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Migratory birds can learn to adjust to early spring

Climate change has brought about various significant shifts in the global environment, and one such change has been the early onset of spring. This phenomenon is impacting migratory birds, which are struggling to keep pace with the changes in their natural environment. 

In Sweden and its surrounding regions, migratory birds are arriving later than the peak availability of caterpillars, their primary food. 

Flight adjustment

Researchers from Lund, Sweden, and the Netherlands have found a potential solution to this pressing issue: encouraging migratory birds to fly further north. 

The experts believe that by directing these birds slightly further north, it could offer their offspring a more favorable start in life.

“We thought that perhaps the migratory birds could fly further north until they find a place with suitable well-developed caterpillars,” said study co-author Jan-Åke Nilsson from Lund University.

Declining food supply

Due to warmer springs, caterpillars now hatch, grow, and enter the pupal stage sooner than they did a few decades ago. This poses a significant challenge for birds that rely on these caterpillars as a primary food source, as they cannot consume them once they’ve entered the pupal phase. 

The declining food supply leads to an increasing number of chicks starving during their crucial breeding season. This issue is particularly pronounced for migratory birds that winter in Africa, as they remain unaware of the early spring onset in places like Sweden.

Studying the impact of early spring

While non-migratory birds seem to adjust to this change to some extent due to their continuous presence and innate ability to sense early spring’s arrival, migratory birds don’t share this advantage.

To investigate whether migratory birds could simply fly further north to find caterpillars, the research team embarked on an experiment with Pied Flycatchers. 

These birds, which had arrived pre-breeding in the Netherlands, were captured and transported overnight to Vombs Fure, a pine forest region in Skåne. Notably, this area experiences its caterpillar food peak about two weeks later than the Netherlands. 

Promising results

Given the Pied Flycatcher’s ability to cover the 600-kilometer distance from Lund to Skåne in just two nights, the results of the experiment seem to be promising.

“The birds that were given a lift from the Netherlands to Skåne synchronized very well with the food peak! As they started to breed about 10 days earlier than the ‘Swedish’ Pied Flycatchers they had a dramatically better breeding success than the Swedish ones as well as a better success than the Pied Flycatchers that remained in the Netherlands,” said Nilsson.

The offspring of these assisted Pied Flycatchers, when returning from their maiden spring migration, bypassed the Netherlands altogether, instead making their way to their birthplace outside Lund. These chicks consequently had a higher number of well-fed chicks the subsequent year.

“The number of small birds, particularly migratory birds, has decreased drastically throughout Europe,” said Nilsson. “By flying a little further north, these birds, at least in principle, could synchronize with their food resources and there is hope that robust populations of small birds can be maintained, even though springs are arriving ever earlier.”

More about the impact of early spring

Early spring wields a substantial impact on migratory birds, influencing their migration patterns, breeding behaviors, and access to food resources. The occurrence of early spring implies a faster arrival of warmer temperatures, leading to a chain of environmental changes.

Alteration in migration timings

Firstly, early springs trigger migratory birds to commence their journeys sooner. Birds rely predominantly on the availability of food, and a premature spring ensures a quicker abundance of resources.

The change in climate cues such as temperature and food availability lead birds to alter their migration timing. For example, some species, like the Eurasian Blackcap, advance their migration schedules to exploit the early abundance of food and establish territories, and optimize breeding opportunities.

Early spring and breeding behaviors

The breeding behaviors of migratory birds also experience modifications due to early springs. Birds like the Pied Flycatcher, mentioned previously in this article, might start their breeding season earlier to synchronize with the early availability of caterpillars, their primary food source during breeding.

This alteration in breeding times ensures that the young ones have ample food, leading to higher survival rates. However, if the birds do not adjust their breeding times appropriately, they might face a scarcity of food resources, which can impact the survival rate of the offspring.

Impact on food resources

Furthermore, early spring affects the abundance and availability of food resources for migratory birds. Many birds feast on insects that are also responding to the early arrival of spring.

The insects might peak earlier, and if birds do not adjust their migration timings correctly, they can miss this abundance of food. For instance, Arctic shorebirds that fail to sync their arrival with the peak in food resources might face nutritional stress, affecting their reproduction and survival.

Ecological consequences

The impacts of early spring are not isolated to migratory birds alone. They reverberate throughout the ecological web. Predators and competitors of these birds also experience the effects of their altered behaviors. An early migration and breeding might lead to higher competition for nesting sites and food among different species, leading to shifts in species dominance and potentially altering ecosystem structures and functions.

In summary, early spring ushers in a cascade of changes that critically impact migratory birds. The alterations in migration timings and breeding behaviors are imperative for the birds to align with the availability of food resources. While some species successfully modify their schedules and reap the benefits of early springs, others might struggle to adapt, facing nutritional deficiencies and reduced survival rates.

Additionally, the repercussions of these shifts are intricate and extend to other species in the ecosystem, highlighting the interconnectedness of all living organisms. The profound impacts of early spring accentuate the importance of studying climate changes and implementing conservation strategies to protect migratory birds and maintain ecological balance.

The research is published in the journal Nature Ecology and Evolution.

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