ockdown has sparked a renewed interest in our garden birds, with millions of us enjoying watching them from our windows. But could some species – including the common and familiar great tit – vanish from Britain’s gardens by the end of the century? Researchers from the Norwegian University of Science and Technology, working with the University of Oxford, have modelled how great tits are reacting to the climate crisis. Specifically, are the birds able to respond to the earlier emergence of the caterpillars on which they feed their chicks? Birds such as great tits have evolved to time their breeding cycle so it coincides with the peak of moth caterpillars that feed on oak leaves, which traditionally happens in late May and June. But as temperatures rise, so oaks are coming into leaf earlier, and the caterpillars have responded by hatching out earlier too. This means that when the great tit chicks are ready to be fed, the peak of caterpillars is already coming to an end. Because the parent birds need to find 1,000 caterpillars every day for their hungry offspring, any mismatch is likely to dramatically reduce breeding success. The researchers found that although the birds can respond to climatic shifts, they are not doing so quickly enough. Lead author Emily Simmonds estimates that the tipping point comes when oak leaves, and their associated caterpillars, appear 24 days earlier than usual. The discovery that birds can and do respond to climate change by breeding earlier than normal was first made in the 1990s by Dr Humphrey Crick, a scientist working at the British Trust for Ornithology. He was analysing thousands of cards from the BTO’s long-running Nest Record Scheme, which had been filled in by amateur birdwatchers over the previous half-century, detailing the dates when eggs are laid and chicks hatch. Crick noticed a surprising trend: for many species, the date on which they laid their eggs had moved forward by an average of nine days. The resulting landmark paper, “UK birds are laying eggs earlier”, published in the journal Nature in 1997, provided some of the earliest empirical evidence that wild creatures were already responding to a warming climate. A decade later, in 2006, I remember Bill Oddie introducingd Springwatch with the astonishing news that every blue tit nest they were monitoring had already fledged young – several weeks earlier than usual. Because blue tits only have one brood, they must respond very rapidly to changes such as earlier springs. If they fail to do so quickly enough, their numbers will plummet. At the end of his 1997 paper, Humphrey Crick made this prophetic comment: “For birds, earlier nesting could be beneficial if juvenile survival is enhanced by a prolonged period before winter. Conversely, birds may be adversely affected if they become unsynchronised with the phenology of their food supplies.” Less than a quarter of a century later, both parts of that prediction appear to be coming true. In the short term, a longer breeding season has benefits, especially fr birds such as the robin, blackbird and song thrush, which produce two or more broods of young. Starting to nest earlier in the year might allow them to squeeze in an extra brood, and so produce more offspring in total. Professor James Pearce-Higgins, the BTO’s director of science, points out that our smallest birds, such as goldcrests, wrens and long-tailed tits, are benefiting from another aspect of climate change: the much milder winters of recent years. He also points to the positive impact of our habit of feeding garden birds, which helps species such as blue tits, great tits and goldfinches. At present, he suggests that the advantages of higher winter survival rates outweigh the failure to synchronise with the spring food supply, though that may not always be so. Another climate-driven success is the way many species are now expanding northwards. The latest European Breeding Bird Atlas reveals that, on average, the ranges of Europe’s breeding birds have shifted north by 28km (17.5 miles) since the original survey was done in the late 1980s – almost 1km every year. This may not sound like much, but over time it will allow species once confined to continental Europe to cross the Channel and colonise the UK. Indeed, given that some species respond much faster than others, several (including cattle egrets and great white egrets) have already done so. But as our climate becomes less predictable, with more extreme weather events such as storms, droughts and floods, what scientists have called the “honeymoon period” will come to an abrupt end. As Professor Pearce-Higgins notes, ground-feeding birds may not be able to cope with prolonged summer droughts, which make it harder for them to find food: “One potential exception to this positive picture of warmer temperatures is thrushes and blackbirds, which rely on soil invertebrates. We know, from a study we have run recently asking schoolchildren to count earthworms in school playing fields, that the availability of worms – a major food source for many species – declines significantly in summer, particularly when it is dry.” So, as we stand on the precipice of a runaway warming world, the future for many of our best-known and best-loved birds remains in the balance.