"members" also makes sense for object type "collection".
{
"id": "http://photo.example/evanp/san-diego-2011",
"displayName": "Vacation Photos San Diego 2011",
"objectType": "collection",
"objectTypes": ["image"],
"members": {
"totalItems": 26,
"url": "http://photo.example/evanp/san-diego-2011/members.json"
}
}