Before the invasion: Quantifying the level of pre-introduction selection on two Weaver species

Captured birds go through several selective filters before escaping into the wild.

A few months ago, I published a review paper on hybridization in the Anthropocene. Part of this study quantified the most common mechanisms underlying human-induced hybridization events. It turned out that almost half of the studies I looked at reported hybrids due to the introduction of non-native species (34% intentional and 19% unintentional). These numbers indicate that the invasions of exotic species can have substantial consequences. It is no surprise that many biologists have tried to identify the factors contributing to a successful invasion. However, most of these studies focused on the later stages of biological invasions, such as establishment of escaped individuals or population expansions. The very early stages of the invasion have been largely ignored, even though there might be biases in the capture and transport of exotic species. A recent paper in the journal Evolutionary Applications addressed this knowledge gap by focusing on two avian invaders: the Black-headed Weaver (Ploceus melanocephalus) and the Yellow-crowned Bishop (Euplectes afer).

Selection Filters

At the start of the study, Adrián Baños‐Villalba and his colleagues accompanied several professional bird trappers in Senegal. After catching the birds with traditional clap nets, the researchers took several morphological and behavioral measurements and marked each bird with a unique set of numbered rings. This allowed them to follow the individual birds through the different steps from capture to sale, and determine any selection pressures along the way. After spending about one week in traditional storage cages, the birds were transported over 350 kilometers to a bird-trading company in Dakar. According to this company, the birds were typically kept there for one to three months before export. So, the researchers quantified survival rates thirty days after arrival in Dakar. By comparing the morphological and behavioral traits of the surviving individuals at these different stages, it was possible to estimate the strength of selection on certain traits.

An overview of potential selective filters acting during the pre-introduction stages of an invasion process. From: Baños‐Villalba et al. (2021) Evolutionary Applications.

Micro-evolutionary Changes

The analyses revealed significant changes in several morphological and behavioral traits. During the capture stage, for instance, individuals with a smaller head volume (and perhaps smaller brains) were more likely to be caught. This might reflect some variation in cognitive features and the ability to escape. Later on, however, selection seemed to favor larger brains, possibly because these birds could cope better with novel situations. This example nicely illustrates how pre-introduction selection shapes the captive population and can potentially affect the future establishment success of escaped birds. The authors highlight the relevance of this finding at the end of the discussion:

However, as our results suggest, introduced populations may have already undergone micro-evolutionary changes (assuming traits are heritable) through selective filters (even when these are artificial and human-induced) before reaching the establishment stage, and this likely affects all subsequent changes involved in the adaptation to a new non-native area in the subsequent stages.

These insights can be used to better understand the mechanisms behind successful invasions, and to take appropriate management measures. And this study is also very relevant from a hybridization perspective (the main topic of this blog). While working on this blog post, I came across this paper on mussels: “Pre-introduction introgression contributes to parallel differentiation and contrasting hybridization outcomes between invasive and native marine mussels”. The unchartered territory of pre-introduction selection is waiting to be explored.


Baños‐Villalba, A., Carrete, M., Tella, J. L., Blas, J., Potti, J., Camacho, C., Diop, M. S., Marchant, T. A. & Edelaar, P. (2021). Selection on individuals of introduced species starts before the actual introduction. Evolutionary Applications14(3), 781-793.

Featured image: Black-headed Weaver (Ploceus melanocephalus) © Francesco Veronesi | Wikimedia Commons