Resource Allocation During Avian Incubation

Reid, Jane Margaret (2001) Resource Allocation During Avian Incubation. PhD thesis, University of Glasgow.

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Abstract

The ways in which birds allocate resources during the incubation phase of reproduction were investigated using empirical data gathered from populations of starlings (Sturnus vulgaris) breeding in Shetland and Spain, and modeling. The consequences of the allocations made to incubation for the resources allocated to other phases of life are discussed. Experimentally reducing the energetic demand of incubation by reducing the rate of heat loss from within a nest increased the proportion of offspring that fledged during the same breeding attempt, and the proportion of clutches that hatched completely during the subsequent attempt. Thus the energetic demand of incubation was shown to be sufficient to limit parental fitness, and was suggested to influence the way in which parents allocated resources to other reproductive phases, both within and between breeding attempts. Starlings nesting in poorly insulated cavities built larger nests and reduced the rate of heat loss from within the cavity to a greater extent than starlings nesting in well-insulated cavities. However, the rate of heat loss from within a completed nest was still related to the insulative quality of the original cavity. As experimentally reducing heat loss rate increased breeding success, the acquisition of a well insulated nest site was suggested to be an important phase of a starling's breeding attempt. Experimentally reducing the rate of heat loss from a nest also increased mean incubation-bout duration and thus the proportion of the day that female starlings spent incubating. This change is consistent with the hypothesis that parents terminate incubation bouts in response to their own condition rather than to egg temperature. The contradictory results of previous studies may be explained if, given energetic limitation during incubation, parents leave the nest when the costs of doing so are minimised. As a consequence of the time that incubating females allocated to activities away from the nest, eggs experienced mean temperatures that were below the predicted optimum for embryonic development. However, female nest attentiveness and mean egg temperature increased as the incubation period progressed, a change that may have been due to increased allocation to incubation rather than to changes in the demands of incubation or to improved foraging conditions. Experimentally enlarging a clutch for the duration of the incubation period reduced the proportion of the original clutch that hatched, and the proportion of offspring that fledged successfully. This is likely to have been because the addition of extra eggs directly affected the physical conditions experienced by the embryos, and affected a parent's ability to incubate the entire clutch equally. Clutch enlargement may also energetically constrain a parent's incubation ability. However, modeling suggested that the energetic debt accrued by an incubating parent does not necessarily increase with increasing clutch size, with the exact relationship depending on mean incubation temperature and the thermal properties of the clutch. Thus the consequences of incubation demands for optimal clutch size in birds are not necessarily clear. Male assistance with incubation was suggested to increase the proportion of offspring that hatched and fledged successfully and reduce the time that females spent incubating, and therefore to increase female fitness. However, male assistance was associated with monogamy. As male fitness may generally be maximised by polygyny, sexual conflict over male incubation was predicted. Males were suggested to incubate when they were unlikely to attract multiple females. However, primary females may liave increased the chance that a male would incubate by destroying secondary females' clutches. In conclusion, incubating a clutch of eggs can require substantial allocations of parental time and energy, affecting the resources available for and required by other life phases. Thus incubation can be costly, although costs may be state-dependent, and act through parents and/or their current offspring. Incubation demands may therefore be sufficient to influence overall resource allocation patterns and life-history strategies in birds.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Graeme Ruxton
Keywords: Evolution & development, Zoology
Date of Award: 2001
Depositing User: Enlighten Team
Unique ID: glathesis:2001-76140
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 16:35
Last Modified: 19 Nov 2019 16:35
URI: http://theses.gla.ac.uk/id/eprint/76140

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