Effect of long-term temperature changes at non-breeding grounds on phenological shifts in spring migration timing of male and female blackcaps on the southern Baltic coast
This study analyzes 43 years of ringing data to reveal that while both male and female Eurasian blackcaps on the southern Baltic coast have advanced their spring migration timing in response to warming temperatures in Europe and Africa, sex-specific protandry persists only among early arrivals due to male competition, whereas later migration phases have stabilized despite continued warming.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Every spring, millions of birds embark on a journey from their winter homes to the northern forests where they will raise their young. For decades, scientists have watched how these travelers adjust their schedules as the world warms. A key concept in this field is "protandry," a fancy word for a simple pattern: males often arrive at the breeding grounds before females. This happens because early arrival gives males a better chance to claim the best territories and attract mates, while females often prioritize saving energy for egg-laying and may wait for warmer, safer weather. As temperatures rise across Europe and Africa, researchers have long wondered if these two sexes are reacting differently. Do the males speed up their journey more than the females, or do they both shift their timing in the same way? Understanding this helps us see if birds can keep up with a rapidly changing climate or if they are reaching a limit where their natural rhythms can no longer match the seasons.
A team of researchers from the University of Gdańsk in Poland set out to answer these questions by studying the Eurasian blackcap, a small, common songbird. They spent forty-four years, from 1982 to 2025, catching and recording these birds as they passed through a ringing station on the southern coast of the Baltic Sea. This location serves as a major checkpoint for birds traveling from wintering grounds in Africa and southern Europe to their breeding sites in Scandinavia and the Baltic region. The scientists recorded the exact dates when male and female blackcaps arrived, noting whether they were adults or young birds. They then compared these arrival dates against temperature records from five different regions: wintering areas in Western and Eastern Africa, and stopover zones in the Western and Eastern Mediterranean, as well as Central Europe. By breaking their long dataset into two distinct periods, the early decades and the recent ones, they could see if the birds' reactions to warming had changed over time.
The study revealed a clear and consistent pattern in how the birds responded to the climate. Over the entire forty-four-year period, both male and female blackcaps arrived about nine days earlier than they did in the 1980s. Crucially, the males maintained their lead, arriving on average just over two days before the females, and this gap remained steady throughout the decades. This suggests that the pressure for males to arrive early to secure territories is a powerful force that persists regardless of how much the weather warms. However, the story changes when looking at the middle and end of the migration season. While the first wave of birds and the middle group shifted their arrival dates earlier during the first part of the study, this trend stopped completely in the second period, from 2005 to 2025. The birds simply stopped getting any earlier, even though the world continued to warm.
The researchers found that the timing of these arrivals was tightly linked to temperatures in specific places. The earliest birds, which include the most competitive adult males, seemed to use an eastern route. Their arrival was influenced by warm springs in Eastern Africa, the Eastern Mediterranean, and Central Europe. When these regions were warm, the birds moved faster and arrived sooner. In contrast, the last group of birds to arrive, which consists mostly of young and inexperienced individuals, reacted differently. Their arrival was delayed when winters in the Western Mediterranean were warm. The scientists suggest that a warm winter there causes plants and insects to appear too early, so by the time these late-arriving young birds get there, the food they need to refuel has already peaked and disappeared. Struggling to find enough to eat, they are forced to stay longer on their journey, pushing their arrival date later.
Perhaps the most significant finding is that the birds appear to have hit a biological wall. The middle group of migrants, which makes up the bulk of the population, advanced their arrival by ten days between 1982 and 2004, but this shift halted entirely in the following two decades. The researchers suggest that the birds cannot advance their schedule any further without risking a dangerous mismatch with the food available at their breeding grounds. If they arrived too early, they might face cold snaps or a lack of insects for their chicks. This stabilization suggests that while the blackcaps have shown remarkable flexibility in the past, they may now be reaching the limit of how much they can adjust their migration timing. The study indicates that both sexes respond to temperature changes in the same way, and that the complex interplay of winter conditions and spring warmth along their route dictates their speed. As the climate continues to change, these birds may face a future where their natural ability to adapt is no longer enough to keep them in sync with the seasons.
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