Dave Bevan / Female Blackbird (Turdus merula) feeding chicks at nest, removing faecal sacs, Carmarthenshire, Wales, UK, June.

Tui De Roy / Great frigatebird (Fregata minor) male displaying inflated gular sac, Galapagos, Ecuador.

Ingo Arndt / Sea trout (Salmo trutta trutta) alevin just hatched with full yolk sac, Europe

Ingo Arndt / Natterjack toad male calling {Bufo calamita} vocal sac inflated, Germany

Ingo Arndt / Natterjack toad calling, vocal sac inflated {Bufo calamita} Germany

Dave Bevan / Male Great spotted woodpecker (Dendrocopos major) entering nest hole and leaving with a faecal sac, Carmarthenshire, Wales, UK, June.

Yashpal Rathore / Jayarami Bush frog (Raorchestes jayarami), inflating vocal sac, calling to attract mate, Endemic to Western Ghats. Anamalai Wildlife Sanctuary, India.

Alex Hyde / Hourglass treefrog (Dendropsophus ebraccatus) male with inflated vocal sac calling at night, Central Caribbean foothills, Costa Rica

Thomas Marent / Gelada Baboon (Theropithecus gelada) young feeding on lichen, Simien Mountains National Park, Ethiopia

Alex Hyde / Egg sac of European Cave Spider (Meta menardi) suspended from roof of limestone cave. Plitvice Lakes National Park, Croatia. January.

Sean Crane / Red-eyed Tree Frog (Agalychnis callidryas) calling, Tortuguero National Park, Costa Rica

Alex Hyde / Wolf Spider (Xerolycosa nemoralis) female carrying egg sac, photographd on a white background. Aosta Valley, Monte Rosa Massif, Pennine Alps, Italy. July.