Birds that produce milk?

Ndege wanatoa maziwa? (Scroll down for swahili)

These pieces first appeared on a blog that Dr. Colin Beale wrote on safari-ecology.blogspot.com. I’ve combined and shortened the two articles and provided the Swahili translation below. Follow Colin Beale on Twitter @tzbirder for current and great information (#BirdsAtTea).

You’re never far from a dove in East Africa, and if you sit by a waterhole waiting for something exciting to drop in, you might spend a little time point out how birds drink. As they come in to drink, you’ll see them dipping their beak into the water, tipping their head back and letting the water flow back to their crops then repeating the process. Most birds drink like this – they can’t suck as we can, so have to ‘sip and tip’ to do so. But there are some exceptions, one of which is pretty sure to turn up at every waterhole within a few moments of you: Doves.

A lemon dove in Udzungwa.

Watch a dove drinking and you’ll see it open its beak slight underwater, and then just keep its head there as it uses capillary action to pull the water into the beak, then it’s tongue as a piston to pump water into its throat. It’s a very different drinking action that will be immediately obvious.

The other interesting thing is their ability to feed their young chicks what is called ‘pigeon milk’ for the first few days after hatching. Both males and females produce a substance in their crop that is fed to young chicks in place of normal food. Although this process and substance has evolved completely independently from mammalian milk, it has a number of remarkable similarities. One is that the process is regulated by exactly the same hormone called prolactin. While mammalian milk is a liquid secretion that is probably a modified form of sweat, pigeon milk is actually made of specially grown cells that are shed from the lining of the crop. That difference aside, the content is pretty similar: at about 60% protein and the rest mostly fat. Amazingly the similarities don’t stop here. An important function of mammalian milk is to pass on antibodies to the baby, which helps protect the baby as its immune system is developing. It turns out that exactly the same thing happens in pigeon milk. Here’s a link to a great study showing that chickens dfed on pigeon milk developed a better immune system than chickens fed a similar nutritious diet that didn’t contain the antibodies that are in the pigeon milk. Read more here. Moreover, mammalian milk also includes bacteria, which effectively ‘seed’ the guts with useful bacteria that help digestion – the same is true of pigeon milk.

This process is not unique to doves and flamingos and male penguins also produce crop milk, but it is remarkable.

Swahili version

Ndege wanatoa maziwa!

Makala haya mafupi yalionekana mara ya kwanza kwenye blog ya ‘safari-ecology.blogspot.com’ ya Dr. Colin Beale. Nimeyaunga na kuyaifupisha na kuyatafsiri. Mfuate Colin Beale kwenye Twitter @tzbirder upate habari mapya na zaidi. (#BirdsatTea)

Huwa ukiwa popote Afrika mashariki hupo mbali na njiwa. Ukikaa kwenye vyanzo vya maji ukiwasubiria wanyama kushuka kunywa maji unapata nafasi nzuri kuwaangalia ndege wakinywa maji. Wanavyokuja kunywa maji utaona wanavyozamisha midomo yao kwenye maji, halafu kuinua vichwa na kuruhusu maji kutiririka nyuma ili kushuka tumboni, na kurudia rudia. Hawawezi kunyonya maji na kwa hiyo wana tabia na uwezo tu wakuchota na kumimina. Baadhi ya ndege wengi wanakunywa maji kwa njia hiyo isipokua ndege kadhaa hutembelea kwenye vyanzo vya maji mara kwa mara wakiwa tofauti: ni njiwa.

Mwangalie njiwa akinywa maji na utaona anafungua mdomo wake kidogo tu akiwa ameizamisha kwenye maji na anatulia maji yanavyoingia mdomoni kwa kufyonza ‘capillary action’. Baada ya hapo ulimi wake unatumika kama mchi wa mashini ‘piston’ kusukuma maji kwenye koo chake. Ni namna tofauti na ndege wengine ya kunywa maji.

Kitu kingine ajabu kuhusu njiwa ni uwezo wao kuwalisha vifaranga vyao kitu kinachofanana na maziwa ya njiwa (‘pigeon milk’) siku za karibu baada ya kuanguliwa. Dume na jike wote wanazalisha haya maziwa na kuwalisha vitoto vyao badala ya kuwapa chakula cha kawaida. Tendo na namna hii ya kuitengeneza imetokana na njia tofauti ya kuzalisha maziwa kwa ‘mamalia’, ila inafanana pia kiajabu. Sababu mojawapo ni kwamba maziwa haya unatengenezwa na aina moja ya homoni inayoitwa ‘prolactin’. Wakati maziwa ya mamalia ni majimaji na bila shaka yana uhusiano na jasho, maziwa ya njiwa yanatoka chembe pekee ndani ya shingo lake na kumwagwa kwa vitoto. Kuachana na hiyo tofauti, maziwa ya njiwa na mamalia yana kiasi asilimia 60 ya protein na mengine ni mafuta, lakini kufanana hakiishi hapo. Kazi mmojawapo nyeti ya maziwa ya mamalia ni kuwapa watoto chembe za kinga dhidi ya magonjwa (antibodies) ambazo zinamsaidia mtoto kujilinda wakati akikomaa. Kumbe hata maziwa ya njiwa husaidia kifaranga kukinga mwili wake. Ukipenda kusoma kuhusu utafiti uliofanyika jinsi watafiti waliwapa kuku maziwa ya njiwa bonyeza hapa. Pamoja na hayo, maziwa ya mamalia na njiwa huwapa watoto bakteria ambayo inawasaidia kuweza kuyeyusha chakula chao.  

Hii namna ya kutengeneza maziwa sio pekee kwa njiwa. Flamingo na penguin pia wanatengeneza maziwa kwa njia hii.

Sources:

Beale, Colin. Common Birds: The Ring Necked Dove. https://safari-ecology.blogspot.com/2013/01/common-birds-ring-necked-dove.html

Beale, Colin. Drinking Birds. http://safari-ecology.blogspot.com/2011/05/drinking-birds.html

Gillespie, M.J., Haring, V.R., McColl, K.A. et al. Histological and global gene expression analysis of the ‘lactating’ pigeon crop. BMC Genomics 12, 452 (2011). https://doi.org/10.1186/1471-2164-12-452

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