It's good news for birdwatchers - species are being sighted in regions where they have never before been spotted, or at times when they would not normally be seen.

But changing patterns in avian migration, due at least in part to climate change, could make birds more vulnerable to extreme weather events and mismatches in availability of their food, experts say.

More bird-watching opportunities

Matt Williams, climate change policy officer at the United Kingdom's Royal Society for the Protection of Birds, highlighted how the bird-watching landscape in the UK is changing.

Sightings of birds that had never been seen before in some regions in the UK are certainly exciting for wildlife observers, said Williams. Sites that were previously not suitable for bird-watching are becoming more so.

The Dartford Warbler has spread into southern England

"The Dartford warbler has spread to more sites in south England in just a few years," Williams said. "Heathland in the UK, which might not have been suitable 30 or 40 years ago, is now becoming suitable."

Yet there species are also disappearing - the kittiwake gull is in decline because the cold water plankton it feeds on is moving elsewhere as the oceans warm.

Complex changes

In Europe for example, warmer winter temperatures mean that instead of moving south, birds are increasingly staying put, said Bert Lenten, an expert on migratory birds at the United Nations Environment Program (UNEP).

In Lenten's home - the Netherlands - lapwings would normally migrate to West Africa. But this year, they stayed put due to the warm winter, he told DW. "They are clever birds, if there's no need to go away and they can still find food, then they don't migrate."

Lapwings are among the birds that have been sticking around in Central Europe

Such behavior can have a severe negative impact if there were a sudden shift in temperature or an extreme weather event. This could devastate bird populations that are not adapted to such low temperatures.

Alterations in global temperature are also changing the feeding and breeding cycles of the birds themselves - along with the food they and their chicks eat. Plants and insects the birds rely on are arriving earlier in the year - while the period that chicks hatch remains the same.

Williams told DW this represents a mismatch in timing.

For example in the Netherlands, oak leaves are budding sooner than in previous years, so caterpillars are emerging sooner, then great tit chicks are hatching sooner - while sparrow hawk chicks continue hatching at their usual time. "Species are not able to catch up to each other," Williams said.

Great tits are hatching sooner - outpacing their predators

But Lenten said changing the migratory patterns of birds was often too complex to attribute to a single factor. "Climate change can be a factor, but so can access to clean water," Lenten pointed out.

Disappearing habitats

It's also clear that one of the biggest impacts climate change has is on habitats, said Lenten. "There is concern because of the impact on habitat - like some of the changing wetlands that are drying up."

Regions used as stopovers for birds such as some gulls and terns - which migrate over long distances - are also becoming more arid. "The Sahara Desert is increasing. There is the question of how long birds can fly without stopping for water and a break. Lots of research needs to be done," Lenten added.

Evaporating water sources and the spread of deserts, due also to climate change, will certainly affect birds

Prioritizing protection

A survey recently conducted by the British charity the National Trust recorded sightings of birds in particular regions for the first time ever.

Balearic shearwaters were registered as having been seen on the Norfolk coast, while other wildlife was also recorded as being sighted for the first time in more than a century in Ireland.

"The shifting nature of our shoreline means that we need to think ahead about what is happening to coastal habitats and how we might secure the future of the wildlife that lives by the sea," said David Bullock, head of nature conservation at the trust.

It is still unclear what the real impacts of these changes will really mean for bird populations as a whole, both in the countries to which they are no longer migrating, and also those where they are now staying.

Seabirds like the Balearic shearwater particularly face threats, as oceans are affected by climate change and other factors

What is clear, said Williams, is that in countries like the UK - in northern Europe, which is becoming more central for bird species - provisions for protection will have to be made.

"The government and other authorities need to make sure areas are in good condition," Williams said. "Otherwise the birds are not going to cope, they are not going to find good places to feed and breed."