**1. Introduction**

Coloration is one of the most conspicuous traits that varies among organisms. In the case of animals, colour is mainly due to: (i) the presence of pigments (carotenoids, melanin, turacoverdin, biliverdin, protoporphyrin, etc); (ii) light phenomena such as reflection/emission from animal structures (skin, feathers, etc.); (iii) the presence of microscopic structure in scales, bristles, or feathers, which give them brilliant iridescent colours (commonly named "structural colours") [1]; and (iv) general aspects related to genetics [2]. Due to these reasons, animals show different colours, which can slightly vary even between individuals belonging to the same species. Animal colorations are strongly linked to different biological roles: camouflage, sexual, social, and interspecific signalling, physical protection (against UV radiation for instance), and sexual dimorphism [3–6].

In the case of the birds, feathers play a key role in general coloration. Those that are red orange show these colours thanks to the presence of different carotenoids within their structures. Carotenoids are natural pigments widely spread in nature: chloroplasts and chromoplasts of plants, bacteria, archaea, microalgae, 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 as feathers, skin, eyes, etc.

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 coloration [13].

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 tissues [15, 16].

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 feathers of coastal birds are also discussed.
