Growing in the city: Urban evolutionary ecology of avian growth rates

Abstract

Introduction: Rapid environmental change driven by urbanization offers a unique insight into the adaptive potential of urban-dwelling organisms. Urban-driven phenotypic differentiation is increasingly often demonstrated, but the impact of urbanization (here modelled as the percentage of impervious surface (ISA) around each nestbox) on offspring developmental rates and subsequent survival remains poorly understood. Furthermore, the role of selection on urban-driven phenotypic divergence was rarely investigated to date.

Methods and results: Data on nestling development and body mass were analysed in a gradient of urbanization set in Warsaw, Poland, in two passerine species: great tits (Parus major) and blue tits (Cyanistes caeruleus). Increasing levels of impervious surface area (ISA) delayed the age of fastest growth in blue tits. Nestling body mass was also negatively affected by increasing ISA 5 and 10 days after hatching in great tits, and 10 and 15 days in blue tits, respectively. High levels of ISA also increased nestling mortality 5 and 10 days after hatching in both species. An analysis of selection differentials performed for two levels of urbanization (low and high ISA) revealed a positive association between mass at day 2 and survival at fledging.

Discussion: This study confirms the considerable negative impact of imperviousness-a proxy for urbanization level-on offspring development, body mass and survival, and highlights increased selection on avian mass at hatching in a high ISA environment.

Keywords: birds; body mass; growth rate; imperviousness; phenotypic divergence; selection; urbanization.

Figure 1

Study sites in a gradient of urbanization in the capital city of Warsaw, Poland. These include: a suburban village (a), a natural forest (b), two residential areas (c and g), two urban woodlands (d and e), an office area (f) and an urban park (h). The white dot indicates Warsaw city centre (Palace of Culture and Science). The layer visualized on the map is the original raster file used for the analyses described in section 2.2

Figure 2

Nestling growth curves in high (black) and low (green) ISA environments for great tits (left panel) and in blue tits (right panel). Only nestlings that survived until day 15 (or day 14 or 16 in the case of a few broods, which could not be accessed on day 15) were included in this visualization. Dots refer to original mass measurements averaged by brood to control for non‐independence. Curves were drawn using "method = 'gam', formula = y ~ poly (x, 3)" in ggplot2. For visual clarity, ISA was categorized as low and high, specifically: low ISA—mean of 0.64% ISA for great tits and 1.71% ISA for blue tits; high ISA—mean of 13.3% ISA for great tits and 16.83% ISA for blue tits; (n = 15 great tit broods and n = 27 blue tit broods)

Figure 3

Standardized selection differentials (± standard error) on mass shortly after hatching (day 2) in low and high ISA environments for great tits (left panel) and blue tits (right panel). Fitness is computed as survival at fledging. Both the response variable (fitness) and all explanatory, continuous variables were standardized at a yearly level. ISA values were categorized as low ISA: mean of 0.99% ISA for great tits and 2.12% ISA for blue tits; high ISA: mean of 25.85% ISA for great tits and 27.22% ISA for blue tits. Selection differentials confirm positive selection on mass shortly after hatching, and the strength of selection on mass increases with increasing levels of ISA in great tits

Figure 4

Urbanization, measured as the proportion of Impervious Surface Area around a point of interest, negatively impacts body mass and survival in 2 developing passerine birds. At the same time, body mass shortly after hatching positively increases survival till fledging, particularly in high ISA environments