Tortoises love bindweed, and it is a useful addition to the diet later in the summer when other edible plants may be scarce. However don't overdo it, as it can be somewhat purgative and laxative. The smaller Field bindweed, being milder in this respect, is the better one to choose. As always, pick only from unpolluted places and wash well before feeding if picked from outside your garden.

It is advisable to remove the leaves from the vine before feeding to avoid the tortoise getting tangled or even injured.

Note: Take care not to confuse these climbers with the hallucinogenic (blue-flowered) Morning Glory (Ipomea volacea). Do not confuse either with Black bindweed, Polygonum convolvulus (high in oxalic acid) which has no trumpet-shaped flowers but tiny pale ones:

 - or with Black Bryony (Tamus communis) which has toxic elements (saponins in the whole plant and calcium oxalate crystals in the berries). This has much glossier leaves, lacks the trumpet-shaped flowers, and has red berries after flowering.

From Encyclopaedia Britannica:
Bindweeds
Plants of the closely related genera Convolvulus and Calystegia, mostly twining, often weedy, and producing handsome white, pink, or blue, funnel-shaped flowers. Bellbine, or greater bindweed (Calystegia sepium), native in Eurasia and North America, bears arrow-shaped leaves and white to pink, 5-centimetre (2-inch) flowers. This twining perennial grows from creeping, underground stems and is common in hedges, woods, and along roadsides. Its range tends to coincide with that of its principal pollinator, the hawkmoth. Sea bindweed (C. soldanella), with fleshy, kidney-shaped leaves and deep pink, 5-cm blooms, creeps along European seaside sand and gravel. Several Convolvulus species are widespread or conspicuous. The weedy, perennial field bindweed  (C. arvensis), which is European but widely naturalized in North America, twines around crop plants and along roadsides. It bears long-stalked clusters of fragrant pink, white, or striped blooms, 2 cm across, among arrow-shaped leaves.