Seasonal Update (May 2021)

May is the season many of us start to see fledglings appear in our gardens, such as this juvenile European robin (_Erithacus rubecula_). - Credit: Marc Baldwin

March ended on a balmy note with unseasonably warm temperatures across much of England and eastern Scotland. On the 30th, Kew Gardens saw 24.5C (76F), the hottest March day since 1968. This didn't last, however, with a stubborn area of high pressure ushering in generally cold and dry conditions for mid-spring.

The start of April was significantly cooler than average thanks to an area of high pressure sitting just to our north-west and bringing a northerly air mass and winds from the east over the Easter Bank Holiday weekend. The result was that most of the north and east struggled to reach high single figures Celsius (mid-40s Fahrenheit) on the 1st April, while the far south-west hung on to high teens (mid-60s Fahrenheit). By Good Friday, however, everyone shared the cold and cloudy conditions with patchy drizzle, only brightening up during the afternoon in the west.

Most of April was below the seasonal average thanks to a waxing and waning area of high pressure that setup a relentless Arctic airflow. - Credit: Amos Bühler (CC BY 2.0)

Easter Saturday was largely dry and cloudy in the east with a chilly north-easterly wind, again brightening towards the south and west. Easter Sunday was virtually wall-to-wall sunshine and warmth, with light winds and temperatures reaching the mid-teens Celsius (high 50s Fahrenheit) in the south. Come Monday the cloud was back and temperatures had fallen as a strong north-easterly wind took over, and we saw outbreaks of precipitation that manifested as light snow in some parts even at low levels. The rain was confined to fragmented bands in Scotland, Northern Ireland and the south-east, with the south-west and Wales remaining mostly dry.

The first full week of April saw the high pressure move off into the Atlantic, pulling Arctic air down on Easter Monday and dropping temperatures below the seasonal average. A mid-Atlantic ridge of high pressure resulted in a blocking pattern that kept the northerly airflow in place, on and off, for most of the week. Tuesday was bright with some scattered showers in the north that were wintry at times and temperatures were well down on the seasonal average at 3-8C (37-46F). Overnight Tuesday into Wednesday temperatures dropped to -4C (25F) even in the south, bringing the coldest night of spring so far. Things warmed up a little on Wednesday and Thursday for most, with temperatures ranging from 2-10C (36-50F) across the country—still down on the seasonal average of 10-13C (55F)—and with less windchill for most of England. Northern Scotland was the exception here, being subjected to 50mph winds. The Artic airflow was back on Friday, just in time to start the second weekend, which remained cold with sleet and snow widely across the country. This cold spell has been Europe-wide and is the result of what meteorologists call a “tropospheric-led upwell”.

Week two started with the coldest April night since 2013, with -9.4C (15F) recorded in NE Scotland Sunday night/Monday morning. There were also some significant falls of snow in Wales and the Midlands into the Monday morning, some even making it as far south as London, although it was mostly sleet along the coast. High pressure was parked over the UK for the first half of the week, keeping things settled and largely dry with just a few isolated wintry showers, mostly in the east of England. Away from the North Sea coast, temperatures gradually crept up into double figures Celsius as the week drew on. High pressure endured everywhere but the far north-west of Scotland and Northern Ireland, resulting in widespread sunshine and warmth on both days, albeit with chilly nights—daytime temperatures were around the seasonal average of 14-15C (57-59F).

The waxing and waning of the northern blocking persisted for the remainder of April, making it difficult to build any warm weather. After a gloriously sunny middle weekend, the penultimate week started on a misty and foggy note, although this soon burnt back revealing sunshine for all but the far south-east, where it remained relatively cloudy. A cold front moved down on Tuesday into Wednesday, bringing some light rain for much of the country and dropping temperatures a few degrees. The week nonetheless remained mostly dry with high pressure hanging on into the final weekend. Temperatures hit the mid- to high teens Celsius, with a few places seeing 20C (68F) on the Friday, but it still felt chilly in that northerly airflow.

April was an extremely dry month, despite some rain right at the end. During prolonged dry periods garden wildlife can really benefit from having water left out day and night. - Credit: Marc Baldwin

The final week of April began with a low pressure system working its way slowly southwards, bringing cloud and some scattered showers to Scotland, Northern Ireland and northern and midland England. In the south, things remained bright and settled, with less wind than over the weekend. Overnight lows were widely around freezing in the south, holding up into mid-single Celsius from the midlands northwards. Daytime temperatures were widely into the mid-teens Celsius across the country, touching 20C in the sun and shelter. Wales and southern England were host to the low pressure during the middle of the week, although unfortunately most of the much craved rain remained in the west. A northerly airflow picked up in the north dropping temperatures again. The cold Arctic air had covered the whole of the UK by Friday and into the Bank Holiday weekend, pushing the milder air into eastern and norther Europe.

If you're interested in the wildlife to be found this month, including rabbit reproduction, hedgehogs getting frisky and the return of the basking shark, check out my Wildlife Watching - May article.

In the news

A few of the stories that caught my attention this month include the world’s oldest bird raising her fortieth chick, the importance of yawning, and an online database of carnivore diets across the world.

Recent ZSL tracking data sheds new light on how often cattle and badgers meet one another. - Credit: Marc Baldwin
  • Never too old. A Laysan albatross named “Wisdom” by US Fish and Wildlife Service researchers has given birth to a chick at the age of at least 70. This is not only a staggering age for a female to produce a chick, but also an impressive age for an albatross, which typically has a life expectancy of only 40 years. Indeed, Wisdom currently holds the record for the world’s oldest known bird, let alone oldest new mother.
  • Opposites attract. New research by scientists at the Zoological Society of London suggests that, contrary to previous tracking studies, cattle and badgers may interact more often than we thought, with avoidance being short-lived. Rosie Woodroffe and colleagues found that, at their site, cattle met badgers about 6 times per 24 hr period.
  • Copycat cats. We’re all familiar with how contagious yawning can be, and this is also well known among non-human animals. A recent study on lions in South Africa’s Makalali Game Reserve suggests, however, that it may play an important part in group cohesion and vigilance. The researchers found that, if two lions were lying together, when one yawned the other copied the behaviour and when one stood up so did the other. This suggests that yawning may offer some advantages for collective awareness and the detection of potential danger.
  • Dietary database. University of Sussex doctoral student Owen Middleton has created a database with thousands of dietary records for carnivores across the globe. CarniDIET aims to facilitate conservation research by providing a single resource to easily access the ecological requirements of a range of carnivores from red foxes to leopards.
  • Lockdown lacking. Despite the first lockdown in the UK reducing air pollution (i.e. levels of nitrous oxide in the atmosphere) by just over 50%, the most recent lockdown produced only a 28% decrease in pollution.
  • Warm this winter? New analysis from the Copernicus monitoring programme released last month revealed that the autumn and winter of 2020/2021 were the warmest ever recorded in Europe, being 3.4C (6F) warmer than the 1981-2010 average and 1.4C (2.5F) above last winter. This is despite the Arctic blocking pattern that covered Britain for much of the winter, illustrating the large temperature contrasts across Europe.

Discoveries of the Month

Robo-crop: Robotic lawn mowers cannot ‘see’ hedgehogs

The popularity of robotic lawn mowers has increased considerably in recent years, growing at a rate of about 12% annually and currently worth an estimated £935m (€1 bn / US$1.3 bn). The devices are equipped with a variety of sensors for detecting garden perimeters, cables and various other obstacles. Some mow randomly, eventually covering the whole lawn, while more sophisticated models leverage GPS that allows for specific paths to be programmed.

A hedgehog's primary defence mechanism is to curl up, making them vulnerable to mowing machines. - Credit: Marc Baldwin

Several years ago, concerns were raised about the impact that these robotic mowers might have on garden wildlife, including bees, butterflies and hedgehogs. In 2017, Swiss hedgehog carer Erika Heller estimated that nearly half of the hogs brought to their Igelstation Winterthur rescue centre had been injured by robotic mowers and, the following year, Husqvarna, the biggest manufacturer of the devices, announced it was working on ways to prevent hedgehogs falling victim to them. It seems, however, that there’s still a way to go, and a recent study has cast doubt on the ability of modern units to detect and avoid hedgehogs.

Last year, a team led by Sophie Rasmussen at Aalborg University in Denmark tested the ability of 18 models of robotic lawn mower available on the European market to detect and avoid the bodies of 70 hedgehogs, all of which had died having been brought to wildlife rescue centres in Denmark during the summer of 2020. The researchers divided the cadavers up into four age classes according to their size (dependent juveniles, independent juveniles, adults and large adults) and each age class of hedgehog was tested in three positions, resulting in each lawn mower being tested 12 times on a short, flat lawn in Sweden over two days in August.

In no trial did any of the brands detect the dependent juvenile hogs (i.e. those less than 200g), although the very small individuals were shorter than the height of the blades, allowing the mower to pass over them without causing damage. For independent juveniles and adults (i.e. all those over 200g), the mower had to bump into the hedgehog before it changed direction, and this was the case even for those with cameras or ultrasonic sensors. In many cases the mowers stopped immediately upon contact, or shortly after, leaving only minor injuries. With some devices, however, the safety mechanisms appeared to fail and caused substantial trauma, including partial decapitation in one case.

Writing in the journal Animals earlier this year, Rasmussen and her colleagues note that mowers with fixed blades were more likely to cause damage to hedgehogs than those with pivoting blades, which fold into a protective frame when they hit something harder than grass; this was particularly so if the model also contained skid plates. The researchers concede that daytime mowing should reduce the probability of a hedgehog being injured, but point out that hogs can be active during the day for a number of reasons, particularly during the late summer when litters are dispersing. In their paper they suggest:

” ...we encourage manufacturers, distributors, and sellers of robotic lawn mowers to educate customers on the importance of refraining from using robotic lawn mowers at night time and to check the lawn for wildlife species that are potentially vulnerable to the machines, such as hedgehogs, leverets, fledglings, and amphibians, before mowing.”

Robotic lawn mowers typically come with one of two blade types: fixed blades (A) and pivoting blades (B). The latter are less dangerous to hedgehogs, based on the findings of this study. - Credit: MDPI / Animals / Rasmussen et al. (CC BY 4.0)

Source: Rasmussen, S.L. et al. (2021). Wildlife conservation at a garden level: The effect of robotic lawn mowers on European hedgehogs (Erinaceus europaeus). Animals. 11: 1191. doi: 10.3390/ani11051191

Can you dig it? Fox dens improve local soil quality

There's a growing appreciation of the interconnectedness of nature and we're starting to understand how species have a role to play in sustaining the ecosystems of which they’re a part. We know, for example, that healthy soils underpin a variety of human needs, from the crops we plant to the quality of the water and even the air we breathe. Nonetheless, real concerns around soil impoverishment have been raised since at least the mid-1990s and, in 2017, the FAO-led Global Soil Partnership reported that 75 billion tonnes of soil are eroded from arable lands globally each year at a cost of some £287bn (US$400 bn / €330 bn). Here in the UK, in 2010, the Environment Agency calculated that soil erosion cost our economy £1.2 bn (US$1.7bn / €1.4bn) annually.

The activity of animals and birds is crucial to the maintenance of soil nutrient levels and overall condition, from brown bears returning nitrogen back to the soil through their autumnal salmon feasts, to fungi breaking down the tough lignified tissues of trees, allowing other species access to the nutrients locked up within and returning much of it to the soil. In recent years, we've come to understand that fossorial species, those that spend at least some of their time underground and shift substantial amounts of dirt in the process, also have a vital role in boosting soil quality around their homesteads. This is well known in badgers, for example, and now a new study from Canada adds to the evidence that foxes are also important soil guardians.

Red foxes use earths (dens) during the cubbing period and the presence of these earthworks has a substantial impact on surrounding vegetation and soil quality. - Credit: Marc Baldwin

In July 2018, a team of researchers led by Jessica Lang at the University of Manitoba in Winnipeg conducted a vegetation and soil assessment at eight fox earths in Churchill, Manitoba which had been occupied by red foxes since at least 1994. The den sites were compared to control plots in the study that were in areas away from fox activity.

The results show that, compared with the control sites, the organic soil layer at fox earths had 81% more total inorganic nitrogen and 250% more extractable phosphorous, while organic soil respiration was three times higher. The organic layer was also probably deeper at the den sites, but the statistical power was low. Species richness was higher at fox earths, with 17 species found on average compared with 13 at control sites, and the diversity between quadrats was also higher than at controls, although, overall, the vegetation diversity didn't vary between the earth and control sites. Two species of willow, Arctic grey (Salix glauca) and Athabasca (Salix athabascensis), were indicator species for earths, along with an abundance of erect shrubs, grasses and forbs. Control sites, by contrast, tended to contain more prostrate shrubs and lichens.

Writing in the journal Scientific Reports last month, the scientists note that it remains unknown exactly how the foxes exert an influence on the flora. Do they deposit seeds from their fur or droppings? Or does their denning and scent-marking behaviour promote the growth of certain species present in the soil already? Perhaps a combination of both? Overall, however, there is little doubt that foxes make a difference, and the researchers conclude:

We found that through the combined effects of nutrient additions and soil disturbance, which was absent on control sites, red foxes enrich soil nutrient concentrations and increase soil respiration, pH and overall species richness at den sites. Red foxes alter soil resource availability, which has been experimentally shown to affect species composition.”

Source: Lang, J.A. et al. (2021). Foxes fertilize the subarctic forest and modify vegetation through denning. Sci. Rep. 11: 3031. doi: 10.1038/s41598-021-82742-y.

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