Small birds likely face energetic challenges in temperate zone winters posed by cold weather coupled with food scarcity. These challenges are often assumed to occur, but are rarely experimentally tested. I hypothesized that the naturally occurring, lower abundances of food in temperate zone winters limit a bird's ability to acquire optimal energy and ultimately limit fitness. In this dissertation, I show that supplementation of food decreased mortality and improved traits potentially associated with future reproductive success of wild swamp sparrows (Melospiza georgiana), supporting the hypothesis that winter food abundance limits fitness. These results come from a replicated and controlled food supplementation experiment conducted over three years. First, I demonstrated that following food addition, immigration increased, leading to higher densities, and that all age/sex classes experienced higher survival and maintained larger energy reserves (Chapter I). Survival was positively related to energy reserves, indicating that food availability limits survival through a bird's ability to maintain sufficient fat. In addition to causing mortality in winter, food limitation of energy reserves may carry over to affect future reproductive success by influencing timing of preparations for breeding, including migration. In Chapter II, I show that swamp sparrows decreased fat reserves over each winter, despite unlimited food availability, indicating that they adaptively regulated fat reserves, potentially to balance starvation and depredation risks. Fat reserves of control birds tracked recent temperature and control birds lost muscle throughout the winter, indicating that they were limited by food and were unable to reach optimal fat levels on a daily basis. These results suggest that limitation of energy reserves by food availability can be influenced by temperature and predator abundance. Lastly, I demonstrated that food abundance limits the timing of molt in the wild (Chapter III), an unprecedented finding. Because molt, migration, and breeding typically do not overlap, early molt might lead to earlier migration and breeding. Therefore, we hypothesize that timing of molt is another mechanism by which winter food abundance can limit reproductive success. These results provide strong evidence that food availability can limit wintering temperate migrants in a variety of ways. / Ph. D.

Seasonality influences life history through its effect on the availability of essential resources, with birds timing breeding to occur during peak food availability. Due to density-dependence, investment in breeding is determined largely by the seasonality of food availability, with an increased investment being traded-off against adult survival. A bird's mass acts as an index of a species' foraging environment, because a bird bases its foraging decisions on a trade-off between the risk of predation and the risk of starvation. Under constant predation risk a bird increases its mass as insurance against increased foraging unpredictability. In tropical savannahs day length and temperature remains relatively constant, and there is not a season of increased density-dependent mortality which acts across all species. Thus species have evolved a broad range of life history traits under the same environmental conditions, although how a species experiences seasonality depends largely on its foraging niche. This thesis shows that most savannah species varied their mass across the year, having a reduced mass in the non-breeding season which suggests that foraging remained predictable. Independent of gonad or egg growth they then increased their mass as they started to breed, with the timing of breeding coinciding with peak food availability. Across species in the same foraging niche mass acts as an index of breeding investment, with females increasing their mass more than males. While across species in different foraging niches an increased mass response was associated with higher adult survival, probably because breeding strategy and subsequently adult survival are governed by food limitation. This thesis shows that birds adaptively manage their mass during breeding and that mass is not a result of energetic stress, thus under constant predation risk a bird's mass is a result of foraging predictability as a function of competition for available food and investment in breeding.

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The population dynamics and demography of Daphnia galeata was analysed in a five year study in the biomanipulated Bautzen Reservoir. Samples were taken two times a week during the period May-July in the pelagic zone of this highly eutrophic water. Major bottom-up and top-down factors were determined during the study period and analysed with regard to their influence on Daphnia dynamics and life history. Field data on fecundity and population structure of D. galeata were combined with results from life table and growth experiments performed under approximately in situ conditions to gain insight into the mechanisms leading to a midsummer decline of this cladoceran species which dominates the zooplankton community in Bautzen Reservoir. Two main patterns of Daphnia dynamics emerged: In years without a midsummer decline the population increased slowly in spring, starting from low densities. High water transparency was observed already during the build-up of the population of D. galeata. Despite considerable fluctuations, Daphnia abundance remained on a high level throughout summer. In years with a midsummer decline the population started from relatively high densities in early May and more than doubled during one week. Peak densities were reached before the clear-water stage emerged. At the end of this period the population declined to low values which lasted for the rest of the summer. Fecundity of the Population of D. galeata declined, whereas the mean egg volume increased at the beginning of the clear-water stage as a result of declining food levels. The size at maturity (SAM) remained high during this period. Additionally, juvenile growth was reduced and the age at maturity was retarded. Survival probability was low for those daphnids born shortly before or during the clear-water stage compared to those born later. It can be concluded from these results that recruitment to adult stages is strongly reduced during the clear-water stage. The end of this period is marked by an alternation in generations. Only at that time can SAM be reduced because the new generation of adults matures at a smaller size, carrying small eggs. A high impact of non-predatory adult mortality can be expected when the population is dominated by a strong peak-cohort during the clear-water stage according to recruitment patterns during the build-up of the population. The most drastic decline both of Daphnia abundance and SAM was observed in those years when the biomass of juvenile fish exceeded 20 kg ha-1 at the end of the clear-water stage. Due to gape-size limitation juvenile fish mainly feed on juvenile daphnids during this period and thus, they reinforce bottom-up effects on the Daphnia population. When fish change their size selection towards adult daphnids at the time when the new generation takes over, this seems to represent the worst case for the Daphnia population. Consequently, the timing between bottom-up effects and the feeding pressure of juvenile fish determines the extent of the decline. - (This manuscript is also available - in the form of a book - from Shaker Verlag GmbH, Postfach 101818, 52018 Aachen, Germany world-wide web address: http://www.shaker.de, electronic-mail address: info@shaker.de. It has been posted on the web sites of Dresden University of Technology with the permission of the publisher)

The population dynamics and demography of Daphnia galeata was analysed in a five year study in the biomanipulated Bautzen Reservoir. Samples were taken two times a week during the period May-July in the pelagic zone of this highly eutrophic water. Major bottom-up and top-down factors were determined during the study period and analysed with regard to their influence on Daphnia dynamics and life history. Field data on fecundity and population structure of D. galeata were combined with results from life table and growth experiments performed under approximately in situ conditions to gain insight into the mechanisms leading to a midsummer decline of this cladoceran species which dominates the zooplankton community in Bautzen Reservoir. Two main patterns of Daphnia dynamics emerged: In years without a midsummer decline the population increased slowly in spring, starting from low densities. High water transparency was observed already during the build-up of the population of D. galeata. Despite considerable fluctuations, Daphnia abundance remained on a high level throughout summer. In years with a midsummer decline the population started from relatively high densities in early May and more than doubled during one week. Peak densities were reached before the clear-water stage emerged. At the end of this period the population declined to low values which lasted for the rest of the summer. Fecundity of the Population of D. galeata declined, whereas the mean egg volume increased at the beginning of the clear-water stage as a result of declining food levels. The size at maturity (SAM) remained high during this period. Additionally, juvenile growth was reduced and the age at maturity was retarded. Survival probability was low for those daphnids born shortly before or during the clear-water stage compared to those born later. It can be concluded from these results that recruitment to adult stages is strongly reduced during the clear-water stage. The end of this period is marked by an alternation in generations. Only at that time can SAM be reduced because the new generation of adults matures at a smaller size, carrying small eggs. A high impact of non-predatory adult mortality can be expected when the population is dominated by a strong peak-cohort during the clear-water stage according to recruitment patterns during the build-up of the population. The most drastic decline both of Daphnia abundance and SAM was observed in those years when the biomass of juvenile fish exceeded 20 kg ha-1 at the end of the clear-water stage. Due to gape-size limitation juvenile fish mainly feed on juvenile daphnids during this period and thus, they reinforce bottom-up effects on the Daphnia population. When fish change their size selection towards adult daphnids at the time when the new generation takes over, this seems to represent the worst case for the Daphnia population. Consequently, the timing between bottom-up effects and the feeding pressure of juvenile fish determines the extent of the decline. - (This manuscript is also available - in the form of a book - from Shaker Verlag GmbH, Postfach 101818, 52018 Aachen, Germany world-wide web address: http://www.shaker.de, electronic-mail address: info@shaker.de. It has been posted on the web sites of Dresden University of Technology with the permission of the publisher)