Spatiotemporal variation in laying date and its impact on reproductive success in house sparrows (Passer domesticus)
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- Institutt for biologi 
Timing of breeding is an important determinant of reproductive success for animals living in seasonal environments. Understanding the factors affecting reproductive processes are necessary to predict how populations may respond to future changes in the environment. In general, there is a shortage of studies investigating these processes in multi-brooded species in different habitat types. The aim of this study was to investigate the influence of demographic and environmental factors on the variation in timing of breeding and its consequences for reproductive success between two habitat types in a multi-brooded species, the house sparrow (Passer domesticus). Interestingly, the relationships between laying date of the first clutch and both onset of spring and population density depended on habitat type. Increased population density was associated with advanced laying dates in the no-farm populations where sparrows were more exposed to fluctuating weather and food availability, while no relationship was found in the farm populations where sparrows had access to shelter and food provided by farms. Onset of spring had no effect on laying date in the no-farm populations, and a weak negative effect in the farm populations. There were also differences in the effects of population density and season length on reproductive success between habitats. More fledglings were produced in the no-farm populations compared to the farm populations, especially at long season lengths. Reproductive success decreased with higher population densities only in the farm populations. The relationship between laying date and reproductive success were convex in both habitat types, with lower reproductive output in first clutches initiated around the mean laying date. The total production of fledglings through the season was higher for individuals starting their first clutches earlier than conspecifics. These results illustrate spatial variation in environmental and density dependent effects on reproductive traits. Also, several of the reproductive traits and strategies found in these house sparrow populations are in accordance with those found in other multi-brooded species, such as the importance of timing of breeding relative to conspecifics rather than timing to optimal conditions. This study underlines the importance of considering properties of the habitat and the reproductive strategy of a species when trying to understand how populations may respond to future climate change.