**6. Biomonitoring using birds through the non-invasive methods**

### **6.1 Feathers**

Raptors have been the most frequently used species in biomonitoring research on the environment because of their wide geographic spread for foraging purposes and also because of their higher position on the food chain, which can reflect the number of contaminants flowing through it [57]. However, because a number of raptor species are protected, non-invasive methods can be used. According to Dauwe et al. [58], sampling of feathers is simple, and feathers have already been used in numerous research [59]. According to Burger's research, heavy metals have the propensity to bind to protein molecules during the brief embryonic stage of a feather when tiny blood capillaries connect it to the bloodstream. References [58, 59] came to the conclusion that after the formation of the feathers, the blood vessels become weak and physically detach from the bird. Birds shed a sizable amount of heavy metals during the moulting process through their plumage [57]. Some interior tissues begin displaying decreased metal levels as soon as the metals are sewn into the feathers. When a moult is finished, the internal level of a few heavy metals rises until the next moult, which is a constant process [60]. According to research by [61], if the overall amount of heavy metals in a bird's body is reflected in the feathers as they form, the feathers that are moulted at the conclusion of the process should do so with lower concentrations than those that are moulted at the beginning. The concentration of various heavy metals inside the body of the bird during moulting may vary, being higher at the start and lower towards the end of the process. Birds also deposit toxins like Cd, Cr, and Pb into their developing feathers, which attach to the keratin in feathers and become biologically separated [62]. In addition, the accumulation of these metals into the calcareous tissues is a significant way for female birds to eliminate Pb [63], but Zn and Cu are favourably deposited in the feathers when concentrations surpass permitted limits [64]. Studies by [65, 66], revealed that during the brief period of egg hatching and while it matures, hatchling feathers can indicate local contamination that may result from their parents consuming food from the area [65].

*Birds as Intrinsic Bio-Indicators for Probing Heavy Metal Contamination Signatures in Polluted… DOI: http://dx.doi.org/10.5772/intechopen.110449*

### **6.2 Eggs**

Over time, it has been discovered that the presence of metals in feathers, excrement, and eggs also causes harm to the bird population [65]. Hashmi [67] reported using feathers and eggs for biomonitoring soil, but due to their lengthy storage times and ease of sampling, this method is related to one's own. Additionally, it was determined that because eggs are formed at a specific point in a female bird's life cycle, they can act as a superb signal for local exposure in addition to belonging to a particular subset of egg-laying birds. According to Jayakumar and Muralidharan [68], anthropogenic inputs are the cause of the presence of unnecessary trace elements in both domestic and wild birds. The harmful effects of metal pollution might include everything from kidney and testicular damage to reduced egg production [69]. Bird eggs, according to reference [68] are among the most complex and distinctive types of reproductive cells. They have a protective coating (or shell) that encloses albumin and yolk contents that vary greatly in volume, shape, weight, and amount. From tropical to temperate zones, according to reference [57], female bird species tend to forage on their breeding grounds before to egg production after egg laying. According to reference [67], utilising eggs as a biomonitoring technique provides a number of benefits over using feathers since eggs are more stable and do not change with age, size, or body structure. Eggs have recently been employed in a number of biomonitoring studies, and it has thus been demonstrated that the collection of a single egg without harming the environment has a negligible impact on the population of the species [5]. A given metal's circulation concentration in birds, which is then reflected in egg quality, can be closely correlated with recent exposure [67]. According to reference [70], the egg shell and egg content are typically where metals collect throughout the female's metal deposition. In a similar vein, Burger et al. [43] came to the conclusion that heavy metal exposure from the parent's local environment is what puts eggs in the greatest danger of being harmed.

### **6.3 Excreta**

According to reference [47], investigations on heavy metal contamination in birds are extremely rare in nations like India. Additionally, the Wildlife Protection Act of India (1972) forbids the capture and sacrifice of birds, which makes it challenging for scientists to conduct any analytical research on eggs and tissues. Excreta might therefore be chosen as a different source that can provide clear information on environmental contamination [47, 71]. Birds are exposed to heavy metals through contaminated water sources, excessive metal concentrations in the atmosphere, and food sources close to point sources [72]. The rate of heavy metal absorption varies according to the physiology of the species and is influenced by the metals' bioavailability and other factors. By storing them in the salt gland, uropygial gland, or excreta, birds can get rid of heavy metals [47]. Bird excretions are the best indicator of the environment's level of heavy metal contamination since they excrete more metals than other animals [73]. Excreta from birds serve as helpful non-destructive bioindicators and provide information on the bird's diet [74]. It also details the kinds of metals absorbed and removed [75].

A total of five types of pollution indicator species were identified by [76] which are represented in **Table 1**.


**Table 1.**

*Type of pollution indicator species.*
