**2. Methods**

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

are more resistant to water penetration and forceful impact.

Water birds that swim, dive or plunge can be expected to show adaptations in their contour feathers, compatible with their foraging niches, that are absent in land birds that have no interaction with open water as indeed they do [6]. They show a water repellency and a resistance to water penetration in their contour feathers that vary with the family's specific behavioral patterns. Surface feeders tend to have a predominantly water repellent body plumage whereas those of divers and plungers

Birds that swim and dive will also benefit from reduced drag for their locomotion in water, a consideration that applies less to waders and shore birds and not at all to land birds. Viscous drag in water is dependent on the surface microstructure of the distal one-third of the contour feather, but also on the shape of its surface in contact with water, an aspect of feathers that has so far received little or no attention. Drag in air, such as in flight, on the other hand, has been the topic of several

That the shape of the surface area in contact with water varies among bird families has been noticed in the course of previous studies. It was seen to be nearly circular in land birds with a length-to-width ratio (L/W) of approximately 1.0, but oblong with an L/W of about 4 in penguins (*Spheniscidae*), the most aquatic of families. Birds less intimate with open water showed intermediate

In this chapter, we consider the interface between the distal one-third and flowing water to calculate viscous drag for feather shape geometry. Assuming the flow to be parallel to the long axis of the feather, i. e. zero angle of attack, we can derive the total drag coefficient (DC), composed of viscous pressure and frictional drag, from the computational and experimental results of studies on model ship hulls of varying length-to-diameter ratios using solutions to the Reynolds-averaged Navier– Stokes Equations [7]. For the relationship between drag coefficient and L/W, we

for values of L/W less than 7 which is within the range of feather geometry. The equation predicts that oblong shapes of the tips of contour feathers reduce drag facilitating swimming and diving, whereas a more circular shape would cause an increase in frictional drag. A similar reasoning could be applied to the shape of the area that the body of a swimming bird has in contact with water. If this area is assumed to be elliptical, a drag coefficient for body surface area in contact with

In order to establish if niche-specific adaptations in feather microstructure exist among bird species, various statistical approaches should be considered. Generalized least squares estimation of coefficients for linear models have been commonly used to investigate traits within phylogeny [8, 9]. However, statistical inaccuracies due to high type I errors are widespread without accounting for the evolutionary relationships. A more appropriate approach, described by Adams and Collyer (2018), incorporates phylogeny under a Brownian motion model of evolution while performing ANOVA. This phylogenetic-ANOVA approach offers additional advantages by accounting for group aggregation within phylogeny which

Our hypothesis is that water birds have contour feathers that exhibit in their shapes adaptations to reducing viscous drag according to their interaction with

water could be determined using the same equation.

could influence results and overall conclusions.

( ) <sup>3</sup> 0.0595L/W DC 10 4.071e − − = (1)

**40**

open water.

studies.

values for L/W.

then find

The measurements on contour feathers were performed on abdominal feathers as these are considered to be most representative of interaction with water. The primary source of feathers was the same as used for earlier studies which included water bird species from 11 orders and, for comparison, land bird species from 9 orders [10]. The species entered in this study are compiled in **Table 1**, using English names and taxonomic sequence suggested by Handbook of the Birds of the World [11].

The length and the width of the closed pennaceous portion of the contour feathers of the 48 water birds and twelve land birds in this study were measured to the nearest millimeter using a traveling microscope with the mid-part of the vane taken for the width. At least three feather specimens of each species were examined. The drag coefficients, listed in **Table 1**, were calculated from L/W values using the above equation.

Grouping the bird species according to their interaction with open water can be achieved by assigning them to foraging niches as proposed by Pigot et al. [12], using a standardized protocol for foraging niche delimitation. Following this procedure, a total of thirty niches has been identified for all of the approximately 10,000 bird species of the world. Of these six major foraging niches were categorized as Aquatic with two more chosen by us to accommodate the 48 water bird species of this study. The twelve land bird species could be grouped into two niches: Ground Feeding and Aerial/Sally.

All statistical analyses were conducted using the R statistical computer software (version 3.6.0). In addition to the foraging niches proposed [12] for aquatic birds (group 1) and land birds (group 2), four more analyses were performed using the values of L/W and DC for both land and aquatic bird species (consisting of the various foraging niches) categorized as the following independent variables: aquatic versus land birds (group 3), swimmers versus land birds (group 4), waders versus land birds (group 5) and swimmers versus waders (group 6). These groupings can be visualized in the context of a phylogeny in **Figure 1** and **Table 4**. Phylogenetic trees comprising of 60 bird species representatives of the independent groups were obtained from www. birdtree.org [13]. A total of 1000 trees were generated and a representative tree was constructed using the *maxCladeCred* function from the *phangorn* package (version 2.5.3).

The degree of group aggregation was determined in order to establish if the ANOVA methodology would be affected by the association between the independent variable, i. e. foraging niche and the phylogeny. Group aggregation was performed by calculating phylogenetic variance–covariance using the *vcv.phylo* function from the *ape* package (version 5.3), which was followed by performing a two-block partial least squares analysis using the *two.b.pls* function from the *geomorph* package (version 3.2.1). The degree of group aggregation was estimated by the proximity of the R-value to either 1 or 0, where values equal to or larger than 0.6 were considered strong aggregation. Significant group aggregation was considered for *p*-values <0.05.

In order to determine if the foraging niches for aquatic and land birds as well as the other independent variables, explain feather microstructure while accounting for phylogenetic relationships, a phylogenetic ANOVA (*procD.pgls* function from the *geomorph* package), conventional ANOVA (*aov* function) and non-parametric (*kruskal.test* function) equivalent approaches were followed. Significance among all analyses were accepted for *p*-values <0.05.

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


**3. Results**

*names are provided in Figure 1.*

**Table 1.**

aerial or sally sorties.

analysis for our data set.

*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds*

**ID# Bird Name FN Group L/W DC (10−3)** Spotted Dikkop, *B. capensis* Ground Feeding 2.43 3.523 White-fronted Plover, *C. marginatus* Aquatic Ground 1.78 3.662 Eurasian Curlew, *N. arquata* Aquatic Ground 1.94 3.628 Red Phalarope, *P. fulicarius* Aquatic Ground 2 3.615 Pale-faced Sheathbill, *C. albus* Ground Feeding 2.25 3.561 Pomarine Skua, *S. pomarinus* Aquatic Aerial 2.57 3.494 Lesser Black-backed Gull, *L. fuscus* Aquatic Surface 2.36 3.538 Sooty Tern, *S. fuscata* Aquatic Plunge 2.13 3.586 African Skimmer, *R. flavirostris* Aquatic Aerial 2.01 3.613 Common Murre, *U. aalge* Aquatic Dive 3.33 3.339 Namaqua Sandgrouse, *P. namaqua* Ground Feeding 1.2 3.799 Dusky Turtle-dove, *S. lugens* Ground Feeding 1.27 3.775 Brown-necked Parrot, *P. robustus* Ground Feeding 1 3.836 White-browed Coucal, *C. senegalensis* Ground Feeding 1.13 3.807 Rufous-cheeked Nightjar, *C. rufigena* Aerial/Sally 1.22 3.786 White-rumped Swift, *A. caffer* Aerial/Sally 1.18 3.795 Narina Trogon, *A. narina* Aerial/Sally 2.2 3.572 Half-collared Kingfisher, *A. semitorquata* Aquatic Perch 1.87 3.643 European Starling, *S. vulgaris* Ground Feeding 1.33 3.762

The results of the various forms of analyses are collected in the **Tables 2**–**4**. In **Table 2**, the 60 species of our study are presented as four categories. The 48 aquatic birds are subdivided into swimmers and waders. Their values for DC show a viscous drag coefficient for swimmers significantly lower (*p* < 0.05) than that of waders and, predictably, land birds. In **Table 3**, these categories are further subdivided into eight aquatic foraging niches and two terrestrial ones according to Pigot et al. [12]. It is seen that divers have the lowest recorded drag coefficient increasing in order for plungers, surface feeders, aerials, herbivore surface feeders, ground feeders, perchers to herbivore ground feeders. Land birds experience an even higher drag with no significant difference between ground feeders and those that catch their prey by

*Bird species, foraging niches (FN) and drag coefficients (DC) of the 60 species in this study. Their full scientific* 

In **Table 4**, the 60 species are divided among six groups to show the outcomes of the various statistical analyses used in this study. In the phy-ANOVA analysis, the closeness of the phylogenetic relatedness of the groups is accounted for whereas in conventional ANOVA it is not. However, the value of the latter suffers of shortcomings due to lack of equal sample size and equal variance among the populations in groups one to six. The non-parametric variant does not assume the conditions of equal sample size and variance and, for this reason, is a more appropriate method of

Group aggregations were performed to determine if phylogenetic relatedness and independent groupings could influence the reliability of the phylogenetic

*DOI: http://dx.doi.org/10.5772/intechopen.96994*


*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds DOI: http://dx.doi.org/10.5772/intechopen.96994*

#### **Table 1.**

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

**ID# Bird Name FN Group L/W DC (10−3)** Jackass Penquin, *S. demersus* Aquatic Dive 3.4 3.326 Magellanic Penquin, *S. magellanicus* Aquatic Dive 4 3.209 Gentoo Penguin, *P. papua* Aquatic Dive 3.33 3.339 Rockhopper Penguin, *E. chrysocome* Aquatic Dive 3.4 3.326 Great Northern Diver, *G. immer* Aquatic Dive 2.85 3.437 Little Grebe, *T. ruficollis* Aquatic Dive 2 3.615 Black-necked Grebe, *P. nigricollis* Aquatic Dive 1.73 3.673 Yellow-nosed Albatross, *T. chlororhynchos* Aquatic Surface 1.87 3.643 Great-winged Petrel, *P. macroptera* Aquatic Aerial 2.37 3.536 Blue Petrel, *H. caerulea* Aquatic Surface 2.75 3.457 Gray Petrel, *P. cinerea* Aquatic Surface 3.13 3.38 European Storm-Petrel, *H. pelagicus* Aquatic Aerial 2 3.615 Common Diving-Petrel, *P. urinatrix* Aquatic Dive 1.63 3.695 Great White Pelican, *P. onocrotalus* Aquatic Surface 2.68 3.472 Pink-backed Pelican, *P. rufescens* Aquatic Surface 2.17 3.579 Northern Gannet, *M. bassanus* Aquatic Plunge 2.5 3.509 Cape Gannet, *M. capensis* Aquatic Plunge 2.4 3.53 Cape Cormorant, *P. capensis* Aquatic Dive 2.6 3.488 Darter, *A. melanogaster* Aquatic Dive 3.14 3.377 Great Frigatebird, *F. minor* Aquatic Aerial 2.28 3.555 Gray Heron, *A. cinerea* Aquatic Ground 1.46 3.733 Black-headed Heron, *A. melanocephala* Aquatic Ground 1.45 3.734 Little Egret, *E. garzetta* Aquatic Ground 2 3.61 Hamerkop, *S. umbretta* Aquatic Ground 2.33 3.544 Yellow-billed Stork, *M. ibis* Aquatic Ground 2.22 3.568 Saddlebill, *E. senegalensis* Aquatic Ground 1.82 3.654 Sacred Ibis, *T. aethiopicus* Aquatic Ground 2.12 3.589 Greater Flamingo, *P. ruber* Aquatic Ground 2 3.615 Horned Screamer, *A. cornuta* H.A. Ground 1.19 3.794 Egyptian Goose, *A. aegyptiaca* Aquatic Surface 1.55 3.713 Yellow-billed Duck, *A. undulata* H.A. Surface 2.08 3.597 Coqui Francolin, *F. coqui* Ground Feeding 1.57 3.708 Blue Crane, *G. paradisea* Ground Feeding 2.69 3.469 Limpkin, *A. guarauna* Aquatic Ground 2.58 3.491 Red-knobbed Coot, *F. cristata* Aquatic Surface 1.5 3.724 African Finfoot, *P. senegalensis* Aquatic Surface 2.89 3.428 African Jacana, *A. africanus* Aquatic Ground 1.73 3.673 Greater Painted-snipe, *R. benghalensis* Aquatic Ground 2 3.615 Crab Plover, *D. ardeola* Aquatic Ground 2 3.615 African Black Oystercatcher, *H. moquini* Aquatic Ground 2.23 3.566 Pied Avocet, *R. avosetta* Aquatic Ground 2.36 3.538

*Bird species, foraging niches (FN) and drag coefficients (DC) of the 60 species in this study. Their full scientific names are provided in Figure 1.*

### **3. Results**

The results of the various forms of analyses are collected in the **Tables 2**–**4**. In **Table 2**, the 60 species of our study are presented as four categories. The 48 aquatic birds are subdivided into swimmers and waders. Their values for DC show a viscous drag coefficient for swimmers significantly lower (*p* < 0.05) than that of waders and, predictably, land birds. In **Table 3**, these categories are further subdivided into eight aquatic foraging niches and two terrestrial ones according to Pigot et al. [12]. It is seen that divers have the lowest recorded drag coefficient increasing in order for plungers, surface feeders, aerials, herbivore surface feeders, ground feeders, perchers to herbivore ground feeders. Land birds experience an even higher drag with no significant difference between ground feeders and those that catch their prey by aerial or sally sorties.

In **Table 4**, the 60 species are divided among six groups to show the outcomes of the various statistical analyses used in this study. In the phy-ANOVA analysis, the closeness of the phylogenetic relatedness of the groups is accounted for whereas in conventional ANOVA it is not. However, the value of the latter suffers of shortcomings due to lack of equal sample size and equal variance among the populations in groups one to six. The non-parametric variant does not assume the conditions of equal sample size and variance and, for this reason, is a more appropriate method of analysis for our data set.

Group aggregations were performed to determine if phylogenetic relatedness and independent groupings could influence the reliability of the phylogenetic

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

#### **Figure 1.**

*Phylogenetic tree depicting the phylogenetic relationships between the 60 bird species. The various foraging niches are displayed at the tree tips. Land bird species are illustrated on the tree edges as dashed lines and aquatic birds as solid lines. Swimming characteristics are illustrated by the right-hand bar between land birds (black), waders (gray) and swimmers (dark gray).*


#### **Table 2.**

*Sample size with L/W and DC values (including means (+/*− *standard deviation) of the various independent categories used in this study.*

ANOVA analysis. The results revealed the presence of a relatively strong (r > = 0.6) and significant (*p* < 0.05) group aggregation for groups 2, 4 and 6, thus showing its limiting effect on the reliability on the outcome of the ANOVA analysis. Groups 1, 3 and 5 reveal weaker group aggregation (r < 0.6) but significance (*p* < 0.05) only for group 3.

The results of statistical significance for LW and DC values are comparable for all groups and analyses and therefore significance among groups will be discussed as a single result. Results among the various independent groupings yielded inconsistent results between the three statistical approaches. Results of the phylogenetic ANOVA approach indicated that no significance was observed for all groups

**45**

**4. Discussion**

*aggregation.*

**Table 4.**

*1*

*2*

**Table 3.**

*niche.*

*Aquatic Niches.*

*Terrestrial Niches.*

*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds*

**Foraging Niche Sample Size LW DCf Aquatic Dive1** 11 2.855 +/− 0.739 3.439 +/− 0.156 **Aquatic Plunge1** 3 2.343 +/− 0.156 3.542 +/− 0.032 **Aquatic Surface1** 9 2.322 +/− 0.557 3.548 +/− 0.118 **Aquatic Aerial1** 5 2.246 +/− 0.218 3.562 +/− 0.046 **H.A. Surface1** 1 2.080 +/− NA 3.597 +/− NA **Aquatic Ground1** 17 2.036 +/− 0.264 3.615 +/− 0.063 **Aquatic Perch1** 1 1.87 +/− NA 3.643 +/− NA **H.A. Ground1** 1 1.19 +/− NA 3.794 +/− NA **Ground Feeding2** 9 1.652 +/− 0.596 3.693 +/− 0.130 **Aerial/Sally2** 3 1.533 +/− 0.472 3.718 +/− 0.103

(*p* < 0.05). Parametric results were highly contrasted against this result in that all groups with the exception of land birds indicated significant differences in feather microstructures. The non-parametric equivalent results in significance for groups 3, 4 and 6 and therefore corresponds with the results of phylogenetic ANOVA for groups 1, 2 and 5. The only consistent result across all analyses was group 2, the foraging niches of land birds, which indicated non-significance (*p* < 0.05).

*Summary of the outcome of the statistical analyses used in this study.*

**Group Phy-ANOVA Parametric Non-Parametric 1** Aquatic Birds NS S NS **2\*** Land Birds NS NS NS **3** Aquatic vs. Land NS S S **4\*** Swimmers vs. Land NS S S **5** Waders vs. Land NS S NS **6\*** Swimmers vs. Waders NS S S *S: Significant (p < 0.05). NS: Non-significant (p > 0.05). \*: Significant (p < 0.05) and strong (r > = 0.6) group* 

*Sample size with L/W and DC values (including means (+/*− *standard deviation) of the eight aquatic and two terrestrial foraging niches according to Pigot et al. [12]. Figure 1 Lists the birds that belong to each foraging* 

The present study has shown that adaptations in feather microstructure and body surface area in contact with water that bring about a reduction in viscous and frictional drag while swimming increase according to the bird's intimacy with open water. Swimming and diving birds, such as penguins and grebes, benefit the most from reduced viscous drag, more so than plungers such as gannets. Aerials such as terns even less so, but much more than herbivore surface feeders such as ducks.

*DOI: http://dx.doi.org/10.5772/intechopen.96994*

*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds DOI: http://dx.doi.org/10.5772/intechopen.96994*


#### **Table 3.**

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

ANOVA analysis. The results revealed the presence of a relatively strong (r > = 0.6) and significant (*p* < 0.05) group aggregation for groups 2, 4 and 6, thus showing its limiting effect on the reliability on the outcome of the ANOVA analysis. Groups 1, 3 and 5 reveal weaker group aggregation (r < 0.6) but significance (*p* < 0.05) only for

*Sample size with L/W and DC values (including means (+/*− *standard deviation) of the various independent* 

**Category Sample Size LW DCf Aquatic Birds** 48 2.304 +/− 0.587 3.56 +/− 0.124 **Swimmers** 30 2.484 +/− 0.625 3.515 +/− 0.130 **Waders** 18 1.986 +/− 0.325 3.625 +/− 0.074 **Land Birds** 12 1.623 +/− 0.570 3.699 +/− 0.125

*Phylogenetic tree depicting the phylogenetic relationships between the 60 bird species. The various foraging niches are displayed at the tree tips. Land bird species are illustrated on the tree edges as dashed lines and aquatic birds as solid lines. Swimming characteristics are illustrated by the right-hand bar between land birds* 

The results of statistical significance for LW and DC values are comparable for all groups and analyses and therefore significance among groups will be discussed as a single result. Results among the various independent groupings yielded inconsistent results between the three statistical approaches. Results of the phylogenetic ANOVA approach indicated that no significance was observed for all groups

**44**

group 3.

**Table 2.**

**Figure 1.**

*categories used in this study.*

*(black), waders (gray) and swimmers (dark gray).*

*Sample size with L/W and DC values (including means (+/*− *standard deviation) of the eight aquatic and two terrestrial foraging niches according to Pigot et al. [12]. Figure 1 Lists the birds that belong to each foraging niche.*


*S: Significant (p < 0.05). NS: Non-significant (p > 0.05). \*: Significant (p < 0.05) and strong (r > = 0.6) group aggregation.*

#### **Table 4.**

*Summary of the outcome of the statistical analyses used in this study.*

(*p* < 0.05). Parametric results were highly contrasted against this result in that all groups with the exception of land birds indicated significant differences in feather microstructures. The non-parametric equivalent results in significance for groups 3, 4 and 6 and therefore corresponds with the results of phylogenetic ANOVA for groups 1, 2 and 5. The only consistent result across all analyses was group 2, the foraging niches of land birds, which indicated non-significance (*p* < 0.05).
