Responses of Barn Swallows to Eggs, Young, Nests, and Nest Sites

Condor, 81236~246 0 The Cooper Ornithological Society 1979

RESPONSES OF BARN SWALLOWS TO EGGS, YOUNG, NESTS, AND NEST SITES

JOSEPH A. GRZYBOWSKI

Aspects of egg, young, nest and nest site gentutus) nests moved about 30 cm were recognition have been investigated or ob- accepted if they contained eggs and the served in several species of birds (e.g., Cul- original nest site was covered with sand len 1957, Davies and Carrick 1962, Beer (Tinbergen 1953). In experiments with 1969, Peek et al. 1972). In general, selection Sooty Terns (Sternufuscutu), birds returned appears to favor recognition of as few char- to their nest sites rather than to nests dis- acteristics as necessary to insure a high placed short distances (Lashley 1915). From probability of caring for the correct eggs or these studies, it appears that selection fa- young. Birds that build nests respond to vors more rigid responses to changes in nest their nest sites, and later their young (usu- and nest site for colonial than for non-colo- ally when mobile), but show little evidence nial species. of specifically recognizing their own nests In the present study, I evaluated re- or eggs (Nice 1943, Tinbergen 1953, Beer sponses of Barn Swallows (Hirundo rustica) 1970). Common Murres (Uris aalge; John- to their eggs, young, nests, and nest sites. son 1941, Tschanz 1959), which do not Barn Swallows are facultative colonial nest- build nests, and Royal Terns (Sterna maxi- ers and unique among the above species in ma; Buckley and Buckley 1972), which nest that their colonies are passive aggregations in dense colonies where their eggs may be of birds (Snapp 1976). Barn Swallows have displaced, recognize both their eggs and recently used man-made structures for nest- laying sites. ing (Bent 1942), and such behavior may in- Among passerines, Rothstein (1975) found fluence the size of their aggregations. Do that some species (“rejector species”) can swallows respond to aspects of the nesting recognize foreign eggs placed in their nests, situation in the same way as colonial or non- while others (“acceptor species”) do not. colonial species? Bank Swallows (Riparia rip&a; Hoogland and Sherman 1976), Red-winged Blackbirds METHODS (Ageluius phoeniceus; Peek et al. 1972), and Carrion Crows (Corvus corone; Yom-Tov This study was conducted in southern and central Oklahoma during June and July of 1974, 1975 and 1976) do not discern their eggs, or their 1977. Swallow colonies were located in rectangular young until near fledging, at an age when cement culverts under heavy-duty roads. In 1974, two an “error” in progeny care is possible. Tri- culverts were used for the behavioral part of the study; colored Blackbirds (Ageluius tricolor), which one about 2.5 km southwest of Madill, Marshall County (left culvert in Fig. l), and one under State Highway nest in dense colonies, accept eggs or young 99 adjacent to Lake Texoma, Marshall County. The in- placed in their nests (Emlen 1941). Many side culvert dimensions were 183 cm (height) x 244 non-passerines are similar in their varied cm x 25.2 m (excluding triangular corner; see Fig. 1) ability to recognize their eggs or young and 213 cm x 152 cm x 48.0 m, respectively. In 1975 (Tinbergen 1953, Davies and Carrick 1962, and 1977, a culvert under State Highway 9 near Lake Thunderbird, Cleveland County, was used. It mea- Beer 1970). sured 183 cm x 183 cm x 49.6 m. Nests in culverts Red-winged Blackbirds recognize their were numbered sequentially, whether in use or not, nest sites specifically, but accept substitute and their locations described in relation to: (1) distance nests (Peek et al. 1972). Nero and Emlen from the entrance of culvert and/or nearest nest(s); and (2) shortest distance from outer lip of nest to ceiling of (1951) found that female Red-winged Black- culvert. birds would follow displaced nests even Mist nets were placed on the ends of the culverts, across territorial boundaries of males. Meise opened at night, and checked at sunrise. Captured (1933) moved the nest of a House Sparrow birds were banded and adults were marked with paint. (Passer domesticus) 1.5 m and observed no All males were marked conspicuously on the back with light blue. Females were painted so as to be individ- response to it by the adults. Among non-pas- ually recognizable by using a variety of colors (other serines, a Coopers’ Hawk (Accipiter coop- than light blue) on various parts of the body including erii) nest moved in small increments from left wing, right wing, back of neck, throat and rump. its original location in the crotch of a tall The paint did not appear to impair any individual, and paint-marked birds were captured in successive years. tree to a bushel basket, and then to the Sex was determined by measuring tail and wing chord ground, was accepted at each step by the lengths (Wood 1969), and by the presence or absence adults (Allen 1951). Herring Gull (Lurus ar- of a brood patch.

12361 BARN SWALLOW NESTING BEHAVIOR 237

” .5 1.5 2.5 3.5 4.5 5,5 6.5 15 >8 METERS FIGURE 1. Nesting site of Barn Swallows in culverts FIGURE 2. Frequency distribution of distances be- under heavy-duty road near Madill, Marshall County, tween nests of Barn Swallows nesting in culverts (mea- Oklahoma. There were 17 active nests in culvert on sured to nearest cm). The white bars refer to distances left and 4 active nests in culvert on right. Note closed between all adjacent nest pairs (N = 137) while the mist-nets at entrance of culvert. shaded bars indicate distances between active nests (N = 84).

I watched the birds, with the aid of 7 x 35 binocu- lars, from an automobile or burlap blind positioned the nest was 58 cm below the ceiling. The near the entrance of the culvert. A small mirror was mean distance between nests, for I37 adja- used to shine light into the culvert to confirm the iden- tity of birds sitting on nests. Marked birds were cent nest pairs in 11 culverts, was 2.79 m watched as they entered the culvert and went to their (SD = 2.27 m). Mean distances between 84 respective nests. After nests were identified with in- occupied nest pairs (i.e., nests with eggs or dividually-marked birds, I initiated the experiments. young) was 3.66 m (SD = 3.12 m). The latter Often, only the female using a particular nest was marked, but in no case did two birds identified as fe- represents a maximal value because young male use the same nest. may have recently fledged from “unoccu- Experimentation involved: (1) moving nests horizon- pied” nests located between the occupied tally or vertically from their original location; (2) add- ones. Frequency distributions, converted to ing nests; (3) removing nests; and (4) switching nests, proportion of total, were prepared for dis- and/or eggs and young. Nests to be moved were care- fully pried off the wall and reattached by laying down tances between nests of both data sets and an adhesive mud base to the new location and pressing are shown in Figure 2. With one exception, the moistened back of the nest onto the new location. the lowest distance between occupied nests Details of each manipulation are given in the results. was 0.73 m. Barn Swallows maintain some I observed the responses of Barn Swallows for at least 0.5 h after each manipulation. Observations were minimum distance between nests, but 60% extended up to 1.5 h in cases when the final response of the occupied nests were spaced less than was in doubt. The responses to nests (or nest sites) 3 m apart. were assigned to one of four categories: (1) no response-when a swallow appeared to ignore the nest RESPONSE TO EGGS (or nest site); (2) not accepted-when a swallow investigated the nest (or nest site) by hovering in front of it, I evaluated whether or not Barn Swallows but did not land (occasionally, a swallow in this cate- are able to: discern their own eggs from oth- gory actually touched a nest, but did not perch); (3) er Barn Swallow eggs; distinguish Barn weak response-when a swallow attempted to incu- Swallow eggs from eggs of other species; bate or feed young (or perched at nest site), but rejected these nests (or nest sites) during the observation and/or distinguish eggs from other objects. period (a swallow who sporadically fed young in a nest To test if females could distinguish their was placed in this category); and (4) accepted-when eggs from other Barn Swallow eggs, several a swallow continuously performed incubation or feed- manipulations were performed. When ing activities at a nest without apparent hesitation or clutches with identical numbers of eggs distress. Often, swallows in this category initially exhibited hesitation or distress when exposed to the ma- were substituted for the original eggs, fe- nipulated situation. males accepted the new set in all eight cases tested. In two of these, both the eggs RESULTS and nest were replaced. In two other in- In south-central Oklahoma, Barn Swallows stances, a females’ nest and eggs were replaced their nests linearly on the wall just moved and replaced by two new nests im- below the ceiling of the culvert. The outer mediately ladjacent to the original location; lips of the nests were located 4 to 6 cm be- one of the nests contained her own eggs low the ceiling, except for one case where while the other included a clutch with the 238 JOSEPH A. GRZYBOWSKI

iJ NA NB ORIG ONLY EGGS ORIG YA uk_J NA NB Y-Z Y-A MANIP MANIP I cJx& uu .:>. ... .: . RESULT ? (N=2) i RESULT I 6 80

IJ NA NC NB Y MANIP 2 Y-A Y-Z ORIG ONLY YOUNG uu xu y I---lmm--l MANIP ;/x & ,:::. . ‘.‘.. RESULT 2 6 8 ? RESULT 0 (N=l) ; ! NA NC NB MANIP 3 Y-S Y-Z Y-A ? (N=l) U UU u ‘r ?

FIGURE 3. Responses of Barn Swallows to their (A) eggs and (B) young. Abbreviations and symbols indicate the following: ORIG, linear arrangement of nests on culvert wall before manipulation; MANIP, arrange- NA NC NB ment after manipulation; RESULT, choice of swallow; MANIP 4 Y-S Y-A Y-Z E, eggs; Y, young; semicircle, symbol for nest; dotted U LA-J u arrow, response where nest (or nest site) not accepted ,+, by swallow (see text); solid arrow, response where nest ? (or nest site) accepted by swallow (see text); X, original RESULT 4 0 location of nest. Member of pair responding, d or 9. 6 Sample size given in parenthesis for each result. FIGURE 4. Responses of Barn Swallows to their young (see text). Abbreviations and symbols: NA, NB same number of eggs (Fig. 3A). The females and NC designate particular nests; Y-A, Y-Z, and Y-S in both cases did not incubate either clutch. indicate particular broods; dashed arrow, weak re- These experiments indicate that females sponse of swallow (see text); other symbols as in Fig- could not recognize their own eggs specif- ure 3. ically. When I substituted House Sparrow eggs er, females visited the nests and perched on for the original eggs (3 cases), the females the lips for several minutes during each vis- accepted them. In two instances, I substi- it. These females occasionally looked dow.n tuted cold white stones (approximately the into the nests. As long as their nests were same size as Barn Swallow eggs) for the still present at the original locations, the fe- eggs. The females accepted and incubated males did not attempt to locate other nests them, occasionally prodding (perhaps roll- with eggs. When eggs were later replaced, ing) them with their bills. These results the females accepted and incubated them. demonstrate that Barn Swallows cannot dis- Apparently, the drive to incubate in this tinguish conspecific eggs or any eggs spe- species is a behavioral state controlled in- cifically, and that they will respond to stim- ternally or, at least, not requiring eggs to uli approximating their eggs. initiate the response. The tactile stimulus of Response to eggs does not end with rec- eggs, however, is necessary to maintain in- ognition. Incubation behavior may be trig- cubation behavior. gered internally or by the external stimula- RESPONSE TO YOUNG tion of eggs in the nest. To determine which was the case, only the eggs were removed In these manipulations, I tested Barn Swal- from the nest in four instances. Females at- lows to see if they could distinguish be- tempted to incubate in the nests, shuffling tween their young and other Barn Swallow downward into the nest bowls. Females young, or distinguish the developmental then left the nests and returned several stage of their young from other stages, in- times, attempting to incubate several times cluding eggs. In eight cases, when young during each visit and for several minutes up to 11 days of age were substituted for the during each attempted incubation bout. Lat- original young of similar age, females ac- BARN SWALLOW NESTING BEHAVIOR 239 cepted the change. In two cases, a female 4J with young was presented with the choice of her young, or another brood in a new nest ORIG adjacent to the original site (the original & nest was removed). One female investigated both nests but did not feed either set of young; the other fed both sets of young (Fig. MANIP 3B). ;vCJ I conducted one set of manipulations on ::. a natural “double nest” (Fig. 4)-an active nest (labeled NA) with a second nest (NB) RESULT 9(N=2) i ’ immediately adjacent to it. Nest A (NA) con- 1‘ ‘I tained young (Y-A) and NB was empty. In 3-l manipulation 1, Y-A young were moved to NB, and alien young (Y-Z) of similar age ORIG were placed in NA (see Fig. 4). Both the CJ female and male fed the young in NA. In a second manipulation, NA and young (Y-A) were moved about 1 m to the left and MANIP another nest (NC) with young (Y-Z) was ;/“CJ placed immediately adjacent to it, simulat- ing the original situation but 1 m displaced. The female fed both sets of young equally. RESULT 9 (N=2) The male also fed both sets of young regu- larly, but fed the Y-A young more often. In FIGURE 5. Responses of Barn Swallows with (A) a third modification, Y-A young (from NA) young and (B) eggs to choices of eggs or young (see were moved back to NB and Y-Z young text). Symbols as in Figures 3 and 4. moved to NA at the displaced location. The female fed the Y-Z young and once fed young birds in the next occupied nest (la- strongest response at various stages of the beled Y-S in Fig. 4). The male fed the Y-Z breeding cycle. Two females with young young. For manipulation 4, the Y-A and Y- were provided with choices of nests with Z young were switched again (see Fig. 4). eggs or nests with young, both immediately The female fed the Y-A young in the dis- adjacent to their own nests which were left placed nest (NA) and again fed young in the empty (Fig. 5A). Females selected the nests next occupied nest (Y-S). The male fed with young which they fed, ignoring the young only in NA. These experiments and nests with eggs. They investigated, but did those previously described indicate that fe- not land on their original nests between vis- male Barn Swallows do not recognize their its to the nests with young. When females own young (up to 11 days of age). I was un- with eggs were given the same choice (two successful in conducting tests involving cases), they incubated the eggs, and brood- young older than 11 days because they ed and fed the young (Fig. 5B); initially fledged prematurely when disturbed. they appeared startled by the begging The responses of females to young of dif- young. Females with eggs attempted to in- ferent developmental stages than their own cubate in their original nests, but soon aban- young were evaluated. With notable differ- doned their nests for other choices. In these ences in the size of the young switched, fe- latter manipulations, it appeared that new males showed some reluctance, “distress,” stimuli of young presented to females with or hesitation, but accepted the change in 12 eggs triggered appropriate feeding re- of I3 cases. If eggs and newly hatched sponses without affecting their incubation young were switched, females receiving drive. However, after young have hatched, eggs or young (two cases each) accepted the females searching for young do not respond change, sometimes exhibiting distress, but to eggs. Stimuli indicating advanced stages other times without hesitation. These re- of nesting are accomodated while stimuli of sponses indicate that females are able to previous stages are rejected. distinguish between stages of development, but that similar stimuli in their nests are ac- RESPONSE TO NEST cepted. I evaluated whether or not Barn Swallows Further manipulations were undertaken recognize the nest itself or various charac- to determine which stimuli evoked the teristics of it. When the character of the nest 240 JOSEPH A. GRZYBOWSKI

q Y either at the old locations or immediately or ORIG adjacent to the original locations. Females 4 chose their own nests in three cases; in the OTHER fourth, the female exhibited attraction to her MANIP NESTS nest, hovering in front of it more often. In ,e, a similar experiment, the two nests (original and replicate) were placed 0.5 m (to the left I RESULT 0 (N=3) and right, respectively) from the original lo- 3J cation. The female still selected her nest. ORIG These findings indicate that females can 6 recognize their own nests. MANIP X l--upl$+ RESPONSE TO NEST SITE ,$. The most distinctive attribute of the nesting RESULT f(N=4) j situation is the nest site. Barn Swallows are able to locate their nest sites, but the cues used might differ from one bird to another. I conducted removal experiments to determine if females could locate the original ORIG nest sites if no nests were present. When b “NA nests, with eggs or young, were removed X entirely (four cases), or displaced horizon- Vi; tally (six cases) or vertically (three cases), ,A .+. 1 females would locate the original sites, hov- er in front of them, and often cling to the RESULT $! (N=I) i wall at the original nest locations. Nests displaced vertically downward 0.7 m or more FIGURE 6. Responses of Barn Swallows to nest displacements (see text). Long dashed arrow in A indi- below the original nest site were almost cates positive response to nest site. Other symbols as completely ignored (Fig. 6A). Although the in Figures 3 and 4. females would cling to the original nest locations and look down on their nests, they never landed on or hovered in front of nests was changed by hanging paper towels from vertically displaced. They visited other the lip (two cases), or adding excessive mud nests nearby, but were chased from active (two cases), females exhibited some distress nests by residing adults. These manipula- but accepted the nests. When the lip of the tions indicate that female Barn Swallows nests was altered (three cases), no distress can orient to their nest site with no nest or hesitation was exhibited. When the lin- present. Vertical displacements indicate ing of the nests was switched (three cases), that nests must be in an appropriate position females appeared “uncomfortable” at first, relative to the culvert ceiling to be accept- but accepted the change. In one case, a ed. prominent guinea fowl feather (part of the I also compared the relative attachment lining of the nest) was moved to an empty of females to their nest site with that to the nest nearby. After settling on her nest, the combination of eggs and nest. As indicated female rose, flew to the nest with the guinea earlier, when eggs were removed, but nests fowl feather, picked at the feather, and then were still present (four cases), females returned to incubate on her nest. The male would not attempt to find new nests with of the pair flew into the culvert and perched eggs. When nests with eggs (three cases) or on the lip of the nest with the guinea fowl eggs and newly-hatched young (one case) feather. These results indicate that females were moved 2-3 m and replaced with new are able to recognize gross changes in the nests (with identical contents), three of four nest and may recognize distinctive features females accepted the replacement nests of their nest. (and contents). These results indicate that Because cues used to recognize a nest the females were more strongly attached to may vary among individuals, females with the nest-any nest-present at the original eggs (two cases) or young (two cases) were nest site. Also, as indicated earlier, females given choices between their nests displaced provided with choices of their nests and immediately adjacent to the original loca- other nests near their nest sites selected tions, and nests with identical contents their own nests. How far removed from the BARN SWALLOW NESTING BEHAVIOR 241 original nest sites would females accept NA nests? ORIG When nests and eggs were removed with b no replacement (six cases), females would NA NB orient to original nest locations and then MANIP I search adjacent areas, generally accepting CJXCJ the nearest nests with eggs (to a distance of up to 5 m) from which they were not chased RESULT I ? (four cases; Fig. 6B). Two females did not accept nests at new nest locations (Fig. 6B). NA NB In two cases (one each for a female accept- MANIP 2 c_J xc/J ing and not accepting a nest), the females’ original nest was not used in the manipu- +-I lations. In one instance, a females’ nest and eggs were relocated 5 m, then 4 m, then 3 RESULT2 ? m, and then 2 m from the original location toward the closest culvert entrance. She did NC NB not accept the nest in any of these cases. MANIP 3 ?+A E When this nest was then moved 2 m into u XCJ the culvert from the original location, the b z~m~lm+j nest was immediately accepted. Another nest moved 4 m into the culvert from its RESULT 3 ? original location was not accepted; the female had been incubating at least 11 days. NC NA NB MANIP 4 In the case where a nest with eggs was CJ &xcJ moved 5 m to a point where an empty nest was between the new location and the original location, the female located and ac- RESULT4 $? cepted the original nest (Fig. 6C); this female had been incubating at least seven FIGURE 7. Responses of Barn Swallows to nest and days. These last two experiments suggest nest site (see text). Symbols as in Figures 3 and 4. that no increased attachment occurs with time. In general, nests displaced as much as 2 m horizontally were found and accepted. the nest are secondary in importance to an Sometimes this occurred for nests (with attachment to the nest site. eggs) displaced more than 2 m. Although females oriented to their nest In one series of manipulations (Fig. 7), a sites, they could find nests (with eggs) that female (with nest containing eggs) was pre- had been moved. These females continued sented with a choice of her nest and a sec- to orient to the original nest sites before ond nest, both with eggs and immediately going to the relocated nests in subsequent adjacent to the original location. She inves- visits (see Fig. 8A, manipulation 1). Fe- tigated the second nest, but selected her males may find new locations by using cues nest. When her nest was moved 1 m away for orientation to the old nest sites, or may from the original location (manipulation 2), still be attached to the original sites. To test the female investigated both nests but did which was the case, three females whose not accept either. When her nest (at the new nests had been moved to new locations location I m away) was moved 2.5 m farther without replacements were given replace- from its original location and replaced by an ment nests with eggs at the original location unfamiliar nest (manipulation 3), the female of their nests. (Remember from previous ex- selected the nest nearer the original loca- periments that females provided with sub- tion. In manipulation 4, her nest was moved stitute nests at the original locations did not back to the position of the first manipulation. attempt to find nests elsewhere.) The three The female investigated all three nests be- females tested continued to orient to the fore selecting her own. In two other cases, original locations (with nests present), but females ’ nests with eggs were moved more then flew to the relocated original nests to than 2 m and replacement nests with eggs incubate (see Fig. 8A, manipulation 2). In were placed at the original locations. The manipulation 3, eggs were removed from females accepted the replacement nests at nests at the new locations. The females ori- the original locations. These results indi- ented as usual, flew to the nests at the new cate that attachment to and recognition of locations, and attempted to incubate. After 242 JOSEPH A. GRZYBOWSKI

iJ nately, perching on the nest lips for several ORIG minutes at a time. When the eggs were re- CJ stored to the nest at the new location (manipulation 6), the female came into the cul- MANIP I X :: vert, checked the nest at the original location, flew to the nest at the new loca- RESULT I $? .: ’ T tion, and immediately incubated. She raised her young in this nest. MANIP 2 b CJ In any situation, when the eggs were re- .::.. . . moved, the females first attempted to incubate on the last sites where they had suc- RESULT 2 ? : 1 ‘ cessfully incubated eggs. If these were new locations, they would then accept eggs at MANIP 3 CJ u the original locations; if old locations were .% involved, females would accept eggs at the I I I new locations. Barn Swallows can reorient RESULT 3 ? 1 I very quickly to new nesting situations, but 3J probably use old cues from previous situa- MANIP 4 u \; tions. However, attachment to nest sites can /:.. be weakened when no stimulus for incu- RESULT 4 ? i bation is present, at least for females with I ? eggs. To test what cues may have been em- ployed in orienting to nest site, several manipulations were performed. In one instance mentioned previously, where a “double nest” occurred naturally, a female was mis- MANIP 6 led into picking the wrong nest site by sim- u ,::.. ulating a “double nest” nearby (Fig. 4). In this case, the females’ original nest was RESULT 6 ? ; used in the simulated “double nest.” In a second case, the simulated double nest FIGURE 8. Responses of Barn Swallows to nest site (nests NB and NC in Fig. 9A) included the and eggs (see text). Manipulation 1 refers to the same unused member of the “double nest” (NB). experiment depicted in Figure 6B. Unlike Figure 6B, The females’ original nest (NA), with eggs, however, the result shown for manipulation 1 indi- was left in place (Fig. 9A); the female first cates the response of female Barn Swallows after perched on the next nest nearest the culvert initial acceptance of the displacement nest. Symbols as in Figures 3 and 4. entrance (ND in Fig. 9A) facing her nest location. She then flew to her nest (NA) and incubated. In one instance, where an arti- attempting incubation, they left the dis- ficial series of three nests was previously placed nest and went to the one at the orig- arranged (NC, NB, and NA in Fig. 9B), later inal location and incubated. In subsequent manipulations misled a female (with origi- visits, these females went directly to the nal nest, NA) into temporarily selecting the original nest sites and incubated. wrong nest. Her first choice was ND (the Extended experiments were performed middle nest in the new triplicate nest series with one female (Fig. 8B). In manipulation NA, ND and NF) from which she was 4, her eggs were again moved to her original chased by a second female (the residing nest at the new location. She tried to incu- adult). She then momentarily landed on NA bate on the nest at the original location, (her original nest now in the new triplicate then finally settled on the nest at the new series) before going to NE (the replacement location. In subsequent visits, this female nest at the original location) to incubate. first oriented on the original nest site, and The second female made the correct choice then flew to the new location. For manipu- of her nest the first time, indicating that she lation 5, her eggs were removed so that both was using different cues than the first fe- nests were empty; the female attempted to male. In another case, where a nest (with incubate on the nest at the new location, eggs) was moved 3 m with a replacement then moved to the old location, attempted nest at the original site (Fig. 9C), the female to incubate, and then alternated between selected her relocated nest; in this case she nests. Finally, she visited the nests alter- may have been using a characteristic of the BARN SWALLOW NESTING BEHAVIOR 243

nest to recognize the nest site. Cues for rec- q ND NB NA ognition of nest site appear to be learned ORIG u WcJ and these cues differ among females. Char- ND acteristics of the nest are among the cues MANIP SYJ used in nest site recognition in many cases. u RESULT 0 $ ’ My observations of Barn Swallows ’ be- I havior during removal experiments gave ad- I t ditional insight as to the nature of cues used I* ’ CHOICE to orient to nest sites, and the locations of 3J ORIG NC NB “E” “E” these cues. Females whose nests were ma- wwu v nipulated often hovered at various locations NC NB NE NA ND NF NG MANIP near the nest sites (sometimes in front of the WWfJ b.55 ti opposite wall of the culvert). When new ..‘ .‘ nests were placed on culvert walls, some RESULT ? I f :r I swallows occasionally investigated noisily, IS+CiiOlCE attracting other swallows to these recently Q 2 modified locations. These observations (and T N results of manipulations given above) indi- NA cate that cues inside the culvert are used to OR,,‘ orient to the nest sites. The following ob- eJ NB NA servations suggest that cues outside the cul- MANIP vert may also be used. Females whose nests fi c.J were manipulated often flew in and out of the culverts several times, flew through the RESULT Q T culverts or hovered at the culvert entrances. One isolated experiment was conducted on FIGURE 9. Responses of Barn Swallows to cues used a culvert of north-south orientation which in nest site recognition (see text). ND, NE, NF and NG designate particular nests; other symbols as in Figures contained only two nests located directly 3 and 4. opposite each other; one with eggs on the west wall, and a nest with large young on the east wall. I switched the eggs and ous elements of the nest linings were young. The female of the nest with eggs moved, males in three of four cases tended flew in the north entrance and up to the nest to find the nests with the original linings (now with large young), was startled by the and perch on them. In these instances, I rec- young, and left by the north entrance. She ognized males belonging to a nesting pair proceeded through a series of maneuvers behaviorally and by compatability with fe- alternating between short circular paths just males, as males were not individually outside the culvert and approaches to and marked. In a few cases, where both the retreats from her nest site. She exhibited young and the nests were new, males distress during the first approaches, but showed no special ability to select nests. never landed on her nest during the hour of observation. Sometimes she flew out for DISCUSSION some distance from the culvert, and then If the context of the situation precludes back in, always approaching her nest on the making “errors,” no selective pressure for west wall and never orienting to the oppo- egg or chick recognition should exist. Few site wall. She never accepted the young, but species of birds recognize their own eggs accepted her eggs when they were reintro- specifically. Where recognition has been duced into her nest. demonstrated, the species have (at best) ru- dimentary nests and nest in dense colonies, HESPONSE OF MALES where eggs can easily be displaced (John- Although extensive manipulations were not son 1941, Buckley and Buckley 1972). made to test specifically the responses of These species possess highly variable eggs, males to nesting situations, observations a condition which undoubtedly aids in egg made during the experiments imply that at- discrimination. Some passerines can dis- tachment to and ability to recognize young criminate between eggs of their species and and nests are perhaps greater in the males those of other species or of a different color than the females. I observed males landing (Rensch 1925, Rothstein 1975). Many bird on their nests proportionately more often or species, both passerine and non-passerine, quickly than females during various manip- accept eggs or egg-like objects placed in ulations involving young. When conspicu- their nests. 244 JOSEPH A. GRZYBOWSKI

Parasitism by conspecifics is highly un- mental stage of young, but accepted young likely among Barn Swallows. Copulation in their nests even if these young were at a and egg laying are part of a sequence of be- different stage. Fledged or nearly fledged haviors which includes nest building. young were not introduced into nests with While it is possible for an occasional nest young at earlier stages in either Burtts’ or accident to create a situation for egg dump- my study. ing, such behavior would not persist be- In most species tested, the nest site was yond that generation or that incident. Para- more important than any other aspect of the sitism by Cuckoos (Cuculus CU~OTUS) has nesting situation (Lashley 1915, Meise been observed in Barn Swallows in Europe 1933, Allen and Mangels 1940). Nest site (Dod 1892, Atkinson 1894, Heath 1973). would certainly be the most distinctive as- Friedman (1963), citing only five cases, in- pect of the nesting situation. Proper orien- dicated that the Barn Swallow is an infre- tation to the nest site would avoid contrib- quent host of Brown-headed Cowbirds uting to competing genotypes in both (Molothrus ater). I saw no evidence of par- colonial and non-colonial species and asitism by cowbirds in my study. Parasitism would be favored by selection. Colonial by cowbirds is unlikely in Barn Swallow species are expected to be most rigid in colonies, since some swallows are always their response to nest site. Black-crowned present. Barn Swallows have been seen Night Herons (Nycticorux nycticorux) pre- mobbing Starling (Sturnus vulgaris) intrud- ferred to incubate blocks of wood rather ers (Snapp 1976) and would probably do the than their own eggs in nests placed nearby same to a cowbird. Thus Barn Swallows, as (Allen and Mangels 1940). Negative re- expected, do not recognize their own eggs sponses to nests moved short distances have specifically and accept eggs of another also been recorded for Sooty Terns (Lashley species and stones placed in their nests. 1915) and House Sparrows (Meise 1933). These results are in agreement with those Herring Gulls did accept nests displaced by Burtt (1977) on Barn Swallows and Tree about 30 cm (Tinbergen 1953). However, Swallows (Zridoprocne bicolor), and Hoog- Peek et al. (1972) and Nero and Emlen (1951) land and Sherman (1976) on Bank Swal- found Red-winged Blackbirds, a non-colon- lows. ial species, would find nests displaced con- Recognition of young has been demon- siderable distances. Responses of Barn strated in several studies (e.g., Beer 1969, Swallows to nest site in this study were Buckley and Buckley 1972, Peek et al. 1972) intermediate between those of these colon- and generally occurs just before the young ial species and of Red-winged Blackbirds. become mobile (when the chance for error In a New York study (Snapp 1976), Barn in progeny care is high). Barn Swallows Swallows rarely nested less than 3 m apart. (Burtt 1977), Bank Swallows (Hoogland and I found 60% of the active nests were closer Sherman 1976), and Red-winged Blackbirds than 3 m to another active nest, indicating (Peek et al. 1972) recognize their own that swallows were more densely packed in young near fledging age. However, Hoog- the Oklahoma culverts. Barn Swallows pre- land and Sherman (1976) found that Rough- fer to feed along edge habitats, where wind winged Swallows (Stelgidopteryx ruficol- speeds are reduced and insects are more Zis), a non-colonial species, could not dis- abundant (Samuel 1971). Heavy-duty roads criminate their young from other young able in Oklahoma provide a continuous wind to fly. The authors suggested that these dif- break (whether in open or wooded terrain), ferences between Bank and Rough-winged and swallows may be attracted to these swallows were accounted for by colonial strips of suitable habitat. Swallows nesting and non-colonial habits. Nesting pairs of in the longest culverts, or in colonies of less Rough-winged Swallows may occupy habi- than 10 pairs, seldom occupied the middle tats disjunct enough so that mixing of young sections of culverts; nests were concentrat- is less likely than for other species tested. ed near the entrances. This crowding may Fledglings in colonial situations could have been a factor in my study, modifying benefit from attention of several adults. In responses to nest site which may be more Barn Swallow colonies, such behavior could similar to responses of non-colonial species be most advantageous for young of early in other situations. nesting individuals. Burtt (1977), however, While kinesthetic memory may be cred- presented evidence of aggression toward ited for locating nest sites, Barn Swallows alien fledged young by adult Barn Swal- were occasionally misled by changes in the lows. In my study, Barn Swallows demon- position of various cues. It is likely that re- strated an ability to recognize the develop- sponse to the nest site in this species is de- BARN SWALLOW NESTING BEHAVIOR 245 termined by several, if not many, visual Female Barn Swallows do not recognize cues which vary among individuals. Barn their own eggs specifically and cannot dis- Swallows reoriented quickly to moved tinguish conspecific eggs from other eggs or nests. The birds in this study usually ori- egg-like objects placed in their nests. In- ented to the original locations first, and then cubation behavior appears to be controlled to new locations, a simple adjustment. The internally and the presence of eggs is not new location then became the nest site and required to initiate the behavior. Tactile reorienting to the original situation meant stimulation of eggs, however, is necessary readjusting in the same manner as after the to maintain incubation behavior. first manipulation. Nests left in new loca- Barn Swallows could not distinguish their tions for several days were approached di- own young, up to at least 11 days of age. rectly. Such response abilities are probably They could distinguish young of different advantageous for locating nests in situations developmental stages but accepted such where cues for nest site recognition are al- young placed in their nests. A few females tered or destroyed. The possibility of con- with eggs could be coerced to attend two tributing to foreign genotypes is precluded nests simultaneously, and such behavior by the nest defense of residing adult Barn may reflect an adaptive response of a mul- Swallows. tiple-brooded species. Attachment to nest and nest site may be Females selected their own nests in mul- interrelated. Two female Red-winged tiple-nest situations if nests were placed Blackbirds did select their nests 0.5 m away near the original nest sites, but they did not from their nest site where a replacement show any preference for their nests (with nest was located (Peek et al. 1972). How- eggs) displaced more than 2 m from the ever, this occurred after one day of incubat- original nest sites. The swallows were ing on the replacement nest at the original strongly attached to their nest sites and rec- nest site. Barn Swallows could locate their ognition of nest is part of recognition of the nests when given choices near their original nest site. nest sites. When their nests were displaced Swallows oriented to their nest sites with more than 1 m, with no replacement, the no nest present. Manipulation of prominent birds usually found their nests. One female nest site features misled some swallows into went directly to her nest displaced 3 m with selecting nests at sites other than their own, a replacement at the original site (Fig. 9C). indicating that visual cues are used for rec- This may indicate that the nest itself is ognition of the site. These cues varied great- among the cues used to orient to the nest ly among individuals. Cues both in and out- site. Barn Swallows recognize the nest, and side of the culverts were used for orientation. it may be used to recognize the nest site. The swallows quickly adapted to new nest- Barn Swallows could be coerced into tak- ing situations. ing care of two nests under certain circum- Barn Swallows responded most strongly stances (Figs. 3B, 4 and 5B). This capability to the nest site, then nest, and then eggs or has not been tested in other species, but young. They found nests which had been may be common in multi-brooded species. moved, but only if no replacement nest was Several “double nests” (nests placed close present at the original nest site. They did together) may be results of pairs beginning not accept nests displaced vertically down- second nestings before the first has been ward. completed. In only one case was a nest with a complete clutch of warm eggs situated next to a nest with almost-fledged young. ACKNOWLEDGMENTS Since Barn Swallows defend an area near their nests, such a situation is best ex- The initial impetus for undertaking this study was pro- plained by assuming that both nests be- vided by Charles C. Carpenter. I am indebted to D. longed to one pair of adults. Scott Wood for his assistance in the field during the first year of this study. Financial assistance was pro- SUMMARY vided during the first year by Loren G. Hill, Director of the University of Oklahoma Biological Station. Responses of Barn Swallows to eggs, young, Charles C. Carpenter, Gary D. Schnell, George M. but- nests and nest sites were determined in col- ton, Bedford M. Vestal, and my wife Eileen provided onies located in culverts under roads in useful comments for improving earlier versions of this Oklahoma. Active nesting pairs maintained paper. My thanks also go to Donald A. McCrimmon, Jr., Gary D. Schnell, and an anonymous reviewer for some minimum distance between nests, but their worthwhile criticisms of the final draft. Diane L. 60% of the occupied nests were less than 3 Field,s provided assistance in preparing several drafts m from another active nest. of the manuscript. 246 JOSEPH A. GRZYBOWSKI

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