A pair of Common House Martin breeding in winter in south-west Spain.
Common House Martin (Delichon urbicum) is a long-distance migratory species whose Palearctic populations are wintering mainly in Africa south of the Sahara. Some birds in the Iberian Peninsula and Morocco don’t migrate and remain in the breeding areas throughout the year.
However, to not migrate and stay home in winter is one thing, but to breed in winter is totally abnormal for this species.
The photo above was taken at Hornachos, Extremadura, south-west Spain, in early January 2019. It shows an adult House Martin feeding unfledged young in the nest. The chicks were born in December 2018 according to the Spanish organization AMUS (Acción por el Mundo Salvaje) who shared the photo.
As we can see, the birds already succeeded in their first challenge (lying and hatching the eggs), and their only remaining challenge is to raise the young until fledging (and then to ensure juvenile survival until the summer). If they are bold enough to breed in winter, they can succeed!
Population-level behaviour vs. isolated cases
At the population level, breeding in autumn and winter is common in desert areas because it’s an adaptive behaviour to exploit ephemeral resources (whenever they are available). For examples, see this blog-post about autumnal breeding by desert birds and the references mentioned therein.
The House Martin case is different because this species is normally a trans-Saharan migrant (unlike desert birds, which are residents/nomadic). Moreover, it probably involved only a single pair, which makes it likely an anomaly (for now).
However, every novel behavior starts to take hold at the population level if it’s advantageous to the species survival. Remember the Eurasian Blackcaps (Sylvia atricapilla) from Central Europe that winter in Britain instead of their traditional wintering grounds in the Mediterranean area? These pioneer Blackcaps established a new migratory route and new wintering area within less than 30 years, and all this is genetically coded (inherited). See the following references for example.
Only time will tell if the winter breeding of the House Martin is an indication of something (an adaptation to climate warming?) or just an isolated case.
Berthold, P., Helbig, A. J., Mohr, G. & Querner, U. 1992. Rapid microevolution of migratory behaviour in a wild bird species. Nature 360: 668–670.
Helbig, A. J., Berthold, P., Mohr, G. & Querner, U. 1994. Inheritance of a novel migratory direction in Central European blackcaps. Naturwissenschaften 81: 184–186.