| 1. |
- Hjernquist, Mårten B, et al.
(author)
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COMMON GUILLEMOTS URIA AALGE DIFFERENTIATE THEIR NICHE TO COEXIST WITH COLONIZING GREAT CORMORANTS PHALACROCORAX CARBO
- 2005
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In: Atlantic Seabirds. - 1388-2511. ; 7:2, s. 23-29
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Journal article (peer-reviewed)abstract
- Colonization of new species into an established community generally results in interspecific competition over resources between the colonist and existing members of the community. Interspecific competition has been suggested to influence extinction rates, population dynamics, community structure, niche differentiation and evolution. In this study, we observe possible interspecific competition over breeding sites resulting in niche differentiation and coexistence of Great Cormorants Phalacrocorax carbo and Common Guillemots Uria aalge in a seabird cliff community. In Sweden, Great Cormorants have naturally increased and expanded during the last two decades. Here, we show that most Common Guillemots previously bred on cliff ledges with high roof heights before the studyisland was colonized by Great Cormorants, but are now mainly found breeding on cliff ledges with lower roof heights. A temporary decline in the Common Guillemot population coincided with the colonization event and we discuss the potential for this decline to be caused by increased nest-site competition combined with high nest-site fidelity.
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| 2. |
- Hjernquist, Björn, et al.
(author)
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The effects of quantity and quality of prey on population fluctuations in three seabird species
- 2010
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In: Bird Study. - : Informa UK Limited. - 0006-3657 .- 1944-6705. ; 57:1, s. 19-25
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Journal article (peer-reviewed)abstract
- Capsule Population sizes of Common Guillemots Uria aalge, Razorbills Alca torda and Lesser Black-backed Gulls Larus fuscus were associated with prey abundance but not prey quality. Aims To examine how the abundance and quality of prey fish affects seabird population size and to test the 'junk-food' or nutritional stress hypothesis. Methods Analysis of long-term seabird population size data and Sprat Sprattus sprattus biomass and age-related weight data using a correlative approach. Results De-trended seabird and Sprat population data showed that the abundance of Sprat, the main prey species, was associated with the abundance of seabirds, while no effect of age-related size of prey on seabird population size was found. Conclusion As the Sprat population increased so did the seabird populations, regardless of decreases in 'quality' of Sprats, implying that more prey fish simply seem to mean more food in this marine ecosystem. No support for the 'junk-food' hypothesis was found and the results contradict suggestions from earlier studies that prey quality is important to top-predators in the Baltic Sea.
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| 3. |
- Hjernquist, Marten B., et al.
(author)
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Sex allocation in response to local resource competition over breeding territories
- 2009
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In: Behavioral Ecology. - : Oxford University Press (OUP). - 1045-2249 .- 1465-7279. ; 20:2, s. 335-339
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Journal article (peer-reviewed)abstract
- Sex allocation according to local resource competition suggests that investment and offspring sex ratio should be biased toward the dispersing sex to limit the competition among the natal philopatric sex. Conversely, when competition over resources is low, parents should allocate more resources toward the philopatric sex. In this study, this reciprocal scenario of sex allocation is tested. More specifically, the effect of breeding territory availability on primary sex ratio is studied in the collared flycatcher, a migratory passerine bird, where males are the natal philopatric sex. As predicted, primary sex ratios were biased toward males in areas where available territories were abundant (estimated from population growth). No relationship between sex ratio adjustment and adult phenotypes as well as date of first egg was found. We discuss potential explanation for the male-biased broods in areas with many vacant territories and low levels of competition. We suggest that sex ratio adjustment in relation to breeding territory quality and availability could be relatively common in birds.
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| 4. |
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| 5. |
- Brommer, Jon E., et al.
(author)
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Passerine Extrapair Mating Dynamics : A Bayesian Modeling Approach Comparing Four Species
- 2010
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In: American Naturalist. - : University of Chicago Press. - 0003-0147 .- 1537-5323. ; 176:2, s. 178-187
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Journal article (peer-reviewed)abstract
- In many socially monogamous animals, females engage in extrapair copulation (EPC), causing some broods to contain both within-pair and extrapair young (EPY). The proportion of all young that are EPY varies across populations and species. Because an EPC that does not result in EPY leaves no forensic trace, this variation in the proportion of EPY reflects both variation in the tendency to engage in EPC and variation in the extrapair fertilization (EPF) process across populations and species. We analyzed data on the distribution of EPY in broods of four passerines (blue tit, great tit, collared flycatcher, and pied flycatcher), with 18,564 genotyped nestlings from 2,346 broods in two to nine populations per species. Our Bayesian modeling approach estimated the underlying probability function of EPC (assumed to be a Poisson function) and conditional binomial EPF probability. We used an information theoretical approach to show that the expected distribution of EPC per female varies across populations but that EPF probabilities vary on the above-species level (tits vs. flycatchers). Hence, for these four passerines, our model suggests that the probability of an EPC mainly is determined by ecological (population-specific) conditions, whereas EPF probabilities reflect processes that are fixed above the species level.
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| 6. |
- Forsman, Jukka T., et al.
(author)
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Competitor density cues for habitat quality facilitating habitat selection and investment decisions
- 2008
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In: Behavioral Ecology. - : Oxford University Press (OUP). - 1045-2249 .- 1465-7279. ; 19:3, s. 539-545
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Journal article (peer-reviewed)abstract
- The theory of species coexistence predicts avoidance between species that compete for similar resources. Recent studies, however, have suggested that facilitation is also possible if competitor density provides information about resources. Optimal solution to trade-off between competition and facilitation is predicted to occur at intermediate competitor densities. We tested this hypothesis by experimentally creating a density range of resident tit species (Parus spp.), and measured the response of a competitively subordinate migratory bird, the collared flycatcher (Ficedula albicollis) in terms of habitat preference (settlement order and density), offspring investment (clutch size and primary sex ratio of offspring), and reproductive success (number and condition of nestlings). We show that most habitat choice and investment decisions of flycatchers were unimodally related to tit density, whereas reproductive success decreased linearly with increasing density. Flycatchers thus made mismatched investment decisions at the artificial tit densities because manipulation disassociated the natural correlation between habitat quality and population density. Apparently low and high tit densities were perceived as indication of poor quality habitat in terms of low amount or quality of resources/high mortality risk and high costs of competition, respectively. This demonstrates that competitor density can be used in assessing overall habitat quality in habitat selection and offspring investment decisions, integrating information on resources and competition.
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| 7. |
- Forsman, Jukka T., et al.
(author)
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Experimental evidence for the use of density based interspecific social information in forest birds
- 2009
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In: Ecography. - : Wiley. - 0906-7590 .- 1600-0587. ; 32:3, s. 539-545
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Journal article (peer-reviewed)abstract
- Reproductive success and habitat preference are generally assumed to be negatively associated with densities of con- and heterospecific competitors. However, recent theoretical studies have suggested that in some cases habitat preference may have a nonlinear unimodal function in relation to con- or heterospecific competitor densities - intermediate densities being preferred. Such a pattern is expected if con- or heterospecific densities are used as a proximate cue in habitat selection, which may produce benefits by reducing searching costs and providing information about current habitat quality and costs of competition. At low density the use of such cues, and hence habitat selection, are hampered, whereas at high density costs of competition exceed the benefits of using cues, leading to avoidance. Here, we tested this hypothesis by examining whether arboreal migratory birds use the density of resident titmice (Parus spp.) in habitat selection decisions. Many migrants and titmice species share similar resource needs making titmice density a reliable source of information for migrants. At the scale of habitat patches, we experimentally created a range of titmice densities from low to very high and subsequently measured the density response of migrants. In contrast to the unimodal habitat preference hypothesis, the average species number and total density of migratory birds were positively and linearly correlated with manipulated titmice density. Thus, migrants probably use titmice density as a relative indicator of habitat quality (abundance or quality of food) because foliage gleaners that share similar food resource with titmice, but not ground foragers, showed a positive association with manipulated titmice density. These results emphasize the positive effect of interspecific social information on habitat choice decisions and diversity of migratory bird community.
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| 8. |
- Hjernquist, Marten B., et al.
(author)
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High individual repeatability and population differentiation in stable isotope ratios in winter-grown collared flycatcher Ficedula albicollis feathers
- 2009
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In: Journal of Avian Biology. - 0908-8857 .- 1600-048X. ; 40:2, s. 102-107
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Journal article (peer-reviewed)abstract
- For migrants, we often lack complete information of their spatial distribution year round. Here, we used stable carbon, nitrogen and hydrogen isotope ratios extracted from feathers grown at the wintering sites of the long-distance migratory collared flycatcher Ficedula albicollis, to study how individuals from different breeding populations are distributed at the wintering sites. A sub-sample of birds was also sampled in two consecutive years to test for the repeatability of isotope ratios. Birds from the same breeding populations had more similar isotope ratios compared to birds from other nearby populations (10-100 km apart). Furthermore, isotope repeatability within individuals was high, implying that the observed pattern of isotope variation is consistent between years. We put forward two hypotheses for these patterns; 1) strong wintering site philopatry and migratory connectivity, suggesting that migratory connectivity may potentially be found on a much smaller spatial scale than previously considered, and 2) consistent interpopulation differentiation of feeding ecology at their wintering site.
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| 9. |
- Hjernquist, Mårten B.
(author)
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Living in a Variable Environment : Reproductive Decisions in Wild Bird Populations
- 2008
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Doctoral thesis (other academic/artistic)abstract
- In nature, environments are often variable and heterogeneous influencing ecological and evolutionary processes. This thesis focus on how animals interact with their environment and how that affects the reproductive decisions they make. Using empirical data collected from wild collared flycatcher populations, experiments and molecular approaches I try to unveil some of these relationships and the evolutionary, ecological and conservation implications of these findings are discussed. Firstly, collared flycatchers were shown to use breeding densities of their own and other species using similar resources when assessing costs and benefits associated to breeding in specific habitats. However, species will vary in how informative they are, and the worst competitor – with whom you overlap most in resources needs – also provides the best source of information. Collared flycatcher parents will also benefit differentially from investments in sons and daughters due to habitat characteristics and dispersal differences between the sexes. Here, I show that they will produce more of the sex that will give the highest expected fitness return given the environment they are in. These results also provide a reciprocal scenario to Clark's (1978) classical study of sex ratio adjustment in relation to local resource competition (LRC), as more of the natal philopatric sex is produced when LRC is low. Secondly, the effect of elaborated ornaments on paternity in the socially monogamous collared flycatcher was shown to be of more importance in areas where the intensity of intra- and intersexual conflicts are expected to be elevated. Hence, ornamentation by environmental interactions determines paternity, illustrating that sexual selection through extra-pair paternity is context dependent. Finally, even though the collared flycatcher populations that this thesis is based on have been studied on their breeding grounds for more then 25 years, we know little of where they are when they are not breeding. Here, stable isotope signatures in winter-grown feathers suggests that they may spend their winter with their breeding ground neighbours and do so repeatedly over years. Differences between breeding populations at this small scale should have many impactions for evolutionary and ecological processes as it will, for example, determine with whom individuals interact throughout their life.
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| 10. |
- Hjernquist, Mårten B., et al.
(author)
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Seasonality determines patterns of growth and age structure over a geographic gradient in an ectothermic vertebrate
- 2012
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In: Oecologia. - 0029-8549 .- 1432-1939. ; 170:3, s. 641-649
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Journal article (peer-reviewed)abstract
- Environmental variation connected with seasonality is likely to affect the evolution of life-history strategies in ectotherms, but there is no consensus as to how important life-history traits like body size are influenced by environmental variation along seasonal gradients. We compared adult body size, skeletal growth, mean age, age at first reproduction and longevity among 11 common frog ( Rana temporaria ) populations sampled along a 1,600-km-long latitudinal gradient across Scandinavia. Mean age, age at first reproduction and longevity increased linearly with decreasing growth season length. Lifetime activity (i.e. the estimated number of active days during life-time) was highest at mid-latitudes and females had on average more active days throughout their lives than males. Variation in body size was due to differences in lifetime activity among populations—individuals (especially females) were largest where they had the longest cumulative activity period—as well as to differences between populations in skeletal growth rate as determined by skeletochronological analyses. Especially, males grew faster at intermediate latitudes. While life-history trait variation was strongly associated with latitude, the direction and shape of these relationships were sex- and trait-specific. These context-dependent relationships may be the result of life-history trade-offs enforced by differences in future reproductive opportunities and time constraints among the populations. Thus, seasonality appears to be an important environmental factor shaping life-history trait variation in common frogs.
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