Abstract
In seasonal environments, a temporal decline in breeding performance (e.g. clutch size, nestling condition, and fledging success) of altricial bird species is a well-known phenomenon. In this study, we present the effect of laying phenology on the physiological condition of nestling great tits (Parus major) in 14 consecutive breeding seasons. We used blood haemoglobin and baseline glucose concentrations as indicators of nestling physiological condition. Nestling blood haemoglobin reflects food base quality and availability during the breeding period. Blood glucose concentration can be used as a supplementary reverse index of condition, since it is negatively related to environmental quality. It might be indicative of the stress caused by unfavourable extrinsic factors, though, due to potential confounding factors such as adverse weather conditions, low food quality, or feeding interruptions, glucose levels should be used in this ecological context with caution. Great tit nestlings from earlier broods were characterised by higher mean haemoglobin concentrations, indicating a seasonal decline in food quality and availability. The blood glucose concentration displayed an opposite pattern, with nestlings from earlier broods being characterised by lower mean concentrations than those from later broods. However, very little of the variation in blood glucose concentration can be explained by the variation in laying date, which suggests that blood glucose concentration is of little importance in the context of breeding phenology. Our results show that the physiological condition of nestlings of this species decreases as the breeding season progresses, most probably due to environmental factors.
Highlights
Proper timing of breeding is crucial in highly seasonal environments at temperate latitudes, and is primarily determined by the prevailing weather and trophic conditions[1,2,3]
The blood glucose concentration increased as the breeding season progressed (Table 1)
The R2 value indicating how much of the variation in blood glucose concentration is explained by the breeding phenology in the final mixed model was 0.003
Summary
Proper timing of breeding is crucial in highly seasonal environments at temperate latitudes, and is primarily determined by the prevailing weather and trophic conditions[1,2,3]. Recent advancements in breeding phenology induced by global warming have been studied in many bird species, including tits[6,21,22,23,24,25,26,27] It is not quite clear whether these shifts in phenology have positive or negative consequences for fitness, since rapid shifts to earlier breeding might be disadvantageous if the period of maximum food requirements of nestlings mismatches the main food peak[28]. We focus here on a well-studied European model species, the great tit (Parus major), for which an advancement in egg laying in some areas has been observed[29,30], whereas in other sites, no such effect has been found[6] The occurrence of such an advancement in breeding phenology might lead to fitness consequences and, to selection for earlier breeding. Physiological variation might be even more important in the temporally changing environments where European Parids breed and where decisions of when to initiate broods are of crucial importance
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