If Donald Trump's shambolic presidency has united anyone in America, it is former White House predecessors from both sides of the political divide. Jimmy Carter, George H.W. Bush, George Bush, Bill Clinton and Barack Obama have offered a united front in condemning the volatile rhetoric and lack of decorum that has defined Trump's first 12 months at the helm.

At the dinner in Texas on 21 October, the three most recent leaders of the free world were huddled on stage together, encouraging guests to donate money to victims of Hurricanes Harvey, Irma and Maria. While Clinton addressed the Gala, Bush could be seen cracking a joke with Obama that had him in stitches.

What Bush said, we may never know, but there are a couple of theories and neither look great for the Donald.

Theory #1: Puerto Rico. As Bush leans over to Obama and makes the quip (3.35), Clinton is saying: "Our friends in Puerto Rico and the American Virgin Islands have only begun to dig their way out of what could be still a calamitous disaster."

Trump's response to the devastating effects of a string of hurricanes that have upended the lives of 3.4m Puerto Ricans, who are American citizens but do not enjoy voting rights, has been harshly criticised.

Images of Trump smugly tossing "beautiful, soft" paper towels at crowds of needy people have been mocked at home. The US government response has been seen as too little, too late.

But worst of all, Trump tweeted on 12 October that the government could not provide aid to Puerto Rico "forever" (he has made no such noises about Texas or other storm battered areas). It led some commentators to wonder whether he understood that the territory was actually part of the US.

Was Bush alluding to this when he whispered in Obama's ear?

Theory #2: Birtherism. The other theory, put forward by former UK politician turned journalist Louise Mensch, is that Bush was ribbing Obama about spurious claims that he was not really an American citizen. Claims famously championed by Donald Trump.

About a minute before Bush makes the joke (2.44), Clinton says: "We were all citizens – even us [the ex-presidents] – before we were politicians and after our time was up we became citizens again."

The trio all chuckle a little at this and then Clinton carries on with his speech. But Mensch argues, and our amateur lip-reading skills do not contradict her, that Bush says "except you" to Obama.

If that was what he said, then George Bush has more wit than he has previously been credited with. His successor in the Oval Office certainly found it very funny.

Trump built his political career off the back of the 'Birther Movement' – unfounded claims that Obama was not a US citizen. The property tycoon has since accepted that he was wrong although there are plenty of Americans who believe that the last Commander-in-Chief was not an American. They are typically people likely to sympathise with white nationalist agendas.

Five former US presidents, Jimmy Carter, George H.W. Bush, Bill Clinton, George W. Bush, and Barack Obama attend a concert at Texas A&M University benefiting hurricane relief efforts REUTERS/Richard Carson