Great tits, Parus major, lay too many eggs: experimental evidence in mid‐boreal habitats

• Published in: Oikos
• Main Authors: Rytkönen, Seppo, Orell, Markku
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2001
• Subjects: Parus; Northern Finland
• Online Access: http://dx.doi.org/10.1034/j.1600-0706.2001.930309.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1034%2Fj.1600-0706.2001.930309.x; https://onlinelibrary.wiley.com/doi/pdf/10.1034/j.1600-0706.2001.930309.x

This study shows that great tits lay too large clutches in mid‐boreal habitats. First, breeding success, measured with number of fledglings or proportion of eggs that produce fledglings, in northern Finland (65°N) is much poorer than in central and western Europe. Second, brood size manipulations (ca ±30% of the natural mean) revealed that reduced broods produced equal numbers of and larger‐sized fledglings than control and enlarged broods, giving thus the best fitness value for reduced broods. Third, parents of enlarged broods could not adjust (i.e. increase) their feeding effort to the greater number of nestlings. Fourth, extra feeding (about 1/3 of the theoretical maximal needs of the nestlings) during the nestling period resulted in more numerous and larger‐sized fledglings in comparison to control broods. We suggest that the ultimate explanation for the too large clutches is gene flow from the southern population, which prevents local adaptations in the north. Consequently, the main reason for food limitation during the nestling period is that northern great tits apply “southern” decision rules for timing of breeding, clutch size and foraging behaviour. Thus, they tend to breed too early in comparison to the food abundance peak, lay too large clutches in comparison to the level of resources and, perhaps, forage on a too narrow diet (75% caterpillars). Since the late broods that matched the local food abundance peak did not succeed better than the mismatched earlier ones, the most crucial fault of northern great tits seems to be that they overestimate food abundance during peak demands and lay too large clutches. Another explanation for this could be that northern great tits have adopted a brood reduction strategy. However, the long‐term data reveal that years of high breeding success, which would maintain large clutches in the population, are very rare in the north. Therefore, it is unlikely that a brood reduction strategy per se could explain the phenomenon. Instead, it could work together with the gene ...