The Venice Biennale, Various Venues, Venice

Press Day at the Venice Biennale. A hundred yards apart, two people pound away on step machines.

One, male, is perched on an upside-down tank, its tracks spinning helplessly as if propelled by his running. This, you will have guessed, is art – American art, in fact, the jogger-and-tank being in a show in the US Pavilion by the collaborative duo, Jennifer Allora and Guillermo Calzadilla.

The other runner is female. Her Stairmaster, too, is atop a mode of conveyance – a yacht this time, moored by the Giardini and happily the right way up. She is also part of a double-act. With her husband, the yacht's owner, the expensively Lycra'd woman is not an exhibition entry but one of the biggest contemporary collectors in England: a patron of the Tate, a rival to Charles Saatchi.

As with the tank-man – as with so much here – I just don't get it. If you're that rich, why run? Surely a servant will do it for you. And if you're American, why obsess about it? Dissing US values in the US Pavilion smacks of having your cake and eating it. But as the ranks of private jets at Venice airport suggest, the Biennale isn't just about art. Being divided up by nation, it is about nationalism, which makes you shudder slightly as you approach the Scottish Pavilion – well, not a pavilion, really, Scotland being part of Great Britain when Biennale space was handed out back in 1907. Karla Black, the Scottish entry, gets an airless floor of the distant Palazzo Pisani.

If you're expecting tartan shortbread, though, forget it. Black's patisserie is of a different recipe, pastel-shaded, stiff-peaked, made of sugar paper. There is a whiff of macaroons, cheap soap, the cosmetics counter. For all its frou-frou, Black's work is tough as goats' knees – a Scottish Arte Povera, made of polythene sheeting and rubbed-in pigment, of next to nothing but intelligence and labour. Many Turner Prizes go by when I don't care who wins; but I deeply want Black to get it this year, if only as reward for my happiest half-hour of the 2011 Biennale.

Another was at the New Zealand Pavilion. Michael Parekowhai's installation is tenderly muscular, its objects tending to the large – a big, red piano in the foyer, a sculpted bouncer in the garden – although not always. A pair of Crocs in the grass are of bronze, modelled on those of the artist's little brother, dead at 12; the bouncer is another brother. Parekowhai and his siblings appear as lizards in the lid of a vintage Steinway on which music students play. Four wooden lizards, the living, are on one side of a strut, the fifth is on the other.

The piano, a re-exported import, has been carved with Maori decoration, some real, some of a Europeanised style made for tourists. The carving, at the artist's insistence, was done by the carpenter who did the piano in the film The Piano. Parekowhai's work meditates on the wash of history – on migration and re-migration, the morphing of art, on the impossibility of seeing culture as sharp-edged. His work sits nicely with Mike Nelson's, reviewed last week; and like Nelson's too, its homelessness makes it at home in Venice.

Back in the Giardini, cultural questions come at you from all sides. Is the painting in the Canadian Pavilion bad or Bad? (Clue: bad.) Does the curating of the German Pavilion mar the work of Christoph Schlingensief? (Yes, although it got this year's Golden Lion anyway.) Don't the Belgians have an artist other than Luc Tuymans? (No.) And then there's the French.

Christian Boltanski is an unpredictable artist, sometimes wonderful (his Tasmanian gamble with death), sometimes dreadful (his piece for Monumenta 2010). His work in the French Pavilion is Boltanski at his best, a printworks sketched in metal scaffolding and a press running off a belt of babies, their faces ticked off by digital clocks. It is spare and elegant, also funny – you can stop the machine by pressing a button, then on it whirrs – and scary: Boltanski's baby-factory is a portrait of our Darwinian drive to extinction. Its horror lies in its lightness of touch, a sense of the artist looking at Man's will to over-populate and shrugging.

No Biennale is complete without dead pigeons and this one has 2,000, hung on the rafters of the Palazzo delle Esposizioni by Maurizio Cattelan. Cattelan, who recently started a photography magazine called Toilet Paper, promises that these will be his last sally into sculpture. Boo, hoo. Actually, the birds, called The Tourists, appeared at the 1997 Biennale, so they are by way of being old masters, or at least old hat. They peer down at a suite of Tintorettos, borrowed for the occasion from Venetian museums. The pigeons' look – paint on canvas? – is of blank incomprehension. It mirrors those of many visitors.

Venice Biennale, to 27 Nov

Next Week:

Charles Darwent has a blast with the Vorticists at Tate Britain