| 21. |
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| 22. |
- Nilsson, Jan-Åke, et al.
(författare)
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Causes and consequences of egg mass variation between and within blue tit clutches
- 1993
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Ingår i: Journal of Zoology. - : Wiley. - 1469-7998 .- 0952-8369. ; 230:3, s. 469-481
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Tidskriftsartikel (refereegranskat)abstract
- Causes of egg‐size variation between and within clutches were studied in clutches of the blue tit (Parus caeruleus L.). We measured the mass of each egg in the laying sequence in unmanipulated clutches, in clutches of parents experimentally supplied with extra food before egg‐laying, and in clutches of parents supplemented with extra food after the start of egg‐laying. Hatchlings were weighed at an age of two days and their mass was found to be positively related to egg mass. No general trend of decreasing or increasing egg mass was found within the laying sequence. Females provided with extra food before egg‐laying laid clutches with significantly less variation in egg mass than did control females. The reason for this was that the first‐laid egg of unmanipulated females was lighter than the rest of the eggs in the clutch. This pattern disappeared in clutches of females receiving extra food. Thus, the reduction in egg mass variation among clutches of foodsupplemented females depended on an ability of these females, in contrast to control females, to lay a first egg of the same mass as the rest of the clutch. Eggs laid after the initiation of incubation were significantly heavier than equivalent eggs in those clutches where incubation started after clutch completion. The difference was small, however, and the adaptive significance of the finding is questionable. We argue that intra‐clutch variation in egg mass is connected with greater fitness consequences than in inter‐clutch variation. Furthermore, our results indicate that energetical constraints on the laying female are more important as a cause of the observed intra‐clutch variation in egg mass than are adaptive responses to the environment.
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| 23. |
- Nilsson, Jan-Åke, et al.
(författare)
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Energetic constraints and ultimate decisions during egglaying in the blue tit
- 1993
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Ingår i: Ecology. - : Wiley. - 0012-9658. ; 74:1, s. 244-251
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Tidskriftsartikel (refereegranskat)abstract
- In a population of Blue Tits, Parus caeruleus, we performed two different food provisioning experiments; one starting before clutch initiaition and one starting when the first eggs were laid. We assessed the effect of these feeding experiments on four fitness—related factors, viz., laying date, clutch size, egg mass, and onset of incubation. Given that breeding birds are food and energy constrained, extra food during the breeding season should be invested in the fitness—related factor(s) that is most important for maximization of overall fitness. In the first feeding experiment, when breeding pairs had access to extra food both before and during egg—laying, the female converted this extra energy into earlier laying dates. When food was provided after clutch initiation only, the above preferred option was no longer available. In this situation, females used the extra energy to start incubating earlier in the laying sequence, with no influence on the other factors studied. Thus, both experiments resulted in surplus energy being used to increase the quality of the offspring instead of their number. In both cases the quality was connected with time savings leading to earlier hatching dates.
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| 24. |
- Nilsson, Jan-Åke, et al.
(författare)
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The cost of reproduction: a new link between current reproductive effort and future reproductive succcess
- 1996
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Ingår i: Royal Society of London. Proceedings B. Biological Sciences. - 1471-2954. ; 263:1371, s. 711-714
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Tidskriftsartikel (refereegranskat)abstract
- Although a negative trade-off between current reproductive effort and future reproductive output has for long been hypothesized and in some cases empirically shown, no mechanism with the potential to mediate such a reproductive cost from one season to another has been demonstrated. Here we suggest such a mechanism: the time and/or energy constraints during moult. We manipulated current reproduction of blue tits (Parus caeruleus) by delaying their breeding attempts. This resulted in reproductive costs both in terms of decreased survival and lower subsequent reproductive success in the form of delayed start of breeding (females) or a smaller clutch (males). We found that delayed birds expended about 15% more energy in thermoregulation during the subsequent winter than did control birds. Thus, more time and/or energy directed to reproduction, may constrain moult which results in the production of feathers with low insulating capacity. These feathers are retained during the following winter and breeding season. Thus, this mechanism has the potential to link reproductive activities in one season to future reproductive success.
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| 25. |
- Nilsson, Jan-Åke, et al.
(författare)
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The frequency and timing of laying gaps
- 1993
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Ingår i: Ornis Scandinavica. - : JSTOR. - 0030-5693. ; 24:2, s. 122-126
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Tidskriftsartikel (refereegranskat)abstract
- The frequency of laying gaps was studied in a nestbox breeding population of Blue Tits Parus caeruleus during the breeding season of 1990. In as many as 27% of the 55 clutches studied, laying was interrupted for at least one day. It is concluded that the interruption of egg laying depends on food availability since the frequency of laying gaps decreased in clutches laid by females experimentally provided with extra food. Most gaps occurred after the laying of the first or second egg, indicating that each female started egg laying on the verge of what is possible for uninterrupted laying. The occurrence of laying gaps was not connected with certain dates, female age, clutch size or mean egg mass of the clutch, instead each female seemed to individually optimize the start of egg laying in relation to the risk of interrupted laying. To some extent the female is able to save energy for egg production because an egg laid after a gap was heavier than corresponding eggs laid in clutches without gaps. Furthermore, eggs laid immediatelly before a gap were lighter than equivalent eggs in other clutches, indicating that females, to some extent, are willing to trade egg quality against uninterrupted laying.
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| 26. |
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| 27. |
- Råberg, Lars, et al.
(författare)
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On the adaptive significance of stress-induced immunosuppression
- 1998
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Ingår i: Royal Society of London. Proceedings B. Biological Sciences. - : The Royal Society. - 1471-2954 .- 0962-8452. ; 265:1406, s. 1637-1641
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Forskningsöversikt (refereegranskat)abstract
- We approach the field of stress immunology from an ecological point of view and ask: why should a heavy physical workload, for example as a result of a high reproductive effort, compromise immune function? We argue that immunosuppression by neuroendocrine mechanisms, such as stress hormones, during heavy physical workload is adaptive, and consider two different ultimate explanations of such immunosuppression. First, several authors have suggested that the immune system is suppressed to reallocate resources to other metabolic demands. In our view, this hypothesis assumes that considerable amounts of energy or nutrients can be saved by suppressing the immune system; however, this assumption requires further investigation. Second, we suggest an alternative explanation based on the idea that the immune system is tightly regulated by neuroendocrine mechanisms to avoid hyperactivation and ensuing autoimmune responses. We hypothesize that the risk of autoimmune responses increases during heavy physical workload and that the immune system is suppressed to counteract this.
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| 28. |
- Simonsson, Pär, 1963-, et al.
(författare)
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A comparison of power measuring systems
- 1997
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Ingår i: IEEE Transactions on Instrumentation and Measurement. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9456 .- 1557-9662. ; 46:2, s. 423-425
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Tidskriftsartikel (refereegranskat)abstract
- An automatic zero-power-factor standard has been designed and compared with a digital sampling wattmeter standard. The agreement of the two power measuring standards at 120 V, 5 A, and zero power factor is within a few uW/VA. The paper describes the standards, the phase errors and uncertainties of the two standards
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| 29. |
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| 30. |
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