A swan remained still on a frozen lake for hours. Last week, a couple walking their dog near Schlachtensee in Berlin noticed it and thought the bird was trapped, probably a victim of the sudden cold snap that hit central Europe in early February. They even considered going onto the ice, not knowing that the swan was likely already dying. It’s also possible they didn’t realize that their approach could worsen the situation, not just for that swan but for many others as well.
This winter, a harsh combination of natural selection and virus dynamics is unfolding across Northern Hemisphere lakes. The sight of seemingly frozen birds raises public concern, yet the reality beneath this is much more intricate and significant than the fate of any one animal. Decisions made now—whether to intervene or step back—impact everything from local bodies of water to global poultry markets and the ongoing development of a virus that has drastically altered the life and death of wild birds.
When Cold Becomes a Viral Amplifier
The scenes observed in lakes across Europe and North America are just the visible signs of a larger ecological shift. The highly pathogenic avian influenza H5N1, identified as a global risk nearly three decades ago, has transitioned from a periodic threat to being considered endemic in waterfowl. It now travels continuously through migratory pathways and winter habitats, according to data collected from various states and European veterinary sources.
The frigid temperatures that gripped the continent in early February didn’t cause this issue, but they certainly intensified it. Cold weather helps the virus survive longer in the environment, particularly on ice and in frozen dead animals. Furthermore, cold stress acts as a filter, naturally weeding out weaker individuals, a concept ornithologists term “winter selection.” Virologists now see this as a mechanism that concentrates infections among more vulnerable populations.
In urban waters of Berlin, this pattern becomes clear. Reports show that a noticeable number of wild birds found dead this winter tested positive for avian influenza, with swans representing a large portion of those cases. This aligns with broader findings: waterfowl serve as natural virus reservoirs, shedding it during migration, and clustering on dwindling open water creates optimal conditions for transmission.
The Physiology of Apparent Helplessness
The instinct to rescue these birds stems from a misunderstanding of how they respond to cold. What looks like distress—an immobile bird—can often be a calculated energy-saving tactic.
Birds wintering in temperate climates use various physiological adaptations that make them more resilient than their stillness suggests. For instance, special heat exchange systems in their legs help maintain their body temperature even when standing on ice. So while a duck might have cold feet, its core stays warm.
Additionally, a gland secretes an oil that waterproofs feathers, preventing ice from forming on them. Research from the Leibniz Institute for Zoo and Wildlife Research shows that this oil provides essential protection from the cold.
When birds do freeze, it’s typically considered a postmortem observation rather than a direct cause of death. Often, these birds were already sick, starving, or injured before succumbing to the cold, making freezing the final chapter in a long struggle with illness.
Feeding as a Transmission Mechanism
One of the more surprising aspects of current guidelines is regarding feeding. Historically, park-goers have fed ducks, often viewing it as an innocent act of kindness. But now, such practices create a public health risk based on current epidemiology.
Feeding waterfowl leads to overcrowding. Ducks and swans that would normally spread out gather tight around food, sharing respiratory droplets and contaminating surrounding areas with droppings.
In areas with high rates of virus prevalence, this clustering turns a natural winter gathering into a situation where, as noted by researchers, “the risk of infection for each animal can increase dramatically.”
Berlin’s Landesjagdgesetz, or State Hunting Law, already prohibits this type of feeding, and veterinary authorities across several jurisdictions are reinforcing this guideline. This isn’t merely about the poor nutritional value of bread; it’s more about the mechanics of viral transmission in concentrated groups.
Reporting Versus Rescuing
When people come upon a seemingly stranded bird, the suggested response might feel counterintuitive. Officials recommend keeping distance and avoiding touching any sick or deceased creatures. There are two key reasons: personal safety—since avian influenza poses a zoonotic transmission risk—and the need for epidemiological monitoring, as reporting dead birds can provide early warning signals about the virus’s spread.
Residents encountering dead waterfowl, raptors, or corvids should report them to the local veterinary offices, who decide on testing and arrange for removal when possible. For instance, in Massachusetts, there’s an online form for reporting wild bird mortality. In Pennsylvania, authorities recommend that flock owners eliminate standing water to avoid attracting wild birds.
It’s also advised that dogs be kept leashed near bodies of water, as they can catch the virus and carry it back home on their fur or paws, a pathway that has been documented in previous outbreaks.
The Shifting Epidemiology of H5N1
This winter’s bird mortality should be viewed against a backdrop of significant changes in viral behavior. Analysis of data from the Department of Agriculture shows that January 2025 marked the worst month for poultry losses, with 2025 overall experiencing higher losses than 2024, which in turn outstripped 2023. Commercial layers bore the brunt of these losses, accounting for about 75%, while turkeys were about 11%.
These figures only reflect verified commercial operations; wild bird mortality is likely underreported. Nevertheless, surveillance confirms increased carcass numbers, especially among swans and raptors. The virus is dynamic; although scientists tracking genetic sequences indicate that the circulating strain hasn’t yet mutated for efficient human transmission, the volume of viral replication in bird populations raises stochastic probabilities.
