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| 3. |
- 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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- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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| 8. |
- Svensson, Erik
(författare)
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Costs, benefits and constraints in the evolution of avian reproductive tactics: a study on the blue tit
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study I have investigated the various factors influencing reproductive tactics among breeding blue tits (Parus caeruleus). The study was carried out in a nestbox-breeding population of marked individuals at the Revinge Area, outside Lund, southern Sweden. I have especially investigated the energetic, physiological and temporal factors influencing the reproductive decisions. This included use of various physiological methods such as metabolic measurements of energy expenditure, measurements of immunocompetence using ELISA:s and sexing of nestlings using quantification of DNA-contents using flow cytometry. Three themes were explored: 1/Reproductive effort and costs of reproduction, 2/sex allocation and 3/reproductive timing in a seasonal environment.
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| 9. |
- Svensson, Erik, et al.
(författare)
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Energetic stress, immunosuppression and the costs of an antibody response
- 1998
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Ingår i: Functional Ecology. - : Wiley. - 1365-2435 .- 0269-8463. ; 12:6, s. 912-919
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Tidskriftsartikel (refereegranskat)abstract
- 1. Recently, there has been much interest in physiological trade-offs between parasite resistance and fitness-related traits such as secondary sexual characters or reproductive effort. More specifically it has been suggested that (i) energetically costly activities may suppress the immune system and (ii) that this immunosuppression is caused by costly immune defences competing with other bodily demands for scarce resources, e.g. energy. 2. The possibility was investigated of an energetically based trade-off between humoral (antibody-based) immunocompetence and other costly activities, by immunizing Blue Tits, Parus caeruleus, with novel antigens (proteins) thereby inducing antibody responses, and performing two experiments. In experiment i, one group of birds was subjected to cold stress, thereby increasing their daily energy expenditure and the effect on immune responsiveness was investigated. In experiment 2, the basal metabolic rate (BMR) of immunized birds was measured to investigate the energetic costs of mounting the antibody responses. 3. In experiment I, birds subject to increased energy turnover had significantly lower antibody responses, consistent with the hypothesis that environmental stress could suppress immunocompetence. However, in experiment 2 the energetic costs of these antibody responses were found to be low and at most 8-13% of BMR, indicating that adaptive resource allocation of energy was an unlikely explanation for the lowered immune responsiveness in the cold stress treatment (experiment 1). 4. It is concluded that our data provide some support to the idea that there may be a trade-off between immunocompetence and energetically costly activities such as thermoregulation, reproduction or mate attraction, although this trade-off may not necessarily be based on energy or nutrient limitation (i.e. resource allocation models). Two non-energetic explanations are briefly discussed, one adaptive and one non-adaptive, that could explain the immunosuppression in our study as well as in other behavioural and ecological contexts.
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| 10. |
- Svensson, Erik, et al.
(författare)
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Mate quality affects offspring sex ratio in blue tits
- 1996
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Ingår i: Proceedings of the Royal Society B: Biological Sciences. - : The Royal Society. - 0962-8452. ; 263:1368, s. 357-361
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Tidskriftsartikel (refereegranskat)abstract
- Questions concerning the occurrence of adaptive sex ratio manipulation have a long research tradition in evolutionary biology, but evidence from birds is scarce. It has been suggested that birds are unable to adaptively determine the sex of their young because of the constraints of their sex determination systems. Here we report results from a study of variation in offspring sex ratio in the blue tit, Parus caeruleus, a small passerine bird. Our data suggest that females are able to manipulate their primary sex ratios away from the equal sex ratio predicted from simple chromosomal segregation at meiosis. Contrary to the situation in some other birds, offspring sex ratio variation in this species was not influenced by timing of breeding or brood size. We show that females mated to males with high survival prospects bias the hatching sex ratio of their broods in favour of sons. Although it has previously been proposed that females should produce an excess of sons when mated to high-quality males, our data provide the first evidence that the phenotypic quality of fathers contributes to the sex ratio variation in a natural population.
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