A canary in the coal mine of global equities is beginning to warrant attention, and equity bulls would do well to take notice. The signal: an advance in defensive stocks, according to Bloomberg's Cormac Mullen, who spent more than a decade as an equity analyst, trader and sales trader in Europe before becoming a journalist covering global financial markets.

Mullen explains below:

For a number of years now, defensive stocks have been shunned by investors in favor of cyclical peers that were preferred as the global economic recovery gathered steam. The underperformance of defensive sectors has seen their share in the S&P 500 Index fall to 11% from over 22% in 2009, according to The Leuthold Group.

The past month has seen some change. Even as consumer discretionary and tech stocks led U.S. equities to a 2-percent plus gain, a bunch of defensive stalwarts have been hot on their heels. Consumer staples, real estate, health care, telcos and utilities names have all outperformed cyclical counterparts from industrials, materials and financials to energy stocks.

Those defensive bellwethers, U.S. consumer staples, rose above their 50-day moving average earlier this month for the first time since February, and are close to a two-month high. The long- suffering sector has risen from a more than two-year low reached in May.

And investor demand is picking up -- staples and health-care equity ETFs have seen inflows for the last 10 weeks, with almost $11 billion going to global staples ETFs since the beginning of the year, according to Jefferies.

And a move away from cyclical stocks is already underway in other parts of the world. Japanese defensives have been outperforming cyclical peers since the end of January and are trading at a nine-month relative high. European defensives are also beginning to outperform.

It’s early days, and the shift may not yet be the omen for the end of a historic bull run for stocks. At least in the U.S., economic indicators continue to look solid and the Trump administration’s stimulus boost is coursing its way through the corporate veins of the world’s largest economy

But as trade war rhetoric turns to action, political risk in Europe resurfaces, emerging markets across the globe shudder and the Chinese economy starts to slow, this defensive canary is definitely one investors should be keeping an eye on.