As the weather warms and homes across the UK throw open their windows to welcome spring, many people are finding themselves joined by an unwelcome guest, houseflies.

Despite spotless spring cleaning and wide-open windows, flies often seem determined to stay indoors, circling lamps, buzzing around the ceiling, and ignoring every escape route you try to offer. But according to experts, there's a specific and surprisingly structured reason behind this frustrating behaviour.

Rather than simply being lost or stubborn, flies are performing what neuroethologist Jochen Zeil describes as territorial patrols.

Zeil, who holds a PhD in the visual systems of flies from the University of Sussex, explains that male flies tend to claim defined airspaces beneath overhead "landmarks" like light fittings.

These small zones become prime spots for attracting females, and males will circle them repeatedly, fending off any competitors.

If a rival fly appears, it's chased away to a slightly lower altitude, often just 10 to 30 centimetres beneath the original, where it will start its own patrol. Female flies, meanwhile, are drawn to the busiest airspaces, where the most males are gathered.

Unfortunately for homeowners, once flies are inside, getting them out again is no easy task.

Though their compound eyes offer a wide field of view, flies can only see clearly for short distances and struggle with light perception. This means an open window, particularly if there are artificial lights still on inside, may not register as an exit at all.

Experts suggest that to encourage flies to leave, it’s best to turn off indoor lights and allow the natural light from outside to act as a visual guide.

Otherwise, you may be stuck with buzzing circles until the insects either tire out, or find a new landmark.