Crest Length Research Articles

Radar backscattering from breaking wind waves: field observation and modelling

In this article, the results of a field study of the Ka-band (37.5 GHz) radar backscattering from breaking wind waves are presented. Radar and a video camera were simultaneously used to measure the radar cross section (RCS) of the whitecap zone and the characteristics of wave-breaking events. A comparison is made between absolute geometrical lengths, areas and orientations of the whitecaps and the RCS of the whitecaps at moderate (45°) and high (70°) incidence angles. A thin vegetable oil film covering the illuminated area was made to minimize background Bragg backscattering from the surface free of breaking-wave features. At high incidence angles, a linear-like dependence between breaking crest length and RCS and no significant azimuthal dependence are found. At moderate angles of incidence, a prominent azimuthal variation of the whitecap's RCS is shown, while a linear-like relationship between the active breaking area and the whitecap's RCS is obtained in presence of the film on the sea surface. The results are compared to the model of quasi-specular reflection from the breaking waves. It is shown that accounting for backscattering from the droplets produced by the breaking crest improves prediction of the whitecap's RCS given by this model at high incidence angles.

The crest of the peafowl: a sexually dimorphic plumage ornament signals condition in both males and females

Both male and female peafowl grow crests on top of their head - iridescent blue in males, dull iridescent green and brown in females - but the potential signal function of this plumage ornament is unknown. In this study, peafowl crests were measured in three feral populations, and morphological variation in this ornament was studied in relation to body condition (body mass in relation to tarsus length) and health (white blood cell concentration and ectoparasite load). Prior to the start of the breeding season, male crests are wider with greater pennaceous area, and are more likely to have all feathers grown out compared with female crests. Only crest length changed with measurement date, increasing over time; in males, crest measurements were not related to the extent of train feather development. Crest morphology is a potential signal of individual health and condition in both sexes, but in different ways. In females, the amount of crest plumage grown out to its full extent was related to body condition at the start of the breeding season, whereas in males, the size and pennaceous area of the ornament were related to ectoparasite load. Observations of within-sex agonistic behaviour suggest a possible role for the crest ornament in status signaling in males, because males that engage in more aggressive interactions tend to have wider crests. There was no evidence for a relation between crest morphology and agonistic behaviour in females.

The linkage between fault throw and footwall scarp erosion patterns: an example from the Bremstein Fault Complex, offshore Mid‐Norway

AbstractStudies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault‐related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity‐driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid‐Norway. This 22‐km‐long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first‐order control on footwall erosion patterns is the overall fault‐throw distribution.

Earthquake Behavior of Berke Arch Dam Using Ambient Vibration Test Results

The Berke Arch Dam is the highest arch dam constructed in Turkey. The dam height is 201 m, and the crest length is 270 m. This paper describes the Berke Arch Dam, its finite-element modeling, ambient vibration testing, finite-element model calibration, and earthquake behavior before and after model calibration. First, three-dimensional (3D) models of dam-reservoir-foundation interaction were developed to obtain analytical dynamic characteristics, such as natural frequencies and mode shapes using the Ansys finite-element program. In the analyses, reservoir water was represented by a Lagrangian approach. Then, ambient vibration tests were conducted on the dam on 4 days in May 2009 to obtain experimental dynamic characteristics. In ambient vibration tests, the sensitive accelerometers were placed on several points on the arch dam, and signals were collected from accelerometers. The enhanced frequency domain decomposition technique was used in the extraction of experimental natural frequencies, mode shapes, and damping ratios. After that, 3D finite-element models of the Berke Arch Dam were calibrated using ambient vibration test results. Finally, earthquake behaviors of initial and calibrated models of the Berke Arch Dam were obtained using the Adana-Ceyhan Earthquake in 1998. It was observed that model calibration affects the results considerably.

Discharge Capacity of Piano Key Weirs

In recent years, spillway rehabilitation has increased in importance and become the subject of many projects worldwide. One solution for this problem is the implementation of a new type of labyrinth spillway, called Piano Key Weir (PK-Weir ). This is an excellent alternative for increasing the overflow capacity of existing dams. Similarly to traditional labyrinth weirs, the hydraulic capacity of a PK-Weir is a function of its geometrical characteristics. Currently, there is a lack of systematic experiments, and the existing data does not allow the proposition of a universal design procedure. This paper reviews the previous studies on the efficiency of planned and built PK-Weirs. The results are evaluated by comparing an actual PK-Weir’s discharge to that theoretically obtained for a sharp-crested spillway with crest length equal to the width of the PK-Weir for a given hydraulic head. On the basis of this evaluation, a preliminary design procedure is proposed.

Vocal and Optical Indicators of Individual Quality in a Social Seabird, the Crested Auklet (Aethia cristatella)

Indicators of individual quality in ornamentation or in vocalizations have been reported for different animal species. However, no studies have jointly investigated ornamentation and vocalizations in one species. The crested auklet (Aethia cristatella, Alcidae) is a small colonial seabird of the North Pacific and is unusual for a bird in using optical, vocal, and olfactory signals. We estimated the potential for coding individual quality in vocalizations and plumage ornamentation and the relationship between vocal and optical traits in crested auklets. During the summer seasons of 2008–2009, we recorded 359 trumpet calls from 28 individually marked males and measured indices of body size, condition, and plumage patterns of 58 male and 48 female crested auklets from a breeding colony on Talan Island, Sea of Okhotsk. We found strong interindividual differences in trumpet call characteristics. Furthermore, we found that the maximum fundamental frequency of the loudest notes of the trumpet call is negatively correlated with body condition, and head crest length is positively correlated with body size. However, we found no relationship between vocal and ornamental traits. The results suggest that advertising calls of crested auklets signal caller individuality and quality, but future experimental studies should test whether or not this is indeed case.

Inception Point and Air Entrainment on Flows under Macroroughness Condition

Rock chutes or block ramps are fishway passages with low environmental impact. They also contribute to reaeration of rivers with low dissolved oxygen content, owing to the turbulence enhanced by their three-dimensional macroroughness conditions. This paper analyzes the air entrainment inception in flows over beds in macroroughness condition and the self-aerated flow features of the developing flow downstream of the inception point. Air concentration, inception point locations, and water depth elevations have been measured on two different scaled chutes for slopes ranging between 1V:5.88H and 1V:2.17H. Moreover, two different ogee crest lengths have been tested to assess the role of the inlet conditions on the location of the inception point. New equations have been developed to estimate the location of the point of inception and the respective water depth. Longitudinal variations of the mean air concentration downstream from the inception point have been studied and compared with data from the literature. An expression is presented to estimate the optimum length of the block ramp in natural rivers for maximizing air-water mixing.

Modelling controls on aeolian dune‐field pattern evolution

AbstractA second‐generation, source‐to‐sink cellular automaton‐based model presented here captures and quantifies many of the factors controlling the evolution of aeolian dune‐field patterns by varying only a small number of parameters. The role of sediment supply, sediment availability and transport capacity (together defined as sediment state) in the development and evolution of an aeolian dune‐field pattern over long time scales is quantified from model simulations. Seven dune‐field patterns can be classified from simulation results varying the sediment supply and transport capacity that control the type and frequency of dune interactions, the sediment availability of the system and, ultimately, the development of dune‐field patterns. This model allows predictions to be made about the range of sediment supply and wind strengths required to produce the dune‐field patterns seen in the real world. A new clustered dune‐field pattern is identified from model results and used to propose an alternative mechanism for the formation of superimposed dunes. Bedforms are hypothesized to cluster together, simultaneously forming two spatial scales of bedforms without first developing a large basal dune with small superimposed dunes. Manipulation of boundary conditions produces evolving dune fields with different spatial configurations of sediment supply. Trends of spacing and crest length increase with decreasing variability as the dune field matures. This simple model is a valuable tool which can be used to elucidate the dominant control of aeolian sediment state on the construction and evolution of aeolian dune‐field patterns.

Experimental Studies on Flow over Labyrinth Weir

AbstractLabyrinth spillway is the effective and economical solution to increase the discharge by increasing the spillway crest length without an associated increase in structure width. Use of labyrinth spillway is particularly suited to sites where the spillway width and upstream water surface are limited and larger discharging capacities are required. Weir and spillways designed on the basis of the labyrinth concept may be adopted with facility at sites in which certain constraints render other types unsuitable. Recently, their use is found to be attractive solution for upgrading existing development to satisfy more demanding design flood criteria in the limited waterway. The existing spillway is overcoming its earlier limitations of unusual shape and other complicated flow patterns. Continued efforts are being focused towards development of design curves with different shapes and configurations. The present research primarily aims at evaluating various characteristics of a flow-over labyrinth weir by co...

On the structure and propagation of internal solitary waves generated at the Mascarene Plateau in the Indian Ocean

Analysis of a comprehensive dataset of Synthetic Aperture Radar (SAR) images acquired over the sea area around the Mascarene Plateau in the western Indian Ocean reveals, for the first time, the full two-dimensional spatial structure of internal solitary waves in this region of the ocean. The satellite SAR images show that powerful internal waves radiate both to the west and east from a central sill near 12.5°S, 61°E between the Saya de Malha and Nazareth Banks. To first order, the waves appear in tidally generated packets on both sides of the sill, and those on the western side have crest lengths in excess of 350 km, amongst the longest yet recorded anywhere in the world's oceans. The propagation characteristics of these internal waves are well described by first mode linear waves interacting with background shear taken from the westward-flowing South Equatorial Current (SEC), a large part of which flows through the sill in question. Analysis of the timings and locations of the packets indicates that both the westward- and eastward-traveling waves are generated from the western side of the sill at the predicted time of maximum tidal flow to the west. The linear generation mechanism is therefore proposed as the splitting of a large lee wave that forms on the western side of the sill, in a similar manner to that already identified for the shelf break generation of internal waves in the northern Bay of Biscay. While lee waves should form on either side of the sill in an oscillatory tidal flow, that on the western side would be expected to be much larger than that on the eastern side because of a superposition of the tidal flow and the steady westward flow of SEC. The existence of a large lee wave at the right time in the tidal cycle is then finally confirmed by direct observations. Our study also confirms the existence of second mode internal waves that form on the western side of the sill and travel across the sill towards the east.

Design flood hydrographs from the relationship between flood peak and volume

Abstract. Hydrological frequency analyses are usually focused on flood peaks. Flood volumes and durations have not been studied as extensively, although there are many practical situations, such as when designing a dam, in which the full hydrograph is of interest. A flood hydrograph may be described by a multivariate function of the peak, volume and duration. Most standard bivariate and trivariate functions do not produce univariate three-parameter functions as marginal distributions, however, three-parameter functions are required to fit highly skewed data, such as flood peak and flood volume series. In this paper, the relationship between flood peak and hydrograph volume is analysed to overcome this problem. A Monte Carlo experiment was conducted to generate an ensemble of hydrographs that maintain the statistical properties of marginal distributions of the peaks, volumes and durations. This ensemble can be applied to determine the Design Flood Hydrograph (DFH) for a reservoir, which is not a unique hydrograph, but rather a curve in the peak-volume space. All hydrographs on that curve have the same return period, which can be understood as the inverse of the probability to exceed a certain water level in the reservoir in any given year. The procedure can also be applied to design the length of the spillway crest in terms of the risk of exceeding a given water level in the reservoir.

Discharge Coefficient for Trapezoidal Labyrinth Side Weir in Subcritical Flow

The discharge coefficient of a trapezoidal labyrinth side weir is a function of the Froude number F1, the dimensionless effective crest length L/l, the dimensionless weir length L/B, the dimensionless weir height p/h1, and the sidewall angle α. A labyrinth weir is an overflow weir, folded in plan view to provide a longer total effective length for a given overall weir width. These weirs have advantages compared to the straight overflow weir and the standard ogee crest. Previous studies on the subject have generally focused on rectangular side weirs located on a straight channel. The present study investigates the hydraulic behavior of a trapezoidal labyrinth side weir. The results show that the discharge coefficient of labyrinth side weirs gives a significantly higher coefficient value compare to that of conventional straight side weirs. Discharge coefficient of the trapezoidal labyrinth side weir is 1.5 to 5.0 times higher than the conventional straight side weir. Consequently, an equation for the coefficient of discharge is introduced. The results predicted by the equation were shown to be very satisfactory using root mean square error (RMSE), mean absolute error (MAE) and correlation coefficient (R) statistics.

Observations of Wave Breaking Kinematics in Fetch-Limited Seas

Abstract Breaking waves play an important role in air–sea interaction, enhancing momentum flux from the atmosphere to the ocean, dissipating wave energy that is then available for turbulent mixing, injecting aerosols and sea spray into the atmosphere, and affecting air–sea gas transfer due to air entrainment. In this paper observations are presented of the occurrence of breaking waves under conditions of strong winds (10–25 m s−1) and fetch-limited seas (0–500 km) in the Gulf of Tehuantepec Experiment (GOTEX) in 2004. An airborne nadir-looking video camera, along with a global positioning system (GPS) and inertial motion unit (IMU), provided digital videos of the breaking sea surface and position in an earth frame. In particular, the authors present observations of Λ(c), which is the distribution of breaking wave crest lengths per unit sea surface area, per unit increment in velocity c or scalar speed c, first introduced by O. M. Phillips. In another paper, the authors discuss the effect of processing methodology on the resulting shape of the Λ(c) distribution. In this paper, the elemental method of measuring breaking crests is used to investigate the Λ(c) distributions under a variety of wind and wave conditions. The integral and the first two moments of the Λ(c) distributions are highly correlated with the active breaking rate and the active whitecap coverage. The computation of whitecap coverage yields a larger observational dataset from which the variability of whitecap coverage with wind speed, friction velocity, wave age, and wave slope is presented and compared to previous observations. The dependence of the active breaking rate on the spectral peak steepness is in agreement with previous studies. Dimensional analysis of Λ(c) indicates that scaling with friction velocity and gravity, as in the classical fetch relations, collapses the breaking distributions more effectively than scaling with dominant wave parameters. Significant wave breaking is observed at speeds near the spectral peak in young seas only, consistent with previous studies. The fourth and fifth moments of Λ(c) are related to the flux of momentum transferred by breaking waves to the underlying water and the rate of wave energy dissipation, respectively. The maximum in the fourth moment occurs at breaking speeds of 5–5.5 m s−1, and the maximum in the fifth moment occurs at 5.8–6.8 m s−1, apparently independent of wave age. However, when nondimensionalized by the phase speed at the peak of the local wave spectrum cp, the maxima in the nondimensionalized fourth and fifth moments show a decreasing trend with wave age, obtaining the maxima at dimensionless speeds c/cp near unity at smaller wave ages and moving to lower dimensionless speeds c/cp ≪ 1 at larger wave ages. The angular dependence of Λ(c) is predominantly unimodal and better aligned with the wind direction than the dominant wave direction. However, the directional distribution of Λ(c) is broadest for small c and often exhibits a bimodal structure for slow breaking speeds under developing seas. An asymmetry in the directional distribution is also observed for moderately developed seas. Observations are compared to the Phillips model for Λ(c) in the equilibrium range of the wave spectrum. Although the ensemble of Λ(c) distributions appears consistent with a c−6 function, the distributions are not described by a constant power-law exponent. However, the Λ(c) observations are described well by the Rayleigh distribution for slow and intermediate speeds, yet fall above the Rayleigh distribution for the fastest breaking speeds. From the Rayleigh description, it is found that the dimensionless width of the Λ(c) distribution increases weakly with dimensionless fetch, s/u*e = 1.69χ0.06, where s is the Rayleigh parameter, u*e is the effective friction velocity, and the dimensionless fetch is a function of the fetch X and gravitational acceleration g. The nondimensionalized total length of breaking per unit sea surface area is found to decrease with dimensionless fetch for intermediate to fully developed seas, , where A is the total length of breaking crests per unit sea surface area.

Maternal genetic effects on body weight and breast morphological traits in duck population under selection

Abstract. Direct and maternal additive genetic and mitochondrial variances of duck meat performance traits were estimated using AI-REML algorithm. Records of 3 099 (5 010 pedigreed birds) from six consecutive generations were included. The following four traits were investigated: body weight at 3rd week, body weight at 7th week, sternum crest length at 7th week (in cm), and breast muscles thickness at 7th week (in cm). The data were analysed via three single trait linear animal models: I – additive genetic model, II – model extended to additive maternal effects; III– as model II with mitochondrial effects as random. Adequacy of the models was examined by Akaike´s information criterion (AIC). Relatively high direct additive heritability estimates were obtained for body weight at 3rd week (0.4326–0.4546) and body weight at 7th week (0.5322–0.6088) whereas lower estimates were obtained for sternum crest length (0.1756–0.2744) and breast muscles thickness (0.1369–0.2932). The maternal heritabilities were moderate and also considerably depended on the model used. For all of the studied traits a negative covariance between direct and maternal genetic effects was found. Mitochondrial heritabilities were very low. Generally, on the basis of criteria employed, the model III can be suggested for analysis of body weight whereas for other traits model II seems to be suitable.

Maximum power extraction from ocean wave energy conversion devices.

A generic energy conversion device at the ocean surface is considered. Incident water waves of given frequency and amplitude are incident on the device and one asks just how much power PD can be converted into electricity by the device. A primary descriptor of the water wave is the time averaged power PW carried by the wave, integrated over water depth, per unit length of wave crest. The relevant descriptor of the device’s performance is the capture length, equal to the quotient PD/PW. Basic theory, not taking into account the details of the device’s design, predicts a maximum attainable capture length in terms of the wavelength and of the overall dimensions of the device. For example, the recent work by Thiam and Pierce indicates that the maximum capture length of a Pelamis‐like device is equal to a constant times the area of the device viewed from above divided by the wavelength. Another analysis for a “point” vibrating resonator indicates that the maximum capture length is of the order of 40 times the cross‐sectional diameter of the device. Knowledge of the maximum capture length enables a quick assessment of whether any proposed wave energy farm will deliver a desired amount of power.

The application of 3-D biomodeling technology in complex mandibular reconstruction—experience of 47 clinical cases

3-D biomodeling technology associated with stereolithography has become a fundamental tool in reconstructive surgery in recent years as it enables a highly accurate definition of the defect to be reconstructed. In complex mandibular reconstruction using free flaps, the exact 3-D characteristics of the flap are decisive to restore symmetry and structural integrity, which enables a good functional and aesthetic result. This paper reports the application of an innovative 3-D biomodeling approach for the calculation of the exact contours, angles, length, and general morphology of iliac crest and fibula free flaps for mandibular reconstruction. Between 2003 and 2009, we managed 47 complex mandibular reconstruction cases using this technology. The free flaps used were fibula (n = 29) and iliac crest (n = 18) transferred as osseous or osteocutaneous flaps. From computer tomography scans, virtual 3-D images of the patient's anatomy were converted into 3-D physical models using first Freeform Modeling® software and then selective laser stereolithography technology. Intra-operative bone osteotomies and modeling were guided by these biomodels. Functional and aesthetic end results were at least considered satisfactory in 95.7% of the patients. The majority (80.8%) were in the good and very good end-result categories. This personalized designing of free flaps enabled optimization of pre-operative planning, reduced operative time, and significantly improved the aesthetic and bio-functional outcome. We found it to be particularly useful in major secondary defects (oncological, osteoradionecrosis, trauma) with important distortion of craniofacial structure, congenital malformations, and primary tumor surgery when the dimensions of the tumor have altered normal anatomy considerably.

Dune field pattern formation and recent transporting winds in the Olympia Undae Dune Field, north polar region of Mars

High‐Resolution Imaging Science Experiment (HiRISE) imagery of the central Olympia Undae Dune Field in the north polar region of Mars shows a reticulate dune pattern consisting of two sets of nearly orthogonal dune crestlines, with apparent slipfaces on the primary crests, ubiquitous wind ripples, areas of coarse‐grained wind ripples, and deflated interdune areas. Geomorphic evidence and dune field pattern analysis of dune crest length, spacing, defect density, and orientation indicates that the pattern is complex, representing two constructional generations of dunes. The oldest and best‐organized generation forms the primary crestlines and is transverse to circumpolar easterly winds. Gross bed form‐normal analysis of the younger pattern of crestlines indicates that it emerged with both circumpolar easterly winds and NE winds and is reworking the older pattern. Mapping of secondary flow fields over the dunes indicates that the most recent transporting winds were from the NE. The younger pattern appears to represent an influx of sediment to the dune field associated with the development of the Olympia Cavi reentrant, with NE katabatic winds channeling through the reentrant. A model of the pattern reformation based upon the reconstructed primary winds and resulting secondary flow fields shows that the development of the secondary pattern is controlled by the boundary condition of the older dune topography.

Effect of upstream crest length on flow characteristics and discharge capacity of triangular labyrinth side weirs.

A series of laboratory experiments were performed within this study in order to investigate the hydraulic characteristics and the discharge capacity of sharp-crested triangular labyrinth side weirs with various upstream crest lengths. To conduct the experiment series, twenty seven different labyrinth side weir models having triangular plan view that are manufactured from steel were tested in the laboratory flume. A labyrinth weir is an overflow weir that is folded in plan view to provide a longer total effective length for a given overall weir width. To estimate the outflow over a side weir, the discharge coefficient in the side weir equation needs to be determined. A comprehensive laboratory study including 679 tests was conducted to determine the discharge coefficient of the side weirs. It was found that the discharge coefficient of triangular labyrinth side weirs was higher than that of classical side weirs. In addition, reliable equations to calculate the discharge coefficient of side weirs are presented.

Aeolian dune interactions and dune-field pattern formation: White Sands Dune Field, New Mexico

Pattern formation is a fundamental aspect of self-organization in fields of bedforms. Time-series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off-centre collision; (vi) defect creation; and (vii) dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off-centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of dunes, crest length, defect density, crest spacing and dune height), dune migration rates, and the type and frequency of dune interactions within a 3500 m box transect from the upwind margin to the core of the dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo-lake topography. Pattern evolution is most strongly coupled to the pattern parameters of dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the dunes become further apart and have fewer defects.

Origin and maintenance of mechanosensory feather ornaments

Mechanosensory use is a seldom-mentioned function for feather ornaments, yet recent experimental evidence showed that the elaborate facial plumes of crevice-dwelling whiskered auklets, Aethia pygmaea, have just such a sensory role. Here we explored the evolutionary patterns of mechanosensory function of similar facial feather ornaments in related species. In an experimental chamber (maze) devoid of visible light, crested auklets, A. cristatella, a close relative of the whiskered auklet, showed an increase in head bumps (262%) after trial flattening of their forehead crest. The frequency of head bumps in the absence of the crest was positively correlated with the natural crest length of the crested auklet. There was no correlation between crest length and head bumps when we added an artificial crest to least auklets, A. pusilla, which do not have a natural crest. Thus, only the ornamented Aethia species that breed in deep-crevice appear to have detectable mechanosensory ability. A pairwise analysis across all nonpasserine bird families further revealed a greater frequency of elongated facial plumes in birds that live in complex habitats and are active during low light conditions. We suggest that selective pressure enforced by complex habitats may trigger facial feather exaggeration for mechanosensory use. Once the primordial sensory structures evolved, sexual and other social selection processes could act on these traits and lead towards further exaggeration.