, infected by a tick‐borne virus, Great Island virus, to explore how age‐related spatial structuring can influence the infection costs borne by different members of a breeding colony. Previous work has shown that the per‐susceptible risk of infection (force of infection) is different for prebreeding (immature) and breeding (adult) guillemots which occupy different areas of the colony. We developed a mathematical model which showed that this difference in infection risk can only be maintained if mixing between these age groups is low. To estimate mixing between age groups, we recorded the movements of 63 individually recognizable, prebreeding guillemots in four different parts of a major colony in the North Sea during the breeding season. Prebreeding guillemots infrequently entered breeding areas (in only 26% of watches), though with marked differences in frequency of entry among individuals and more entries toward the end of the breeding season. Once entered, the proportion of time spent in breeding areas by prebreeding guillemots also varied between different parts of the colony. Our data and model predictions indicate low levels of age‐group mixing, limiting exposure of breeding guillemots to infection. However, they also suggest that prebreeding guillemots have the potential to play an important role in driving infection dynamics. This highlights the sensitivity of breeding colonies to changes in the behavior of their members—a subject of particular importance in the context of global environmental change.     

At-sea distribution of seabirds and marine mammals around Svalbard,summer 1991

At-sea distribution of seabirds and marine mammals was quantitatively determined during the Arctic EPOS cruise of RV Polarstern, from 21 June till 28 July 1991, during 377 half-an-hour counts. Data were expressed as numbers per count and as density, and daily food intake was calculated using allometric equations from literature. Mean densities for the whole expedition were 29 seabirds per km² (mainly little auk,Alle alle: 8.7, kittiwake,Rissa tridactyla: 8.2, Brünnich’s guillemot,Uria lomvia: 6.5 and fulmar,Fulmarus glacialis: 3.4), 0.06 pinnipeds, 0.01 cetaceans and 0.002 polar bears. Total food intake by seabirds and marine mammals was 3.9 kg fresh weight/km² per day, with extreme values of 6.6 in the northern west-to-east transect and 2.5 in the Storfjorden. The major ecological influence were fish eaters (1.7), and more especially Brünnich’s guillemot (1.2). Geographic differences were also detected: food intake by Brünnich’s guillemot represented 62% of total intake in Storfjorden, and by the kittiwake, 45% in the first eastern transect. The first and last transects in the western Barents Sea are described and discussed in more detail. Within different sectors, high concentrations of seabirds were noted, corresponding to hydrological features such as fronts between Atlantic and polar waters, as well as ice edges. Values of seabirds’ density and food intake are higher than in the Greenland Sea, even than at the biologically very active ice edge there. Figures for pinnipeds and cetaceans are similar; numbers of polar bears were higher around Spitsbergen. The data presented were collected during the European “Polarstern” Study (Arctic EPOS) sponsored by the European Science Foundation     

Within and between species competition in a seabird community: statistical exploration and modeling of time-series data

In a changing environment, the maintenance of communities is subject to many constraints (phenology, resources, climate, etc.). One such constraint is the relationship between conspecifics and competitors. In mixed colonies, seabirds may have to cope with interspecific and intraspecific competition for both space and food resources. We applied competitive interaction models to data on three seabird breeding populations: black-legged kittiwake (Rissa tridactyla), common guillemot (Uria aalge) and Brünnich's guillemot (Uria lomvia) collected over 27-years at Kharlov Island in the Barents Sea. We found a competitive effect only for the kittiwake breeding population size on the common guillemot breeding population size when kittiwakes were abundant. The timing of kittiwake breeding negatively affected the number of breeding Brünnich's guillemots. The timing of breeding was negatively correlated to biomass of the main pelagic fish in the Barents Sea, the capelin (Mallotus villosus), which suggests an indirect action. The community matrix shows that the community was not stable. The kittiwake population did not decrease as seen in north Norwegian populations. Likewise, the common guillemot population, after a crash in 1985, was recovering at Kharlov while Norwegian populations were decreasing. Only the Brünnich's guillemot showed a decrease at Kharlov until 1999. We suggest that the stability of the kittiwake and common guillemot populations at Kharlov is due to better feeding conditions than in colonies of the Norwegian coast, linked to a possible eastward shift of the capelin population with the temperature increase of the Barents Sea.     

Variance in prey abundance influences time budgets of breeding seabirds: evidence from pigeon guillemots Cepphus columba

We use data on pigeon guillemots Cepphus columba to test the hypothesis that discretionary time in breeding seabirds is correlated with variance in prey abundance. We measured the amount of time that guillemots spent at the colony before delivering fish to chicks ("resting time") in relation to fish abundance as measured by beach seines and bottom trawls. Radio telemetry showed that resting time was inversely correlated with time spent diving for fish during foraging trips (r=−0.95). Pigeon guillemots fed their chicks either Pacific sand lance Ammodytes hexapterus , a schooling midwater fish, which exhibited high interannual variance in abundance (CV=181%), or a variety of non-schooling demersal fishes, which were less variable in abundance (average CV=111%). Average resting times were 46% higher at colonies where schooling prey dominated the diet. Individuals at these colonies reduced resting times 32% during years of low food abundance, but did not reduce meal delivery rates. In contrast, individuals feeding on non-schooling fishes did not reduce resting times during low food years, but did reduce meal delivery rates by 27%. Interannual variance in resting times was greater for the schooling group than for the non-schooling group. We conclude from these differences that time allocation in pigeon guillemots is more flexible when variable schooling prey dominate diets. Resting times were also 27% lower for individuals feeding two-chick rather than one-chick broods. The combined effects of diet and brood size on adult time budgets may help to explain higher rates of brood reduction for pigeon guillemot chicks fed non-schooling fishes.     

Non-random, intra-colony distribution of bridled guillemots Uria aalge

Within the Common guillemot Uria aalge colony, at the Gannet Clusters, Labrador, Canada, there was no evidence of heterogeneity in the proportion of bridled birds in different parts of the colony. Nor was there any evidence for assortative mating among bridled guillemots. Examination of the distribution of bridled birds in rows of guillemots however, showed a significant degree of clumping. Two hypotheses are presented to explain this distribution pattern: (a) the environmental hypothesis, which proposes that bridled birds preferentially select particular micro-habitats, or choose to breed where there are already bridled birds. (b) the return hypothesis, which proposes that one sex (probably female) returns to breed very close to where it was reared.     

Recruitment to a seabird population depends on environmental factors and on population size

1. A novel capture-mark-recapture (CMR) method was used to build a multistate model of recruitment by young birds to a breeding population of common guillemots Uria aalge on the Isle of May, Scotland. Recruitment of a total of 2757 individually marked guillemots over 17 years was modelled as a process where individuals had to move from an unobservable state at sea, through a nonbreeding state present in the colony, to the breeding state. The probabilities of individuals returning to the colony in a given year, at age 2 and 3-4 years, were positively correlated with an environmental covariate, the winter North Atlantic Oscillation index (WNAO) in the previous years. 2. For 2 year olds, there was a negative relationship with breeding population size, suggesting that density dependence operated in this colony through limitation of food or some other resource. 3. Survival over the first 2 years of life varied with cohort, but was unrelated to the WNAO. Mean survival over this 2-year period was high at 0.576 (95% CI: 0.444; 0.708). 4. This high survival, combined with a low 'local' survival after age 5 years of 0.695 (0-654; 0.733) and observations of Isle of May chicks at other colonies, suggests that most surviving chicks return to the natal colony before deciding whether to recruit there or move elsewhere.     

Declining trends in the majority of Greenland’s thick-billed murre (Uria lomvia) colonies 1981–2011

Large population declines were reported for the thick-billed murre (Uria lomvia) in Greenland for the period 1930s–1980s, but no national status has been published for the past 20 years. Meanwhile, the murres have gained more protection and several human-induced mortality factors have been markedly reduced. Here, we give an updated status based on the past 30 years of murre count data. The total Greenland population in 2011 was estimated to 468,300 birds (95 % CI 430,700–505,900) or around 342,000 breeding pairs, distributed within 19 colonies. This represents an overall reduction of 13 % since the mid-/late 1980s. In the same period, five colonies went extinct. Large and apparently stable colonies in Qaanaaq (Northwest Greenland) account for more than half the population (68 %), but most other colonies declined heavily, with up to 6 % p.a. in the most critical areas. So far, nothing indicates that food is a limiting factor in Greenland during the breeding season, although rather few colonies have been studied in details. In contrast, illegal hunting and disturbances during the breeding season are still a problem in Greenland, despite more restrictive hunting regulations, and may explain much of the continued population decline. In addition, recent studies from Svalbard indicate that a large-scale deterioration of the marine environment in the North Atlantic, due to oceanographic changes, may impact recruitment to some of the Greenland colonies. Murre colonies in southern Upernavik, Disko Bay, South Greenland and East Greenland are in urgent need of additional conservation initiatives to avoid further declines and local extinctions.     

How large an area of sea do Helgoland seabirds use for foraging during the breeding season?

Conclusions Of the species examined, only the guillemot remained within the study area, while other species ranged much further from the colony to forage. The number of guillemots present at sea was 2943±222 birds as calculated by Ordinary Kriging method (van der Meer & Leopold, in prep.). This corresponds to the number expected from the size of the colony and the attendance patterns of the birds (2889: Grunsky, 1992). Based on (1) metabolism of adults, (2) cost of egg production, (3) amount of food delivered to chicks, and (4) population parameters according to Grunsky (1992), we calculated the total energy consumption during the breeding season (cf. Cairns et al., 1992). The length of the breeding season was taken from 1st April (10 days before the first egg) to 16th June (mean day of leaving). In total, the guillemots used 1015 million kJ of energy to complete the 1991 breeding season. The diet (Grunsky, 1992) consisted, for 68.6%, of sandeel (7.8 kJ/g wet mass) and for 31.4%, of clupeoids (10.8 kJ/g), so in total 81 tonnes of sandeel and 37 tonnes of clupeoids were needed to support the colony of guillemots.     

Population Regulation in Magellanic Penguins: What Determines Changes in Colony Size?

Seabirds are often studied at individual colonies, but the confounding effects of emigration and mortality processes in open populations may lead to inappropriate conclusions on the mechanisms underlying population changes. Magellanic penguin (Spheniscus magellanicus) colonies of variable population sizes are distributed along the Argentine coastline. In recent decades, several population and distributional changes have occurred, with some colonies declining and others newly established or increasing. We integrated data of eight colonies scattered along ∼ 600 km in Northern Patagonia (from 41°26´S, 65°01´W to 45°11´S, 66°30´W, Rio Negro and Chubut provinces) and conducted analysis in terms of their growth rates, production of young and of the dependence of those vital rates on colony age, size, and location. We contrasted population trends estimated from abundance data with those derived from population modeling to understand if observed growth rates were attainable under closed population scenarios. Population trends were inversely related to colony size, suggesting a density dependent growth pattern. All colonies located in the north—which were established during the last decades—increased at high rates, with the smallest, recently established colonies growing at the fastest rate. In central-southern Chubut, where colonies are the oldest, the largest breeding aggregations declined, but smaller colonies remained relatively stable. Results provided strong evidence that dispersal played a major role in driving local trends. Breeding success was higher in northern colonies, likely mediated by favorable oceanographic conditions. However, mean foraging distance and body condition of chicks at fledging were influenced by colony size. Recruitment of penguins in the northern area may have been triggered by a combination of density dependence, likely exacerbated by less favorable oceanographic conditions in the southern sector. Our results reaffirm the idea that individual colony trends do not provide confident indicators of population health, highlighting the need to redefine the scale for the study of population changes.