Climate Change and Pine Martens Tangle Goldeneye's Breeding Clock
In the northern boreal lakes of Finland, the Common Goldeneye (Bucephala clangula) has long been a harbinger of spring—arriving with the thaw to stake out nesting sites in tree cavities and artificial nest boxes. But new research reveals that this ritual is undergoing subtle but significant shifts, as warming springs and the ever-watchful eyes of pine martens alter the rhythms of reproduction.
A 28-year study led by Hannu Pöysä, published in the Journal of Avian Biology, provides the most detailed picture yet of how climate change and nest predation combine to affect not only when goldeneyes breed, but how many of their ducklings survive to independence. Crucially, the study analyses both successful and failed nests—offering a more nuanced view than previous work based only on successful breeding attempts.
Ice-Out and Lay-In: A Climate-Driven Advance
The timing of ice breakup on boreal lakes—an indicator of spring onset—was found to be a primary driver of when breeding begins. Over the nearly three-decade span, goldeneyes have advanced the start of their breeding season by an average of 13 days.
“This is a clear signal of climate change in action,” notes Pöysä. “Goldeneyes rely on open water not just for feeding but for courtship and nesting. Earlier ice-out means earlier laying.”
However, while the start of breeding is shifting, the end is not keeping pace. As a result, the breeding period is lengthening, albeit weakly—contrary to some earlier findings that suggested contraction.
The Shadow of the Pine Marten
Interestingly, nest predation—particularly from pine martens—plays a greater role in shaping the end of the breeding season. Females that lose their nests are more likely to attempt later renesting or switch to parasitic laying strategies, which extends breeding into the summer.
Moreover, nest predation in one year influences breeding phenology in the next. Successful females often return to their previous nest sites and breed early, while those who were predated must search anew—delaying the process.
These subtle lag effects underscore the interplay between local ecological pressures and broader climatic trends.
Fewer Ducklings Taking Flight
Perhaps the most sobering finding is that while the nest-stage success (eggs producing fledglings) is mainly affected by predation, final reproductive success—the number of ducklings surviving to independence—declines with earlier springs.
“This suggests a mismatch,” explains Pöysä. “Earlier laying may lead to hatching during suboptimal weather. Newly hatched ducklings, while hardy, are still vulnerable to cold snaps and food shortages.”
Increased lake browning and perch competition may also be reducing the availability of aquatic invertebrates—critical food for ducklings.
Rethinking Breeding Metrics
Pöysä’s findings also challenge conventional wisdom on how bird breeding is tracked. Many long-term datasets, such as those based on nestling ringing, exclude failed nests. This may mask true trends in breeding timing and duration, particularly in species with complex renesting behaviour like goldeneyes.
For ecologists, the takeaway is clear: local pressures like predation must be considered alongside global changes when interpreting phenological trends. For conservationists, it’s a reminder that the effects of climate change are not uniform—and may manifest most acutely during the hidden weeks after hatching.
3 Apr 2025
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