An analysis of passerine egg traits across the city mosaic: Urbanisation does not affect egg size and pigmentation patterns

Ignacy Stadnicki , Michela Corsini, Klaudia Szala, Andrew Gosler, Marta Szulkin

1. Rapid urbanisation provides remarkable opportunities to study how sudden, extreme changes impact wildlife. Compared to natural areas, cities are characterised by factors affecting both abiotic (e.g. climate, pollution, habitat fragmentation) and biotic (e.g. Normalized Difference Vegetation Index (NDVI), species composition, phenology) components of the ecosystem, ultimately changing the ecological and evolutionary dynamics of those habitats. Analogously to many other taxonomic groups, urban birds differ from rural birds in morphology, behaviour and reproductive patterns. Yet potential links between urbanisation and avian egg traits — a key aspect of the avian life-cycle — remain under-researched. Given the limited availability of primary natural calcium sources (snails) in cities, eggs from heavily urbanised areas were expected to be smaller and more pigmented, indicating thinner shells and lower overall egg quality.

2. To better understand how urbanisation affects eggs, data on 718 great tit (Parus major) eggs from 90 clutches, spread across eight study sites in a city mosaic, were collected for two breeding seasons. All clutches were photographed using a standardised approach, and further analysed using digital imaging and visual scoring to assess egg volume and pigmentation patterns. Urbanisation was quantified as the percentage of Impervious Surface Area (ISA) in the vicinity of each clutch via satellite imagery.

3. In line with some of the earlier studies conducted on semi-natural bird communities, egg volume covaried with lay date and female body condition, while for both egg volume and egg pigmentation (spots percentage) — a year effect was detected. However, in contrast with predictions, there was no association between urbanisation and the examined egg traits.

4. While urban clutches are consistently smaller, this study shows that eggs as such are similar to those found in rural habitats in terms of volume and pigmentation patterns. These results suggest that urban-driven environmental pressures may not be as strong or directional during the egg laying phase as they are at later stages of reproduction, and highlight the complexities of urban ecological and evolutionary dynamics.