Ardea
Official journal of the Netherlands Ornithologists' Union

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Tinbergen J.M. (1987) Costs of reproduction in the Great Tit: intreasonal costs associated with brood size. ARDEA 75 (1): 111-122
In order to estimate within season reproductive cost (in terms of future reproductive output) associated with brood size in the Great Tit, brood size experiments were performed in three successive seasons. Broods with the same hatching date and approximately the same number of young were selected. Half of the young were taken from one brood (reduced) and added to another (enlarged) while one brood was kept as a control. The adults were individually marked. Subsequent breeding attempts of the same parents were identified within the study area (160 ha). Nestbox checks were done every week. Nestling weight was taken on day 14 of nestling life. Nest failure was not dependent on manipulation but differed greatly between years. The number of fledglings and the body weight of those broods that produced at least one fledgling differed between years and between manipulation categories. The fraction of birds that relayed after nest failure was not dependent on the brood size manipulation. However, for the birds that fledged young there was a definite effect of manipulation on the production of second broods. Parents of enlarged broods were less likely to produce second clutches than were those of control broods, while parents with reduced broods were only slightly more likely to produce a second clutch. The relationship between the fraction of second broods produced and the manipulated brood size suggests a concave-up cost function of brood size within the season. Clutch size and hatching success of the second brood were not related to the manipulation of the first brood. However, there was a relationship between the manipulation and the interval between the first and the second brood. The birds that had most young fledged started their second clutch later. Parental condition, in terms of body weight corrected for size, could not be shown to be related to manipulation, and thus was unlikely to be an important cause of the above effects. Comparison of the results of Slagsvolds (1984) experiments with this study and analysis of the correlations between both nestling weight and the number of young with the inter-brood intervals, suggest that the number of nestlings and/or their body weight in the very last phase of the nestling period or after fledging is of importance in determining the inter-brood interval, and possibly the occurrence of second broods. The effect of manipulation on the occurrence of second broods is consistent with the hypothesis that parents are faced with a trade-off between tending the fledglings of the first brood and starting a second one. The intra-seasonal cost, in terms of the future reproductive output, of having a large brood is shown to be a lowered probability to produce a second clutch. No indications were found that manipulation affects the ability of the parents to produce a second clutch.


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