**2. The colour of bird feathers**

Bird feathers have been the aim of several works during the last two centuries. Thus, the first reports on bird plumage listed in databases like PUBMED, Web of Science or Scopus analysed aspects focused on the muscles in charge of the feathers movement [17] or their growth [18]. Other aspects of bird feathers related to biological roles like sexual selection, colonization strategies or signalling have also been extensively explored [19–23]. These aspects are intricately connected to the coloration of avian plumage.

The first detailed studies about the colour of bird feathers were published in indexed scientific journals in the middle fifties last century. Since then, around 500 manuscripts have been reported on this subject (**Figure 1**). It is worthy to note that the number of studies about the colouration of plumage significantly increased at the beginning of XXI century (**Figure 1**). However, the number of publications focused on the presence of carotenoids in bird feathers is lower compared to those related to other issues affecting the phenotype of birds (**Figure 1**). Bird coloration (mainly in feathers) is one of the most studied topics to elucidate the role of natural and sexual selection in the evolution of phenotypic diversity. Thus, the variety of vibrant plumage colours has evolved as a direct result of social and environmental pressures.

**27**

**Table 1.**

of both [26, 27].

**Melanins**

**Carotenoids**

**Porphyrins**

*carotenoids. Revision date: 10th January 2021.*

**Figure 1.**

*Haloarchaea May Contribute to the Colour of Avian Plumage in Marine Ecosystems*

The colour of plumage and other structures in animals and plants is due to the presence of pigments (pigment-based coloration) or the presence of microscopically structured surfaces fine enough to interfere with visible light (structural coloration) [24]. Iridescence for instance, is one of the better-known examples of it [25]. In some cases, feather colours are the result of a combination

*Graph representation of the total number of publications per year from the database PubMed concerning the combination of the following keywords: ( ) bird feathers AND colour; ( ) bird feathers AND colour AND* 

**Table 1** summarizes the most representative pigments already identified as part of the colour of bird plumage. The most abundant are melanin and

**Name Colour References** Eumelanin Grey/Black [28–31] Pheomelanin Brown [28, 29, 31, 32]

Zeaxanthin Yellow [2, 33, 34] Lutein Bright Yellow [2, 33–35] β-carotene Yellow [2, 35] β-cryptoxanthin Yellow [2, 35] Canthaxanthin Orange Red [2, 33–35] Astaxanthin Red [2, 34–36] Rhodoxanthin Purple-red [34, 37, 38]

Turacoverdin Green [39] Coproporphyrin III Red Brown [40] Turacin Red [41]

*Summary of the main features characterizing the most abundant pigments in bird feathers.*

*DOI: http://dx.doi.org/10.5772/intechopen.96414*

*Haloarchaea May Contribute to the Colour of Avian Plumage in Marine Ecosystems DOI: http://dx.doi.org/10.5772/intechopen.96414*

#### **Figure 1.**

*Birds - Challenges and Opportunities for Business, Conservation and Research*

as feathers, skin, eyes, etc.

coloration [13].

tissues [15, 16].

feathers of coastal birds are also discussed.

**2. The colour of bird feathers**

coloration of avian plumage.

fungi and even phytoplankton [7–9]. All the mentioned organisms can synthesize carotenoids, but animals in general are not able to produce them *de novo* (aphids and spider mites are an exception, and it is assumed that they acquired this ability thanks to genes transferred from fungi [10]). Thus, animals obtain carotenoids from diet. After food uptake, they are mainly metabolized by the liver and intestinal epithelium [11] to be further incorporated into fatty tissues or other structures such

There are over 600 known carotenoids classified into two classes: xanthophylls (which contain oxygen) and carotenes (which are hydrocarbons without oxygen). Thanks to their chemical structure, they absorb wavelengths ranging from 400–550 nanometres (violet to green light) [12]. Consequently, these pigments are deeply coloured yellow, orange or red. Some carotenoids have vitamin A activity (they can be converted into retinol) and most of them can also act as antioxidants. Recently, it has been stated that cytochrome P450 enzymes are also involved in red carotenoid

Red coloured birds inhabiting salted environments such as salt marshes, seaside ecosystems, salted lagoons etc. may often acquire carotenoids by ingesting small organisms or even microorganisms like yeast and algae. Thus, flamingos (*Phoenicopterus sp*.) filter-feed on brine shrimp (*Artemia salina*) and blue-green algae (*Dunaliella salina*) [14], which are high rich sources of carotenoids. They are broken down into pigments by liver enzymes and fully incorporated into

The nature of the colour shown by red-pink feathers is one of the aspects strongly discussed during the last few years. Many works have demonstrated that the colour is due to the carotenoids obtained through the diet, whilst other studies suggested that other external factors like microorganisms or light phenomena could contribute to the final red-orange-pink phenotype. This chapter summarizes recent knowledge about the presence of alive microorganisms belonging to the Archaea domain on the surface of red-pink feathers thus may contributing to their colour. General aspects related to the carotenoids produced by haloarchaea inhabiting

Bird feathers have been the aim of several works during the last two centuries. Thus, the first reports on bird plumage listed in databases like PUBMED, Web of Science or Scopus analysed aspects focused on the muscles in charge of the feathers movement [17] or their growth [18]. Other aspects of bird feathers related to biological roles like sexual selection, colonization strategies or signalling have also been extensively explored [19–23]. These aspects are intricately connected to the

The first detailed studies about the colour of bird feathers were published in indexed scientific journals in the middle fifties last century. Since then, around 500 manuscripts have been reported on this subject (**Figure 1**). It is worthy to note that the number of studies about the colouration of plumage significantly increased at the beginning of XXI century (**Figure 1**). However, the number of publications focused on the presence of carotenoids in bird feathers is lower compared to those related to other issues affecting the phenotype of birds (**Figure 1**). Bird coloration (mainly in feathers) is one of the most studied topics to elucidate the role of natural and sexual selection in the evolution of phenotypic diversity. Thus, the variety of vibrant plumage colours has evolved as a direct result of social and environmental

**26**

pressures.

*Graph representation of the total number of publications per year from the database PubMed concerning the combination of the following keywords: ( ) bird feathers AND colour; ( ) bird feathers AND colour AND carotenoids. Revision date: 10th January 2021.*

The colour of plumage and other structures in animals and plants is due to the presence of pigments (pigment-based coloration) or the presence of microscopically structured surfaces fine enough to interfere with visible light (structural coloration) [24]. Iridescence for instance, is one of the better-known examples of it [25]. In some cases, feather colours are the result of a combination of both [26, 27].

**Table 1** summarizes the most representative pigments already identified as part of the colour of bird plumage. The most abundant are melanin and


#### **Table 1.**

*Summary of the main features characterizing the most abundant pigments in bird feathers.*

carotenoids [28, 42]. On the one hand, melanin-based coloration switches from brown to black due to the presence of phaeomelanin or eumelanin, respectively, or the number and distribution of the melanosomes [29, 30]. On the other hand, carotenoids-based colorations vary from yellow to red as previously mentioned.

The genetics of coloration in birds remains poorly described. However, it is extensively accepted that its expression is phenotypically plastic with a high sensitivity to variation in environmental conditions. Therefore, the melanin-based colour should be considered the key system to understand the molecular basis of phenotypic variations [43]. Some other pigments are only present in some species. This is the case of psittacofulvins, which are found just in a few species of parrots (*Psittacidae*) or penguins (*Spheniscidae*) [44–46] or turacoverdins, responsible for the bright green coloration of several birds of the family *Musophagidae*, most notably the turaco (*Turac*o sp.; *Musophagidae*). It is chemically related to turacin, a red pigment also found almost exclusively in turacos [39].
