Editing Down feather (section)

==Function==
[[File:Melopsittacus undulatus - male pet preening.jpg|thumb|right|Body down feathers, like these exposed on this adult male [[budgerigar]]'s back, lie underneath the contour feathers and help to insulate birds against heat loss.]]
The loose structure of down feathers traps air, which helps to insulate the bird against heat loss<ref name=Sibley/> and contributes to the buoyancy of waterbirds. Species that experience annual temperature fluctuations typically have more down feathers following their autumn moult.<ref name=Scott>{{cite book |title=Essential Ornithology |url=https://archive.org/details/Essential_Ornithology_by_Graham_Scott |first=Graham |last=Scott |page=[https://archive.org/details/Essential_Ornithology_by_Graham_Scott/page/n45 31] |publisher=Oxford University Press |year=2010 |isbn=978-0-19-856997-8 }}</ref> There is some evidence that down feathers may also help to decrease the incidence of nestling cannibalism among some colonially nesting species, as the stiffness of the feathers make the young more difficult to swallow.<ref name=Scott/> Pollutants can reduce the efficiency of these functions. When oiled, for example, down feathers mat and clump together, which breaks down the bird's insulation and allows water to reach the skin.<ref>{{cite book |title=Effects of Oil Pollution on Waterfowl: A Study of Salvage Methods |first1=Lynn A. |last1=Griner |first2=Robert |last2=Herdman |publisher=EPA Water Quality Office |date= December 1970 |page=18}}</ref>

Female [[wildfowl]] use down feathers plucked from their own breasts to line their [[bird nest#Scrape|scrape nests]]. This process performs the dual function of helping to insulate the [[Bird egg|eggs]] and exposing the female's [[brood patch]]—an area of bare skin, rich in blood vessels, which transmits heat very efficiently.<ref>{{cite book |chapter=Family Anatidae (Ducks, Geese and Swans) |title=Handbook of Birds of the World, Volume 1: Ostrich to Ducks |year=1992 |first=Carles |last=Carboneras |publisher=Lynx Edicions |location=Barcelona, Spain |page=[https://archive.org/details/handbookofbirdso0001unse/page/558 558] |isbn=978-84-87334-10-8 |chapter-url=https://archive.org/details/handbookofbirdso0001unse/page/558 }}</ref> Of the various items birds use to line their nests, down feathers provide the most effective insulation, though only when dry; wet down is the least effective insulator.<ref>{{cite journal |title=Using Artificial Nests to Test Importance of Nesting material and Nest Shelter for Incubation Energetics |first1=Geoff M. |last1=Hilton |first2=Mike H. |last2=Hansell |first3=Graeme D. |last3=Ruxton |first4=Jane M. |last4=Reid |first5=Pat |last5=Monaghan |journal=The Auk |volume=121 |issue=3 |pages=777–787 |date=July 2004 |doi=10.1642/0004-8038(2004)121[0777:UANTTI]2.0.CO;2 |doi-broken-date=5 April 2025 |s2cid=86123694 |issn=0004-8038 }}</ref> Down may also help camouflage the eggs when the female is away from the nest, particularly as the birds often draw the feathers over their eggs before leaving.<ref>{{cite journal |journal=Functional Ecology |title=Nest protection in mallards ''Anas platyrhynchos'': untangling the role of crypsis and parental behaviour|first1=J. |last1=Kreisinger |first2=T. |last2=Albrecht |volume=22 |issue=5 |date=October 2008 |pages=872–879 |doi=10.1111/j.1365-2435.2008.01445.x |doi-access=free |bibcode=2008FuEco..22..872K }}</ref>

Because a bird can eliminate [[heavy metal (chemistry)|heavy metal]]s in its feathers and because feathers can be collected non-invasively and stored indefinitely, down feathers can be used to check for evidence of metal contamination in the bird's environment. Studies have shown a high level of correlation between the level of metal contamination in a bird's diet and the level found in its feathers, with the proportion of the chemicals found in its feathers remaining relatively constant (and relatively high for some metals).<ref>{{cite journal |title=Assessment of metals in down feathers of female common eiders and their eggs from the Aleutians: arsenic, cadmium, chromium, lead, manganese, mercury and selenium |first1=Joanna |last1=Burger |first2=Michael |last2=Gochfeld |first3=Christian |last3=Jeituer |first4=Daniel |last4=Snigaroff |first5=Ronald |last5=Snigaroff |first6=Timothy |last6=Stamm |first7=Conrad |last7=Volz |doi=10.1007/s10661-007-9973-y |journal=Environmental Monitoring and Assessment |year=2008 |volume=143 |issue=1–3 |pages=247–256 |url=http://www.chec.pitt.edu/metals_eggspdf.pdf |pmid=17934788 |pmc=4300135 |bibcode=2008EMnAs.143..247B }}</ref>