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Visual Signals in the Crepuscular CommonNighthawk (Chordeiles minor)Author(s) AARON J ROTH GEORGE C ARGYROSREBECCA B BROWNINGSource The American Midland Naturalist 150(1)191-193 2003Published By University of Notre DameDOI httpdxdoiorg1016740003-0031(2003)150[0191VSITCC]20CO2URL httpwwwbiooneorgdoifull1016740003-0031282003291505B01913AVSITCC5D20CO3B2

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Visual Signals in the Crepuscular Common Nighthawk (Chordeiles minor)

ABSTRACTmdashThe common nighthawk (Chordeiles minor) is a crepuscular species known to usevisual signals for agonistic and sexual communication To test the hypothesis that whiteplumage patches function in visual communication of reproductive quality we measured thesize of white patches present on the wing tail and throat of the otherwise cryptic plumage ofmales of this species belonging to three age classes There was a significant increase in wing-band and tail-band size with age but no significant increase in throat-patch size We arguethat these white patches combined with behavioral displays likely function as visual signals ofmale quality

INTRODUCTION

Aragones et al (1999) proposed that nocturnal bird species rely on visual communication more thanpreviously thought They found that conspicuousness of white spots on the otherwise cryptic plumage ofmale red-necked nightjars (Caprimulgus ruficollis) increased with age They argued that these white spotsare used by this nocturnal species as visual signals to convey reproductive status (Aragones et al 1999)

The common nighthawk (Chordeiles minor) is a crepuscular species that uses visual displays for mateselection and territorial defense (Gross 1940 Weller 1958 Sutherland 1963) Territorial displaysinvolve the execution of a steep aerial dive at the end of which a lsquolsquoboomingrsquorsquo sound is produced byair rushing through the primaries a vocal call note is also given (Miller 1925) Although this dive isexecuted mostly as a display for territorial defense it is also done at the beginning of courtshipand above mates and young (Poulin et al 1996) Courtship displays by males involve fanning the tailand puffing the throat while uttering a croaking note from the ground in front of the female (Gross1940)

The plumage of this species is cryptic but white bands are present on the primaries of both sexesMales also have white bands on the rectrices and a white throat-patch The wing-band is larger on malesthan on females and in males it becomes larger with age (Selander 1954) However it is unknown if thesize of the tail-band or throat-patch increases with age We sought to determine the extent to which ifany the size of each white patch increases with age in male common nighthawks Such size differencescould be an indication of male quality by communicating how many reproductive seasons a potentialmate has survived and how likely a male is to successfully hold a territory We hypothesize that thesepatches function in visual communication of reproductive quality and predict that they should increasein size with age

METHODS

We examined 144 male common nighthawks from collections at the Museum of Comparative Zoologyat Harvard (MCZ) and the North Carolina Museum of Natural Sciences (NCSM) Relative age wasdetermined from plumage using four separate characteristics degree of primary tapering amount ofbuff edging on primaries and secondaries and degree of scapular mottling (see Pyle 1997) Birds wereseparated into three age classes hatch year (HY) second year (SY) and after second year (ASY) Toassess size we measured wing chords

We measured the right wing-band as well as the right half of the tail-band and throat-patch usingFowler Pro digital calipers following a similar protocol to Aragones et al (1999) The wing-band ismade up of white spots on the outermost five or six primaries The tail-band consists of white spots onthe outer four rectrices For wing-band and tail-band we measured the greatest breadth of both lengthand width for each feather spot to the nearest 001 mm and multiplied to determine area Spot areaswere summed to calculate total area of each band Throat-patch measurement was the sum of threemeasurements of the perimeter of the right half of the triangular patch (areas could not be calculateddue to inability to measure exact angles within the triangle) and represents the area visible when thefeathers are laying naturally Measurements of the three patches from each age class were comparedamong age classes using one-way ANOVA Where significant we applied a Tukeyrsquos test to determinewhich age classes were different

191

Am Midl Nat 150191ndash193

RESULTS

The 144 specimens examined included 13 HY 78 SY and 53 ASY (Table 1) Wing chord did notincrease significantly with age (F frac14 203df frac14 2P frac14 014) Wing-band size increased significantly amongthe three age classes (F frac14 2075df frac14 2P 001) No tail-bands or throat-patches are present in HYbirds Tail-band size increased significantly from SY to ASY birds (F frac14 848df frac14 1P 001) butthroat-patch size did not (F frac14 001df frac14 1P frac14 092) Most birds had white spots on primaries 6through 10 however there were white spots on the fifth primary in 3 HY 28 SY and 13 ASY individualsWith the exception of one individual white spots were present only on retrix 2 through 5 in all SYand ASY birds

DISCUSSION

Conspicuousness of plumage signals may be favored by selection pressure suggesting that increases inwing-band and tail-band size communicate which males have survived multiple breeding seasons and aretherefore more desirable as potential mates (Aragones et al 1999) Mean wing chord measurement didnot vary significantly among age classes suggesting differences in wing-band and tail-band size arelinked to age and not body size Throat patch appears to be involved only with gender recognitionbecause size did not vary between SY and ASY birds and is not present in HY birds

The wing-band is the largest of the three patches and the most visible during flight making it themost likely patch to be involved in territorial display The volume of the lsquolsquoboomrsquorsquo and call note atthe bottom of the dive varies with the intensity of the display becoming louder when conspecifics enterthe territory (Roth and Jones 2001) This display is usually restricted to dawn and dusk periods (Poulinet al 1996) however if there is sufficient artificial light the display is given throughout the night (Rothand Jones 2001) These behavioral factors in conjunction with the age-related increase in spot sizesuggests that the visual display of wing-bands and audible component of the lsquolsquoboomrsquorsquo and call notes areall important signals as part of this display Consequently larger more conspicuous wing-bands shouldbe advantageous to the visual component of this display Further males possessing such patches shouldhave a higher rate of success in holding territory adding to the attractiveness of such a male to potentialmates

The throat patch and tail-band are prominently displayed during courtship (Gross 1940) Thesepatches are only present in after hatch-year males suggesting that they play a role in identifying sexuallymature males Similar to the conclusions of Aragones et al (1999) we conclude that the increase in tail-band size is related to signaling function of prospective male mates

Nightjar vision is adapted for greater acuity in low light but it is likely that they have poorly developedcolor vision (Holyoak 2001) Therefore the use of white plumage signals that contrast with otherwisedark plumage should be an effective means of conveying signals in low light situations Age and sexrelated variation in wing-band and tail-band size has been reported for other nightjar species (Aragoneset al 1999) In these species plumage signals are obscured when the bird is perched then amplified inbehavioral displays Also in each case open or semi open habitats are used for transmission of signalsThis trend is present in the common nighthawk as well providing further evidence of the importance ofvisual signals among species active primarily in low-light situations

AcknowledgmentsmdashWe thank Douglas Causey John Gerwin and David Lee for providing specimensfor this study We also thank Mark Brigham and John Gerwin for reviewing early drafts of this paperSamuel Gutmann provided advice on statistical analysis

TABLE 1mdashMean and standard deviation of length (mm) of wing chord area (mm2) of wing-band tail-band and perimeter measurement (mm) of throat-patch in common nighthawks

Age class Wing chord Wing-band Tail-band Throat-patch

HY (n frac14 13) 1912 6 730 11827 6 4103 0 0SY (n frac14 78) 1916 6 104 15038 6 3172 5315 6 1638 752 6 92ASY (n frac14 53) 1946 6 66 17760 6 3521 6193 6 1703 754 6 104

192 THE AMERICAN MIDLAND NATURALIST 150(1)

LITERATURE CITED

ARAGONES J L A DE REYNA AND P RECUERDA 1999 Visual communication and sexual selection ina nocturnal bird species Caprimulgus ruficollis a balance between crypsis and conspicuous-ness Wilson Bull 111(3)340ndash345

GROSS A O 1940 Eastern Nighthawk p 206ndash234 In A C Bent (ed) Life histories of North Americancuckoos goatsuckers hummingbirds and their allies US Natl Mus Bull

HOLYAOK D T 2001 Nightjars and their allies the Caprimulgiformes Oxford University Press NewYork New York

MILLER A H 1925 The boomflight of the Pacific Nighthawk Condor 27141POULIN R G S D GRINDAL AND R M BRIGHAM 1996 Common Nighthawk (Chordeiles minor) In

A Poole and F Gill (eds) The birds of North America No 213 The Academy of NaturalSciences Philadelphia and The American Ornithologistsrsquo Union Washington DC

PYLE P 1997 Identification guide to North American birds Slate Creek Press Bolinas CaliforniaROTH A J AND G S JONES 2001 Territorial behavior of Common Nighthawks in an urban habitat Bird

Observer 29(2)107ndash114SELANDER R K 1954 A systematic review of the booming nighthawks of western North America Condor

5657ndash82SUTHERLAND C A 1963 Notes on the behavior of Common Nighthawks in Florida Living Bird 231ndash39WELLER M W 1958 Observations of the incubation behavior of a Common Nighthawk Auk 7548ndash59

AARON J ROTH1 Department of Biology Northeastern University Boston Massachusetts 02115GEORGE C ARGYROS Center for Vertebrate Studies Northeastern University Boston Massachusetts02115 AND REBECCA B BROWNING North Carolina Museum of Natural Sciences 11 West JonesStreet Raleigh North Carolina 27601 Submitted 8 January 2002 accepted 6 December 2002

1 Corresponding author

193NOTES AND DISCUSSION2003

RESULTS

The 144 specimens examined included 13 HY 78 SY and 53 ASY (Table 1) Wing chord did notincrease significantly with age (F frac14 203df frac14 2P frac14 014) Wing-band size increased significantly amongthe three age classes (F frac14 2075df frac14 2P 001) No tail-bands or throat-patches are present in HYbirds Tail-band size increased significantly from SY to ASY birds (F frac14 848df frac14 1P 001) butthroat-patch size did not (F frac14 001df frac14 1P frac14 092) Most birds had white spots on primaries 6through 10 however there were white spots on the fifth primary in 3 HY 28 SY and 13 ASY individualsWith the exception of one individual white spots were present only on retrix 2 through 5 in all SYand ASY birds

DISCUSSION

Conspicuousness of plumage signals may be favored by selection pressure suggesting that increases inwing-band and tail-band size communicate which males have survived multiple breeding seasons and aretherefore more desirable as potential mates (Aragones et al 1999) Mean wing chord measurement didnot vary significantly among age classes suggesting differences in wing-band and tail-band size arelinked to age and not body size Throat patch appears to be involved only with gender recognitionbecause size did not vary between SY and ASY birds and is not present in HY birds

The wing-band is the largest of the three patches and the most visible during flight making it themost likely patch to be involved in territorial display The volume of the lsquolsquoboomrsquorsquo and call note atthe bottom of the dive varies with the intensity of the display becoming louder when conspecifics enterthe territory (Roth and Jones 2001) This display is usually restricted to dawn and dusk periods (Poulinet al 1996) however if there is sufficient artificial light the display is given throughout the night (Rothand Jones 2001) These behavioral factors in conjunction with the age-related increase in spot sizesuggests that the visual display of wing-bands and audible component of the lsquolsquoboomrsquorsquo and call notes areall important signals as part of this display Consequently larger more conspicuous wing-bands shouldbe advantageous to the visual component of this display Further males possessing such patches shouldhave a higher rate of success in holding territory adding to the attractiveness of such a male to potentialmates

The throat patch and tail-band are prominently displayed during courtship (Gross 1940) Thesepatches are only present in after hatch-year males suggesting that they play a role in identifying sexuallymature males Similar to the conclusions of Aragones et al (1999) we conclude that the increase in tail-band size is related to signaling function of prospective male mates

Nightjar vision is adapted for greater acuity in low light but it is likely that they have poorly developedcolor vision (Holyoak 2001) Therefore the use of white plumage signals that contrast with otherwisedark plumage should be an effective means of conveying signals in low light situations Age and sexrelated variation in wing-band and tail-band size has been reported for other nightjar species (Aragoneset al 1999) In these species plumage signals are obscured when the bird is perched then amplified inbehavioral displays Also in each case open or semi open habitats are used for transmission of signalsThis trend is present in the common nighthawk as well providing further evidence of the importance ofvisual signals among species active primarily in low-light situations

AcknowledgmentsmdashWe thank Douglas Causey John Gerwin and David Lee for providing specimensfor this study We also thank Mark Brigham and John Gerwin for reviewing early drafts of this paperSamuel Gutmann provided advice on statistical analysis

TABLE 1mdashMean and standard deviation of length (mm) of wing chord area (mm2) of wing-band tail-band and perimeter measurement (mm) of throat-patch in common nighthawks

Age class Wing chord Wing-band Tail-band Throat-patch

HY (n frac14 13) 1912 6 730 11827 6 4103 0 0SY (n frac14 78) 1916 6 104 15038 6 3172 5315 6 1638 752 6 92ASY (n frac14 53) 1946 6 66 17760 6 3521 6193 6 1703 754 6 104

192 THE AMERICAN MIDLAND NATURALIST 150(1)

LITERATURE CITED

ARAGONES J L A DE REYNA AND P RECUERDA 1999 Visual communication and sexual selection ina nocturnal bird species Caprimulgus ruficollis a balance between crypsis and conspicuous-ness Wilson Bull 111(3)340ndash345

GROSS A O 1940 Eastern Nighthawk p 206ndash234 In A C Bent (ed) Life histories of North Americancuckoos goatsuckers hummingbirds and their allies US Natl Mus Bull

HOLYAOK D T 2001 Nightjars and their allies the Caprimulgiformes Oxford University Press NewYork New York

MILLER A H 1925 The boomflight of the Pacific Nighthawk Condor 27141POULIN R G S D GRINDAL AND R M BRIGHAM 1996 Common Nighthawk (Chordeiles minor) In

A Poole and F Gill (eds) The birds of North America No 213 The Academy of NaturalSciences Philadelphia and The American Ornithologistsrsquo Union Washington DC

PYLE P 1997 Identification guide to North American birds Slate Creek Press Bolinas CaliforniaROTH A J AND G S JONES 2001 Territorial behavior of Common Nighthawks in an urban habitat Bird

Observer 29(2)107ndash114SELANDER R K 1954 A systematic review of the booming nighthawks of western North America Condor

5657ndash82SUTHERLAND C A 1963 Notes on the behavior of Common Nighthawks in Florida Living Bird 231ndash39WELLER M W 1958 Observations of the incubation behavior of a Common Nighthawk Auk 7548ndash59

AARON J ROTH1 Department of Biology Northeastern University Boston Massachusetts 02115GEORGE C ARGYROS Center for Vertebrate Studies Northeastern University Boston Massachusetts02115 AND REBECCA B BROWNING North Carolina Museum of Natural Sciences 11 West JonesStreet Raleigh North Carolina 27601 Submitted 8 January 2002 accepted 6 December 2002

1 Corresponding author

193NOTES AND DISCUSSION2003

LITERATURE CITED

ARAGONES J L A DE REYNA AND P RECUERDA 1999 Visual communication and sexual selection ina nocturnal bird species Caprimulgus ruficollis a balance between crypsis and conspicuous-ness Wilson Bull 111(3)340ndash345

GROSS A O 1940 Eastern Nighthawk p 206ndash234 In A C Bent (ed) Life histories of North Americancuckoos goatsuckers hummingbirds and their allies US Natl Mus Bull

HOLYAOK D T 2001 Nightjars and their allies the Caprimulgiformes Oxford University Press NewYork New York

MILLER A H 1925 The boomflight of the Pacific Nighthawk Condor 27141POULIN R G S D GRINDAL AND R M BRIGHAM 1996 Common Nighthawk (Chordeiles minor) In

A Poole and F Gill (eds) The birds of North America No 213 The Academy of NaturalSciences Philadelphia and The American Ornithologistsrsquo Union Washington DC

PYLE P 1997 Identification guide to North American birds Slate Creek Press Bolinas CaliforniaROTH A J AND G S JONES 2001 Territorial behavior of Common Nighthawks in an urban habitat Bird

Observer 29(2)107ndash114SELANDER R K 1954 A systematic review of the booming nighthawks of western North America Condor

5657ndash82SUTHERLAND C A 1963 Notes on the behavior of Common Nighthawks in Florida Living Bird 231ndash39WELLER M W 1958 Observations of the incubation behavior of a Common Nighthawk Auk 7548ndash59

AARON J ROTH1 Department of Biology Northeastern University Boston Massachusetts 02115GEORGE C ARGYROS Center for Vertebrate Studies Northeastern University Boston Massachusetts02115 AND REBECCA B BROWNING North Carolina Museum of Natural Sciences 11 West JonesStreet Raleigh North Carolina 27601 Submitted 8 January 2002 accepted 6 December 2002

1 Corresponding author

193NOTES AND DISCUSSION2003