The Science of Spring

Human beings have an innate connection to nature and seasonal cycles, even if modern lifestyles mean that we spend little of our time outdoors. Our spirits are automatically lifted when we see carpets of snowdrops appearing and the first swallows arriving back in spring as it signals warmer days to come. In meteorological terms, spring starts on 1^st March, whereas in astronomical terms, it starts between March 19^th and 21^st on the vernal equinox. This is when the sun crosses the earth’s equator, making day and night almost exactly the same length. We associate spring with hope and renewal, but for many plant and animal species it is a delicate balancing act between leaving winter dormancy at the right point, and initiating reproduction.

The study of these seasonal cycles in nature is called phenology and it provides fascinating insights into how animals and plants weigh their survival against producing successful offspring. Phenology also enables us to understand how different species are dependent on each other in complex food webs, as predators rely on peaks in seasonal prey abundance to feed their offspring. Recording phenological events such as the tree budburst and first egg dates of birds has also given us vital information on how the natural world is adapting to climate change and warming spring temperatures. These spring events are occurring earlier each year and jeopardising the seasonal cycles of many species as animals try to keep up with the changes. Phenology allows us to track these fluctuations and also to understand the complexity of seasonality within ecosystems.

Spring is one of the most fascinating times of year to observe nature, so we have taken a look at the amazing adaptations that different species groups have to enable them to make the most of spring in the UK.

What happens to trees in spring?

Our deciduous trees have spent the winter in a dormant state, without leaves but bearing freeze-tolerant buds. In early spring they switch from a deep endodormancy to a more responsive state (ecodormancy), waiting for the right combination of increasing daylength and warmer temperatures to trigger leaf budburst. Trees will not switch into ecodormancy unless they have reached a threshold number of cold days, which avoids the risk of the buds developing too early and being caught out by a late freeze, although the mechanism underlying this is unclear. Once the temperatures warm up, trees start moving the sugar that has been stored in their roots overwinter back up into their xylem for transport to the buds. The rising sap fuels the development of the buds, which is regulated by hormones that control shoot growth such as auxin and cytokinin.

Elder is one of the earliest trees to unfurl its leaves, starting as early as January, with hawthorn not far behind in February. Trees such as hazel, willows, and blackthorn flower before their leaves develop, and the clouds of delicately scented blackthorn flowers are one of the highlights of March. These early flowering species are essential sources of nectar for insects when they emerge and seeing a goat willow tree that is humming with hundreds of bees visiting the fluffy catkins is one of the highlights of spring. Most other tree species flower in April or May, with dog rose flowering in May or June. The timing of budburst and flowering is closely linked to spring temperatures, and in a warmer year, spring will start earlier, whereas in a colder year (as 2023 has been so far), spring will be delayed as the trees are prevented from developing. This leads to significant interannual variation in the timing of budburst, which has downstream effects for many animal species.

The budburst of oak is a cornerstone of woodland phenology, as many caterpillar species are dependent on the emergence of oak leaves. The relationship between oaks, winter moth caterpillars, and great tit breeding is one of the most studied phenological interactions in ecology, and provides a great example of how intricate food webs can be and how important seasonal timing is. It is critical for the caterpillars that they emerge at the correct moment to coincide with the appearance of young leaves, as the oak leaves develop unpalatable tannins as they get bigger. The great tits then need to time their breeding to this annually shifting peak in caterpillar numbers, which only lasts a few days, so that their chicks are the optimal age when the peak occurs. The complexity of this relationship is due to the fact that the timing of maximum demand in great tit chicks is set weeks before the appearance of the caterpillars when the females start incubating the eggs. There is an additional link to this food chain as sparrowhawks time their breeding to coincide with the population explosion in young blue and great tits emerging from their nest.

There are constraints on the physiology of the birds (e.g. daylength, temperature) during the egg laying and incubation period that prevent them breeding earlier, and which can lead to phenological mismatches between the predators and their prey source. The oak budburst begins in late March, with the leaves developing fully in April, but the phenological records of oak leaf emergence are one of the indicators that climate change is making spring earlier, as oak is coming into leaf three weeks earlier than in the 1950s. Although the birds have shifted their breeding season forward, in some places there is an increasing mismatch between the birds and the caterpillars, leading to reduced breeding success.

Spring flowers

Tree phenology also has impact on the appearance of woodland flowers such as wood anemones, bluebells, and wood sorrel. These ephemeral woodland flowers have to emerge before the canopy above them closes and shuts out the sunlight. This means that climate change is also affecting the timing of woodland flowers, with many flowers now emerging earlier in response to warming spring temperatures, on average a month earlier in the UK. Similar to the birds, there are constraints on how early plants can emerge as they rely on a shift in photoperiod to trigger growth. The larger the gap between the flower emergence and the tree leaf-out dates, the greater the period of time that these plants have to photosynthesise and be pollinated, so maintaining the gap between the flowers and the trees is critical. There is evidence that European flowers are succeeding in maintaining a two-week gap, although US flower species are struggling to advance as quickly as the trees. Flowers and insects are mutually dependent on each other so any shift in the timing of flowers has the potential to have a significant impact on insect emergence and survival.

The wildflowers of spring are a beautiful display of whites, yellows, and blues, starting with snowdrops in February and running into the pink hues of red campion and foxgloves in May. Although snowdrops are not native and were first reported in the wild in the 18^th century, they have naturalised here, and their hopeful appearance at the end of winter is the first sign of spring being imminent. February often sees the first primroses and native wild daffodils appearing, as well as lesser celandines, dandelions, and wood anemones. These all provide vital early sources of nectar for emerging insects in woodland and our gardens.

April and May bring one of our greatest woodland spectacles, as carpets of beautifully scented bluebells appear in woodland and hedgerows. Bluebells are one of the indicators of ancient woodland, and the UK has impressive bluebell displays because we do not have a large wild boar population digging up the bulbs. Bluebells are under threat, however, from hybridisation with the Spanish bluebell that is common in gardens, but it is easy to tell the difference as native bluebells are much more delicate, darker blue and bend over at the top rather than being straight. Woodland flowers are also under threat from poor woodland management, where insufficient light reaches the floor, which in turn has a negative impact on butterfly numbers. Well managed woodland has a mixture of dense canopy and open glades, where light can reach the ground flora. This can be achieved through selective thinning or coppicing, which replicates the natural breaks in the canopy which occur when a tree falls over. Bluebell carpets are often interspersed with the lemons, whites and pinks of yellow archangel, lesser stitchwort, and red campion flowers. Once the bluebells fade, the lanes and woods are filled with towering foxgloves, a favourite with bees.

Birds - breeding and birdsong

Many of our resident birds begin their breeding season in February by finding a mate and establishing territories, then they start looking for nest sites in March. The increase in breeding season activity is accompanied by the startup of the dawn chorus, which reaches its peak in April and May. The mistle thrush and song thrush are two of the first species to start singing, followed by blue tits, great tits, dunnocks, wrens, chaffinches, and later blackbirds. The only species that sings year-round is the robin as they maintain their territories during the winter, which is a significant investment in territory defence. Spring is the best time of year to hone birdsong identification skills as the birds are still easily visible in the bare branches, and also the best time to see elusive resident species such as lesser spotted woodpeckers.

Passerine (perching) bird species commonly show two types of nesting strategy, cavity nesting and open nesting. Cavity nesting species such as tits, sparrows, nuthatches, woodpeckers, starlings, pied flycatchers, and redstarts would naturally find a hole in a tree or building and build their nest inside, protected from predators. This has the advantage of allowing them to raise larger broods and keep their nestlings in the nest longer, but It does mean that they are limited by suitable nest sites. Modern woodland management techniques tend not to leave much standing dead wood, so nestboxes can compensate for a lack of natural cavities. In contrast, species such as robins, blackbirds, thrushes, long-tailed tits, wrens, goldcrests, bullfinches, and chaffinches build their own open nests from a variety of materials in a secluded location, commonly hidden in bramble and ivy. Their nestlings fledge much earlier at around 12 days and are usually less noisy than the cavity nesting chicks. The advantage of open nesting is that they are less limited by nest site availability and many open nesting species have multiple broods of chicks, although they are at more risk of predation. In many bird species of both nesting types, there is a seasonal decline in long-term offspring survival, with the offspring from early broods being much more likely to survive to breed than the offspring of later broods.

Migrant species such as warblers, flycatchers, swallows, martins, and swifts have additional challenges in breeding as they have to time their migration to arrive in the UK at the right moment. The summer migrants usually start arriving in April and May, with swifts being the last to arrive and the first to leave again in July or August. For cavity nesting migrants such as redstarts and pied flycatchers, their arrival after the resident species causes competition for nesting sites and there can be some deadly battles. With spring starting earlier each year because of climate change, natural selection is putting pressure on migrants to arrive on their breeding grounds and start egg laying as early as possible. Inevitably some species are able to adapt more quickly than others, and species such as pied flycatchers are already struggling to adapt.

Reptiles and amphibians

Reptiles and amphibians (herpetofauna) have been hibernating or dormant over the winter and begin emerging and accelerating their metabolism in February. Common frogs are the first to migrate to nearby ponds to start mating and producing frogspawn. If you see frogspawn in a pond it is easy to differentiate between frog and toad spawn as frogs lay their eggs in big clumps and toads in long strings. Frogs prefer more shallow bodies of water, whereas common toads migrate to their ancestral breeding grounds slightly later and prefer deeper bodies of water. The toad migration results in the amazing spring spectacle of hundreds of toads moving back to their breeding grounds. Teams of volunteers across the country assist with protecting toads on migration as they cross roads and anything in their path.

Newts emerge from their winter dormancy in March and head to a pond for the breeding season, migrating at night, when there are fewer predators around. There are three species of native British newt, the smooth newt (Lissotriton vulgaris), palmate newt (Lissotriton helveticus), and great crested newt (Triturus cristatus). Newts are a type of salamander and have a life cycle that is mostly terrestrial and aquatic only during the breeding season. The males perform elaborate courtship dances for the females, who then lay 4 or 5 eggs every day for a month. Females wrap the eggs individually in plant leaves in a painstaking process, carefully folding them with their back legs.

The eggs hatch into larvae, which can be differentiated from tadpoles by their feathery external gills and because their front legs develop first. The baby newts leave the ponds in summer at the same time as the adults, when they are known as efts, and do not return until the following spring. The great crested newt has significant legal protection, and it is an offence to damage or obstruct their habitat. There is considerable ecological surveying in the spring to assess whether they are present on sites with proposed construction projects.

Some reptile species begin emerging from hibernation as early as January if the weather is warm, although most species emerge and begin basking in April. Slow worms tend to warm up underneath things such as sheets of metal, whereas the other reptiles bask in the open. Although male adders emerge early in the year, they do not eat until May, even if their last meal was in September. The females emerge later in April or May and then they move to the mating grounds where the male adders duel with elaborate, twisting dancing. Most of our reptile species are ovoviviparous, meaning that the females incubate their eggs internally and give birth to live young. Grass snakes are our only egg-laying (oviparous) species, burying 30-40 eggs in compost heaps, rotting vegetation, or manure. There are two native lizard species, common lizards who give birth to live young (viviparous) and sand lizards who lay eggs.

What are mammals doing in spring?

Bats begin emerging from their overwinter hibernation in March, feeding on the increasing numbers of insects, but will return to torpor if the temperature drops. The females become pregnant after emerging from hibernation, using sperm that was stored from mating in the previous autumn. They form communal maternity roosts, giving birth to a single pup in June and providing milk until it until it weans six weeks later. Bat species are protected in the UK and there is extensive surveying during the spring and summer to ensure that any building or development work takes bat roosts into account. Bats tend to use dedicated summer roosting sites, which are separate from their hibernation sites. They will use a variety of summer roosts depending on the temperature and microclimate within. This is why it is recommended to put up a number of bat boxes if you are installing them on a site, with different locations in and out of shade.

Hedgehogs emerge from hibernation in March after losing up to one third of their body weight. They spend April fattening back up and looking for nest sites ready for breeding in May. Dormice also emerge from hibernation in March, spend some time regaining body condition by feeding on buds, insects, and flowers before breeding mostly in June and July.

Other mammals do not hibernate, but many species use spring as an opportunity to raise their young, synchronising their breeding seasons so that their young are born at the period of peak food availability in a similar way to bird species. For deer this means that the rut happens in late summer or autumn and then the females are pregnant overwinter. Badgers can mate at any time of year, and they use delayed implantation to ensure that their cubs are born in January of February, so that they emerge to begin foraging out of the sett in April and May when food is more plentiful. Stoats also use delayed implantation, giving birth to young in spring after mating the previous summer. The main mating season for foxes is in January, with females giving birth in March and cubs emerging in April or May, again when food sources are easy to find.

Rabbits and hares also spend their spring producing young, with breeding starting in February and continuing until late summer. Rabbits are born entirely dependent on their mother, blind and with no fur, but young hares (leverets) are independent as soon as they are born and only get fed once a day by the female at sunset. March is famous as the month when hares can be seen ‘boxing’, but this is females testing the stamina of males rather than a competition between males.

Insects

Insects vary in their overwintering strategies, with most overwintering in the egg or larval stage and some spending the winter as adults. The insects that spend the winter in torpor as adults tend to be the first to emerge in spring, with 7-spot ladybirds, queen wasps, queen bumblebees, and hoverflies being some of the first to appear. The first butterflies of the year are also those that have overwintered as adults, and the first sightings of a brimstone, peacock, small tortoiseshell, red admiral, or comma butterfly are one of the highlights of spring. They emerge from torpor needing food and water and the fluffy catkins of goat willow (Salix caprea) are an essential early source of pollen and nectar, along with cherry plum, blackthorn, celandines, and dandelions. The first butterflies to emerge from chrysalises are the orange-tip and the tiny holly blue, which both emerge in April. Many butterfly and moth species time the emergence of their caterpillars to the budburst of their host plant.

Dragonflies and damselflies do not start emerging until late spring or summer, with some such as the banded demoiselle, common blue damselfly, broad-bodied chaser, and emperor dragonfly starting to emerge in May. This emergence is triggered by a combination of day length and temperature, as the larvae climb out of the water to begin their transformation. Dragonfly and damselfly species spend most of their life cycle in water as a larval form, with some living as nymphs for more than five years. Prior to their incomplete metamorphosis (going straight from larva to adult without a pupal stage), the larvae spend several days in warm, shallow water, adjusting to breathing air. Then they climb out onto vegetation and break their adult form out of the larval casing, at which point they are very vulnerable to predators. The newly emerged adults, known as tenerals, are pale in colour and weak fliers, and they spend some time drying off before they begin hunting and looking for a mate. Most adult dragonflies only live for a week, although some can live for up to two months.

One of the most famous insect emergences that starts in spring is that of mayflies. These ephemeral insects are on the wing for a matter of hours, during which time they do not eat, just mate, lay eggs, and die. This leads to familiar clouds of mayflies that have synchronised their emergence and formed big mating aggregations. Similar to dragonflies though, they spend most of their life cycle in the water, living as a nymph for two years.

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Spring is one of the most important times of year for animals and plants, with many species focusing on timing their reproduction to peaks in food abundance. There is a seasonal decline in the success of offspring survival across many animal species, meaning that offspring from earlier broods or litters are more likely to survive than later ones. Animals must balance their breeding season timing with that of other seasonal events such as moult to ensure they and their offspring have the best chance of survival. It is critical that we monitor these seasonal cycles to assess and predict the likely effects of climate change on our wild populations.

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