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Description popularizing the research project
In early spring, in March, tiny nimble birds start bustling among the long twigs of weeping willows over the rivers and ponds. They are males of penduline tits. They have a lot of work to do and very little time. They are supposed to build one of the most intricate constructions in the world of birds – a soft and fluffy mitten-like nest suspended from a thin twig of a willow. They have to hurry up, as other males do not cease to collect willow and poplar catkins weaving their elaborate nests nearly touching the surface of water. After some time, when green covers the plants, the nests will not be so easy to spot among the leaves. Even if somebody succeeded in finding the nest, it could turn out to be uninhabited in spite of the fact that it is the middle of breeding season. It happens that the males, once having finished one nest, start building another. The extra ones come in handy as the birds often abandon their nests, even with eggs already laid, and start a new clutch with new partners. Unfortunately, none of the parents does come back to the abandoned eggs.
Briding system of the vast majority of birds is social monogamy with biparental care. Penduline tit, which is the object of our research, is one of exceptions. Breeding system of this species is ambisexual polygamy with uniparental care. Each individual faces a dilemma whether to take care of the offspring or desert them and look for a new mate. Our research is conducted at Milicz ponds in the SW Poland. Penduline tits are individually marked with a combination of three colour rings and a metal ring with an individual code. We search the study area for new nests and control previously found ones. We determine the number of eggs, nestlings and fledglings, which allows determining reproductive success of the individuals. We analyzed the strategies of 142 males and 94 females. 39% of males and 45% of females mated with more than one partner in the season, and 14% of males did not attract any partner. Parental care was provided most often by the female (46% of broods), while the male care was observed at 17% of clutches. 37% of clutches was deserted by both parents before incubation had started. Clutch desertion is a common strategy of both male and female penduline tits. This behaviour allows an individual to renest with another partner, and to avoid costly parental care. On the other hand, there is the risk that its partner will also desert the clutch. Females minimize the cost of desertion by abandoning the incomplete clutch. This tactic also allows them to leave the nest before their males. Although desertion is a common strategy of females, most of them took care of at least one brood (65%) in the season. 15% of females provided parental care at two nests. These females had the highest annual reproductive success.