Brian Bevan / Pair of Reed warblers (Acrocephalus scirpaceus) feeding young at nest, Bedfordshire, England, UK, July.

Michael Quinton / Northern Flicker (Colaptes auratus) carrying fecal sac from nest cavity in forest, Alaska

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

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

John Chan / Female Bushtit (Psaltriparus minimus) returning to nest to feed chicks, carries away a fecal sac, Southern California, USA, April.

Kim Taylor / European Robin (Erithacus rubecula) flying from nest with faecal sac, Surrey, England, UK.

Nick Upton / House sparrow (Passer domesticus) male perched on a roof gutter near its nest entrance under old tiles, with a faecal sac it has just brought from the nest, Wiltshire, UK, June.

Philippe Clement / Great tit (Parus major) bringing food to nestbox to feed chicks, leaves with fecal sac, Belgium, April.

Dave Bevan / Swallow (Hirundo rustica) removing fecal sac from nest in barn filmed using infra red, Carmarthenshire, Wales, UK, September.

Alan Murphy/ BIA / Northern Flicker (Colaptes auratus) male carrying fecal sac from nest cavity, British Columbia, Canada

Dave Bevan / Slow motion clip of a Pied wagtail (Motacilla alba) entering nest site in old tractor with food, before leaving with a faecal sac, Carmarthenshire, Wales, UK, July.

Dave Bevan / Slow motion clip of a Song thrush (Turdus philomelos) feeding chicks, takes faecal sac and flies out of frame, Carmarthenshire, Wales, UK, June.