This blog post is provided by Lucyna Halupka and tells the #StoryBehindThePaper for their article “Fitness consequences of longer breeding seasons of a migratory passerine under changing climatic conditions“, which was recently published in Journal of Animal Ecology

Most changes in the world have opposing effects, positive or negative, on different individuals or groups. This concerns for example the current situation during the Covid-19 pandemic, as well as climate change. Many organisms suffer due to climate change, but some of them benefit. In our new paper published in Journal of Animal Ecology we describe the case of a winner of climate change: a passerine bird, the Eurasian reed warbler.

The Eurasian reed warbler is a small bird (12 g, about 2-3 times smaller than a house sparrow) breeding in reedbeds of Europe and Asia, and wintering in sub-Saharan Africa. The birds have a relatively long breeding season (May-August) but suffer high nest losses, and frequently re-nest laying up to 4 clutches annually. Reed warblers are monogamous and both parents care for the young. However, they sometimes divorce and hence we focused on breeding data for females only.

Our earlier research provided evidence that reed warblers started breeding earlier during the last few decades, which corresponded with the rise in ambient temperatures during the breeding season – such an effect is often observed in many bird species. However, the end of laying did not change which resulted in the lengthening of the breeding season of our study population. This observation inspired us to ask a question about fitness consequences of longer breeding seasons.

Theoretically a longer breeding season should result in, so called, ‘higher re-nesting potential’, or higher chances for laying a replacement clutch (when the first one has been lost) or a second brood (when the first clutch has been successful). For example, during a very short summer (and breeding season) in the Arctic birds never have second broods and only rarely lay replacement clutches. We were interested to know whether a longer breeding season of our population of reed warblers corresponded with longer breeding seasons of individual females (this was not obvious, as breeding of reed warblers is very asynchronous in the study population, and an average female stays in the study area much shorter that the whole population), more clutches laid by individual females during the breeding season, and higher proportion of second broods (the ones laid after successful rearing of the first brood). Because more clutches should result in more young, we also wanted to learn whether the annual number of young produced by a breeding female increased when breeding seasons became longer.

To answer these questions we collected the breeding data of colour-ringed females in the current century (2005–12) and compared them with the data of females nesting on the same study plot (“Stawy Milickie” nature reserve in south west Poland) in the 1980s (1980–83).

We found that the longer breeding season of the whole population in the current century indeed was associated with longer seasons of individual females (the extension was two weeks). Surprisingly, longer breeding season did not result in more clutches laid annually by a breeding female: in both study periods an average female laid two clutches. However, annual productivity (the total number of young that successfully left all the nests of a female) increased enormously: in the 2000s females produced 75% more fledglings annually than females in the 1980s (2.8 vs 1.6, respectively). Furthermore, the proportion of females raising second broods (the one laid after successful raising young in the first brood) increased from 2.7% to 23.6% between the first and the second study period. At the same time the share of females that did not produce any young annually decreased from 48.1% to 15.5%. The higher offspring production in recent years was related to higher nest success in the 2000s and an earlier start of breeding, which secured more time to re-nest.

We conclude that climate change may prolong reproductive seasons in some species (mainly multi-brooded ones) resulting in higher productivity. Such species may benefit from climate change. On the other hand, climatic changes occurring in wintering areas or stopover sites may result in lower survival.

Read the paper

Read the full paper here: Halupka, L., Borowiec, M., Neubauer, G. and Halupka, K. (2021), Fitness consequences of longer breeding seasons of a migratory passerine under changing climatic conditions. J Anim Ecol. Accepted Author Manuscript. https://doi.org/10.1111/1365-2656.13481