Differences in the timing of reproduction between urban and forest European blackbirds (Turdus merula): result of phenotypic flexibility or genetic differences?

Abstract

Species which have settled in urban environments are exposed to different conditions from their wild conspecifics. A previous comparative study of an urban and a forest-living European blackbird population had revealed a three weeks earlier onset of gonadal growth in urban individuals. These physiological adjustments are either the result of genetic differences that have evolved during the urbanization process, or of phenotypic flexibility resulting from the bird's exposure to the different environmental conditions of town or forest. To identify which of these two mechanisms causes the differences in reproductive timing, hand-reared birds originating from the urban and the forest populations were kept in identical conditions. The substantial differences in the timing of reproduction between urban and forest birds known from the field did not persist under laboratory conditions, indicating that temporal differences in reproductive timing between these two populations are mainly a result of phenotypic flexibility. Nevertheless, urban males initiated plasma luteinizing hormone (LH) secretion and testicular development earlier than forest males in their first reproductive season. Moreover, plasma LH concentration and follicle size declined earlier in urban females than in forest females, suggesting that genetic differences are also involved and might contribute to the variations in the timing of reproduction in the wild.

The nest-site characteristics of the forest population of common blackbird(Turdus merula) in Eskişehir, Turkey

“…This theory could be backed up with the fact that pairs in the aforementioned forest population had similar nest characteristics. This study also reveals the behavioral and genetic differences between the forest and urban populations for common blackbird, which are often associated with different ecological factors in a variety of environments (Partecke et al, 2004;Partecke et al, 2006). Some of the common blackbird populations found in Turkey shows seasonal migration inside the country, but others do not migrate during the winter (Kiziroğlu, 2008).…”

Mechanisms Associated with an Advance in the Timing of Seasonal Reproduction in an Urban Songbird

“…To date, there is limited evidence from studies that have compared the endocrine axes of urban vs. nearby rural populations in both field and common garden settings to support the importance of both plasticity and genetics in modulating earlier reproduction and HPG activation in urban habitats (Partecke et al, 2004(Partecke et al, , 2006Schoech, 2009;Atwell et al, 2014;Davies et al, 2015). While these studies highlight the importance of differential HPG activity in facilitating earlier and extended breeding in urban habitats, much remains to be learned about the generality of those results, as well as the specific mechanistic sources of variation across the many "levels" of the HPG axis.…”

Independent colonization of multiple urban centres by a formerly forest specialist bird species

“…This leapfrog model of urban colonization is particularly plausible in migratory or partially migratory species, such as the blackbird, in which occasional long-distance dispersal events of over 300 km have been recorded, even though natal and adult dispersal distances are typically much shorter than this, at approximately 3 km (Paradis et al 1998). While plasticity may sometimes be involved in the colonization of urban areas (Slabbekoorn & Ripmeester 2008), urban blackbirds exhibit some adaptations to city life, which have a partly genetic basis, including changes in stress responses, migratory tendencies and reproductive cycles (Partecke et al 2004(Partecke et al , 2006Partecke & Gwinner 2007). Therefore, urban blackbirds may be more likely to possess the necessary traits for colonization of unoccupied urban areas than rural blackbirds.…”