European breeding phenology of the invasive common waxbill, a sub-Saharan opportunistic breeder

Biological invasions may involve species colonising different climatic regions than those in their native ranges, and it is not straightforward to predict how breeding phenology changes in the invasive ranges. The common waxbill Estrilda astrild is one of the most widespread invasive birds worldwide. It is an opportunistic breeder, adapted to transient breeding opportunities in its native sub-Saharan African range, which is often climatically unpredictable. The least equatorial range of invasive waxbills is now in Europe, in the Iberian Peninsula, where they experience predictably seasonal climate. Previous reports of waxbill breeding phenology in two different regions in Iberia show a very long breeding season, but differ in whether or not peak breeding coincides with the Spring breeding of native passerines. Using field data from over 20 sites across climatically different regions in their Iberian range, we show that waxbills have a long breeding season extending until Autumn, but with a clear Spring peak around May, both in regions with hotter and milder summers. Nest monitoring in a large mesocosm with over 50 waxbills, across 3 years, confirmed these field observations and showed what appears to be year-to-year plasticity in phenology, which may include smaller nesting peaks outside Spring. This behavioural flexibility, together with the long breeding season of waxbills in the Iberian temperate climate, likely facilitates their invasion success.