Editing Bird (section)

===Excretory system===
Like [[reptile]]s, birds are primarily [[Metabolic waste#Uricotelism|uricotelic]]; that is, their [[kidney]]s extract [[nitrogenous waste]] from their bloodstream and excrete it as [[uric acid]], instead of [[urea]] or [[ammonia]], through the ureters into the intestine. Birds do not have a [[urinary bladder]] or external urethral opening. With the exception of the [[Ostrich#Description|ostrich]], uric acid is excreted along with faeces as a semisolid waste.<ref>{{cite web|last=Ehrlich |first=Paul R. |author2=David S. Dobkin|author3=Darryl Wheye |title=Drinking |url=http://www.stanford.edu/group/stanfordbirds/text/essays/Drinking.html |year=1988 |work=Birds of Stanford |publisher=Stanford University |access-date=13 December 2007}}</ref><ref>{{Cite journal|last1=Tsahar |first1=Ella |title=Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores |journal=Journal of Experimental Biology |volume=208 |issue=6 |pages=1025–1034 |year=2005 |pmid=15767304 |doi=10.1242/jeb.01495 |last2=Martínez Del Rio |first2=C |last3=Izhaki |first3=I |last4=Arad |first4=Z |doi-access=free |bibcode=2005JExpB.208.1025T }}</ref><ref name=coprodeum>{{cite journal | doi= 10.1016/S1095-6433(03)00006-0 | last1= Skadhauge | first1= E | last2= Erlwanger | first2= KH | last3= Ruziwa | first3= SD | last4= Dantzer | first4= V | last5= Elbrønd | first5= VS | last6= Chamunorwa | first6= JP | title= Does the ostrich (''Struthio camelus'') coprodeum have the electrophysiological properties and microstructure of other birds? | journal= Comparative Biochemistry and Physiology A | volume= 134 | issue= 4 | pages= 749–755 | year= 2003 | pmid = 12814783 }}</ref> However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.<ref>{{Cite journal|last1=Preest |first1=Marion R. |date=April 1997 |title=Ammonia excretion by hummingbirds |journal=Nature |volume=386 |issue= 6625|pages=561–562 |doi=10.1038/386561a0 |last2=Beuchat |first2=Carol A. |bibcode=1997Natur.386..561P }}</ref> They also excrete [[creatine]], rather than [[creatinine]] like mammals.<ref name="Gill"/> This material, as well as the output of the intestines, emerges from the bird's [[cloaca]].<ref>{{Cite journal|last1=Mora |first1=J. |year=1965 |title=The regulation of urea-biosynthesis enzymes in vertebrates |journal=[[Biochemical Journal]] |volume=96 |pages=28–35 |pmid=14343146 |last2=Martuscelli |first2=J |last3=Ortiz Pineda |first3=J |last4=Soberon |first4=G |pmc=1206904|issue=1|doi=10.1042/bj0960028 }}</ref><ref>{{Cite journal|last=Packard |first=Gary C.|year=1966 |title=The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates |journal=[[The American Naturalist]] |volume=100 |issue=916 |pages=667–682 |doi=10.1086/282459 |jstor=2459303|bibcode=1966ANat..100..667P }}</ref> The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by [[Bird anatomy#Reproduction|joining cloaca]], and females lay eggs from it. In addition, many species of birds regurgitate [[Pellet (ornithology)|pellets]].<ref>{{Cite journal|last=Balgooyen |first=Thomas G. |date=1 October 1971 |title=Pellet Regurgitation by Captive Sparrow Hawks (''Falco sparverius'') |journal=[[Condor (journal)|Condor]] |volume=73 |issue=3 |pages=382–385 |doi=10.2307/1365774 |url=http://sora.unm.edu/sites/default/files/journals/condor/v073n03/p0382-p0385.pdf |via=Searchable Ornithological Research Archive |archive-url=https://web.archive.org/web/20140224142542/http://sora.unm.edu/sites/default/files/journals/condor/v073n03/p0382-p0385.pdf |archive-date=24 February 2014 |jstor=1365774 }}</ref>

It is a common but not universal feature of [[Altriciality|altricial]] [[passerine]] nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a [[fecal sac]]. This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system.<ref name="audubon">{{cite web|url=https://www.audubon.org/news/what-are-fecal-sacs-bird-diapers-basically |first1=Benji |last1=Jones |title=What Are Fecal Sacs? Bird Diapers, Basically|website=Audubon|date=7 August 2018|access-date=17 January 2021}}</ref>