Abstract
Rapidly increasing urbanisation requires mitigation against associated losses of biodiversity and species abundance. In urban-breeding birds, altered food availability for nestlings is thought to reduce reproductive success compared to forest populations. To compensate for shortages of preferred foods, urban parents could increase their search effort for optimal diets or provision other foods. Here, we used telemetry and faecal metabarcoding on blue tits from one urban and one forest populations to compare parental effort and comprehensively describe nestling diet. Urban parents travelled on average 30% further than those in the forest, likely to offset limited availability of high-quality nestling food (i.e. caterpillars) in cities. Metabarcoding, based on a mean number of 30 identified taxa per faeces, revealed that the diets of urban chicks were nonetheless substantially shifted to include alternative foods. While in the forest caterpillars comprised 82 ± 11% of taxa provisioned to nestlings, in the city they constituted just 44 ± 10%. Pre-fledging chick mass as well as offspring numbers were lower in urban than in forest-reared broods. Thus, at least in our comparison of two sites, the hard labour of urban parents did not fully pay off, suggesting that improved habitat management is required to support urban-breeding birds.
Highlights
In-depth studies of the ecology and fitness of urban fauna often focus on birds because they are encountered in cities (e.g. Chamberlain et al 2009; Isaksson 2015; Glądalski et al 2017; Narango et al 2018; Pollock et al 2017; Seress et al 2018)
Forest blue tits bred in 124 nestboxes in mixed deciduous, oak-dominated woodland surrounding the Scottish Centre for Ecology and the Natural Environment, on Loch Lomond, Scotland (56°7.5′ N, 4°37′ W; 280 total nestboxes; Pollock et al 2017; Supplementary Methods)
We report reproductive outcomes for the 130 nonfocal broods in our urban and rural study sites and for the 15 tracked broods used for radio telemetry and metabarcoding
Summary
Chamberlain et al 2009; Isaksson 2015; Glądalski et al 2017; Narango et al 2018; Pollock et al 2017; Seress et al 2018). Urban adapters are of particular interest for efforts to counteract biodiversity loss because populations in urban areas often have lower reproductive success than those in more natural environments (e.g. smaller clutch size, more nest failures and lower nestling weight; Mennechez and Clergeau 2006; Chamberlain et al 2009; Seress et al 2012; Pollock et al 2017). Identifying the drivers of reproductive success in urban birds could allow for targeted management of urban environments to counteract such negative effects
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