An early feast

Today, whilst out birding, something caught my eye. I have started visiting Warwick Racecourse more often this past year, mainly to build up a species list for the site and to get some nice photographs of different birds. 

At this time of year, I am usually focused on the Skylarks and the returning warblers; however, today, what caught my eye first was a flock of Starlings. Starlings are not unusual on site. They are present all year round in small flocks, usually on the short turf by the football ground. Here they potter over the grass, looking for insects and probing for leatherjackets. Today, however, they were further over near the water channel. They sat along the fence line, some of them shooting into the air, hovering and then returning to the fence. If I couldn’t see directly that they were Starlings, I would have identified them as stocky flycatchers, twitching into the air, fluttering and returning to a perch. It was this odd behaviour and that thought of the flycatcher that held the answer. 

Taking a moment to look around me, I noticed that the air was filled with black flies. Flies are often a very overlooked taxon, unless they are flying around your food, but they are incredibly important ecologically. The flies I could see were familiar to me. 

They were jet black, some about an inch long. They floated in the air with their legs dangling, looking like some kind of drone. They seemed to bob on the air currents, settling occasionally on vegetation. These flies are known as St. Mark's Flies (Bibio marci) or the hawthorn fly. They are called St Mark’s flies because their emergence flight period usually coincides with St Mark’s saint day (25th April). 

Image from Warren Photography - https://www.warrenphotographic.co.uk/28027-st-marks-fly-in-flight

The fly is known for its spring appearance. Its larvae live in the soil, feeding on roots and decaying vegetation before pupating into their adult forms, which then take to the air, slowly drifting in swarms with their legs hanging down. This stage is short-lived, lasting perhaps a week as they search out a mate. During this week, they sustain themselves by feeding on the nectar of fruit trees and hawthorn flowers, making them important pollinators for these species. 

In many ways, this display of floating dancing flies is a terrestrial form of that exhibited by Mayflies over our rivers. These swarms form an important food source for many species. The Starlings had identified this and left their usual spot to take advantage of the glut of food.

It's one of nature's miracles, the ability of such tiny organisms to synchronise and emerge together. Sometimes it's the small things that reveal the most.

An upgraded Mostela

 

I have embraced camera trapping as a medium for observing and recording nature. What started as a whimsical addition to my arsenal of ecological tools has turned into a passion. I have been running cameras for over 10 years, have experimented with several techniques, and have even published a book on the topic.

One of the most interesting tools in camera trapping is the Mostela. I have blogged about this device before. Mostelas are designed to improve the detectability of small mustelids such as Stoats and Weasels.

Mostela as detailed by the Vincent Wildlife Trust 2022 (image courtesy of Vincent Wildlife Trust)

They are essentially a box with a drain pipe entrance that contains a camera. They can be baited or unbaited. The theory is that these spaces will be used by small mammals such as voles, mice and shrews; they will visit the site for food if baited or just investigate it. Their continued presence creates a scent trail that Weasels and Stoats will follow when hunting.

Weasel recorded in the current fixed Mostela in 2025

Raw Components

In reality, I have had little success with capturing images of Weasels and Stoats. I am not entirely sure Stoats are resident on the site, in person or on camera. Stoats have only been recorded once since 2015. Weasels, however, are resident, but due to their speed, they are not easily detected on camera. On Otter Cam and the mostelas, they have been recorded once a year, whilst the hedgerow camera recorded them on average twice a year.

The greatest benefit I have found is not in collecting mustelid data but in observing the small mammals. Small mammals can be recorded with standard camera placement, but better identification is achieved with small-mammal rigs and mostelas.

I have used home-built mostelas for over two years, as well as purpose-built ones purchased from the Netherlands (An expensive approach!), and now feel like I want to experiment with the design.

Assembled

One drawback of the mostelas is that, during the daytime, the closed chamber makes it still very dark and often requires infrared to make any visitors visible, making the visitor's pelage colour hard to see. This can be important in identification. To counter this, I am experimenting with a translucent lid that should give better colour clarity during the day.

I have also removed the base plate to reduce costs, provide a more natural view, and allow the camera to be placed more effectively. Often, the best camera positioning involves tilting the camera down. With Gardepro and Bushnell cameras, which are my preferred brands, the lens is often quite high on the device, meaning that individuals moving closer to the camera are often missed or hard to see. Without a base, it is possible to dig a trench for the camera to sit in.

To build this mostela, I have used marineplywood that I bought online pre-cut to size. For the entrance tubing I am experimenting with two entrances opposite to each other using bend fixtures bought from Amazon. This cuts down on the difficulty of cutting and sealing a length of drain pipe.

I have tested the principles in the back garden but this weekend I will install it in the wild to replace my existing mostelas that is slowly disintegrating.