Leptin Affects Life History Decisions in a Passerine Bird: A Field Experiment

BACKGROUND: Organisms face trade-offs regarding their life-history strategies, such as decisions of single or multiple broods within a year. In passerines displaying facultative multiple breeding, the probability of laying a second clutch is influenced by several life-history factors. However, information about the mechanistic background of these trade-offs is largely lacking. Leptin is a protein hormone produced by white fat cells, and acts as a signal between peripheral energy depots and the central nervous system. In addition, leptin affects cells at all levels of the reproductive axis and plays a critical role in regulating the allocation of metabolic energy to reproduction. As such, it is possible that leptin levels influence the decision of whether or not to invest time and energy into a second clutch. Accordingly, we expect a treatment with exogenous leptin to result in an increased number of second broods. METHODOLOGY/PRINCIPAL FINDINGS: At a later stage during the first brood, female great tits were treated either with long-term leptin-filled cholesterol pellets (the experimental birds) or with pellets containing only cholesterol (the control birds). We found that leptin-treated females were significantly more likely to have a second brood and that the earlier females were more likely to lay a second clutch than the late females. CONCLUSIONS/SIGNIFICANCE: As both timing of first brood and treatment with leptin were important in the decision of having multiple broods, the trade-offs involved in the breeding strategy most likely depend on multiple factors. Presumably leptin has evolved as a signal of energy supply status to regulate the release of reproductive hormones so that reproduction is coordinated with periods of sufficient nutrients. This study investigated the role of leptin as a mediator between energy resources and reproductive output, providing a fundamentally new insight into how trade-offs work on a functional basis

Daily foraging routines and feeding effort of a small bird feeding on a predictable resource.

According to theory, a small diurnal bird living in a predictable environment should have the highest feeding effort as late as possible in the day in order to minimize the time it carries large and costly reserves. The feeding effort should also decline with increasing food availability. We tested both these ideas with the lesser spotted woodpecker (Dendrocopos minor). For most of the year, this bird feeds on wood-living insects in dead tree branches. This food supply is likely to be highly predictable on a daily scale. Our results corroborated the theory. We found that the proportion of time spent actively feeding was lower in the mornings (before noon) than in the afternoons. We also found that woodpeckers spent less time feeding the higher their food availability However, for a given food availability they spent more time feeding in the afternoons. This supports the idea that feeding is less and other activities are more valuable in the mornings given a predictable food resource. This is the first demonstration of daily routines in small birds concordant with a predictable environment. In spring, males but not females reduced their feeding time. This difference between the sexes may be related to their sex-specific reproductive effort

Occurrence of the Lesser Spotted Woodpecker Dendrocopos minor in relation to area of deciduous forest

In the springs of 1985 to 1988, amateur ornithologists searched for Lesser Spotted Woodpeckers in 152 census areas in the south and central parts of Sweden. Each census area was 200 ha. The frequency of occurrence of the woodpecker increased with the total area of deciduous woodland; when the census areas contained less than 17 ha, it was 24%, when the areas had 17-38 ha it was 62%, and with >38 ha deciduous woodland it was 80%. The latitude in south and central Sweden had no significant effect on the area requirement, but our results show that the species requires larger areas in managed than in natural forests. A stepwise discriminant analysis revealed that the occupation of census areas by the Lesser Spotted Woodpecker could be predicted with 68% accuracy from the area of nemoral and riparian deciduous woods (positive association with occupation) and the area of mixed coniferous/deciduous wood (negative association with occupation). Non-nemoral deciduous wood, marsh wood, stand age and the density of snags did not enter the discriminant function . The area of nemoral deciduous forest has decreased in Sweden during the second half of this century due to cutting. This is probably one cause of the decline of the Lesser Spotted Woodpecker

Habitat preferences of the Lesser Spotted Woodpecker Dendrocopus minor

We analysed the habitat preferences of Lesser Spotted Woodpeckers Dendrocopos minor in deciduous stands relative to forest type, snag density, tree age, area of the stand and latitude . Data were collected in 1985-1988 in 104 census areas, each of 200 ha, in south and central Sweden. We compared the stands with and without observations of the species using log-linear modelling, with the area of the stands as a covariate. We found that the Lesser Spotted Woodpecker preferred stands with many snags (P = 0.0004) and old stands (P = 0.011), while it avoided stands without snags (P = 0.0011) and with mixed coniferous / deciduous forest (P =0.00004). Whether the area of the stand was less than or greater than the median size was less important (P = 1) . The preferences were the same in all the different regions studied (P = 1) . We regard snag density, stand age and forest type as good indicators of the habitat preferences of the the Lesser Spotted Woodpecker. Since these variables are easily measured in the field, they may serve as a starting-point for preservation of the habitats of the species