Preferred Nest Spacing of an Obligate Cavity-Nesting Bird, the Tree Swallow
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The Condor87:356-363 0 The CooperOrnithological Society 1985 PREFERRED NEST SPACING OF AN OBLIGATE CAVITY-NESTING BIRD, THE TREE SWALLOW ALISON MULDAL H. LISLE GIBBS AND RALEIGH J. ROBERTSON ABSTRACT. - In order to determine the preferred dispersion of a population of breeding Tree Swallows (Tachycineta bicolor) in Ontario, we set up an abundant supply of nest boxes with a variety of distancesbetween them. The 72 boxes were arranged in 12 equidistantly spacedspirals. Within a spiral, the distance between boxes was much smaller than between spirals. Over five breeding seasons,we observed the order and positions in which swallows settled in the spirals. Pairs of swallows usually settled in empty spirals before settling in spirals occupied by conspecifics,but they did not avoid nesting in spirals occupied by Eastern Blue- birds (Siulia sialis). Swallows did not show any spacing preferences when their nearest neighbors were in different spirals, and were therefore more than 36 m away. Within spirals,however, swallowsnested as far as possiblefrom each other when their nests were less than 14 days apart. Swallow nests in the same spiral also tended to be spaced out temporally. We conclude that, over the range of distances within a spiral, Tree Swallows prefer to space their nests as far from conspecificsas possible. The observed spacingpattern probably arises from ter- ritorial behavior that is directed toward defense of a nest site from intruders. Breeding birds dispersetheir nestsin a variety ia; Bent 1942) and White-backed Swallows of patterns ranging from the tightly clumped (Cheramoecaleucosternum), which excavate distribution of colonial speciesto the uniform their own burrows in sandbanks (Serventy and distribution of territorial species(Lack 1968). Whittell 1976). On the other hand, another The spatial distribution of resources such as burrower, the Banded Sand Martin (Riparia food, nest sites, and nest materials; the intra- cincta),nests solitarily (McLachlan and Liv- specificcompetition for theseresources and for ersidge 1965) and another mud-nest builder, mates; and predation pressureare some of the the Barn Swallow (Hit-undorustica), nests sol- factors determining the breeding dispersion of itarily or in loose colonies (Snapp 1976). individuals (Crook 1965, Lack 1968, Hoog- Those speciesthat must nest in existingholes land and Sherman 1976). and cavities have distributions that are deter- In this paper, we report on an experimental mined primarily by the availability of suitable approach to determine the preferred nesting nest sites(von Haartman 1957, Holroyd 1975). dispersionof a cavity-nesting species,the Tree The Tree Swallow, Purple Martin (Prognesub- Swallow (Tachycinetabicolor). Swallows (Hir- is ), and Northern Rough-winged Swallow undinidae) show varying degrees of gregari- (Stelgidopteryxserripennis) are examples of ousnessduring the breeding season. Disper- obligate cavity-nesting swallows. Cavity-nest- sion may depend upon a particular species’ ing species show considerable intraspecific nestingrequirements. Some speciesbuild their variation in their nesting behavior, and may own nests, while others use existing crevices nest either solitarily or colonially, depending in trees, rocks, and walls, or old burrows ex- on the distribution of nest sites (Bent 1942). cavated by other animals. Since spacingpatterns are so diverse in cavity- Those speciesthat build their own nestshave nesters,it is of interest to determine which, if some choice in the nature of their association any, nesting dispersion pattern is actually pre- with conspecifics, and intraspecific interac- ferred. Nest spacingpreferences of cavity-nest- tions are probably the most important deter- ing species can be determined by providing minants of their dispersion patterns (e.g., many nest sites with different distances be- Hoogland and Sherman 1976, Snapp 1976). tween them. Once the preferred spacing is For example, Cave Swallows (Hirundo @vu) known, one can begin to look for the behav- and Cliff Swallows(H. pyrrhonota),which build ioral mechanismsby which the spacingpattern globular nests of mud pellets, breed in dense is achieved, and for any effects of the spacing colonies, as do Bank Swallows (Riparia ripar- pattern on fitness. [3561 TREE SWALLOW NEST SPACING 357 16m 8m FIGURE 1. The arrangement of nest boxes in a spiral cell. The interbox distancesare drawn to scale. We sought to examine the preferred nest spacing of Tree Swallows when their choice was not constrained by the availability of nest sites.Tree Swallows were chosenbecause they commonly breed in Ontario and readily nest in nest boxes; thus, it was easy to manipulate the spacingof available nest sites for a sizable FIGURE 2. Map of the study area showing the three population of swallows. We asked the ques- hayfields with the grid of spiral cells. tions: (1) Do swallowstend to clump together, spaceout uniformly, or settle randomly in the available nest sites when they arrive at the The nest boxes were checked early in the beginning of the season?(2) Over what range afternoon at 1-3-day intervals from mid-April of distances do swallows exhibit any spacing to mid-July each year, to determine the order behavior? and, (3) Are the nesting attempts of in which they were occupied. Since it was usu- neighboringswallows synchronized or are they ally difficult to be certain of the date on which temporally spacedas far apart as possible?We pairs settled in boxes, we used the date when answered these questions by setting up a reg- the first eggwas laid as an index of the settling ular arrangement of nest boxes with a wide date. From our available data, date of the first range of interbox distances,and recording the egg was highly correlated with the date when order and positions in which swallows settled nest material first appeared in the box, which during the breeding season. we assumeto be close to the settling date (Y = 0.677, y1= 41, P < 0.01). First egg date is METHODS therefore a reasonable indicator of the order The study was done at the Queen’s University in which different pairs settled. Biological Station, Chaffey’s Locks, Ontario, The spiral arrangement of nest boxes in the during the summersof 1977 to 198 1, inclusive. three fields enabled us to look at Tree Swallow The study site consistedof three adjacent hay- settling patterns both within single spirals, fields of different sizes but of similar vegeta- where potential nearest-neighbor distances tion. Nest boxes mounted on 1.75-m high poles were relatively short (l-l 9 m), and also be- were erected in a grid of spirals, each spiral tween different spirals,where potential nearest consisting of six boxes placed 1, 2, 4, 8, and neighbors were relatively remote (36-92 m). 16 m from a central box (Fig. 1). This arrange- Within a spiral, there were thirteen different ment provided a variety of distancesbetween permutations of interbox distance. Depending boxes, both within a single spiral and between on which box was chosen by the first pair of spirals. The entrance holes on all the boxes swallowssettling in a spiral, a secondpair nest- were oriented in the same direction so this did ing in an occupied spiral had a choice of five not affect their desirability. of thirteen possible nest-box distances from One of the fields had eight spiralsin a regular the first pair. The settling behavior between grid, with the central boxesof each spiral spaced and within spirals was considered separately. 64 m from neighboring spirals.The two small- Non-parametric statisticswere used to ana- er fields had two spirals each, with the central lyze the data (Sokal and Rohlf 1981). For the boxes again 64 m apart (Fig. 2). The distance analysesof dispersion patterns, we calculated between boxes in neighboring spirals was in a coefficient of dispersion (CD = variance/ the range of 36-92 m, which greatly exceeded mean). The difference between the calculated the distance between boxes in a single spiral CD value and 1.0 indicates whether the birds (1-19 m). were distributed uniformly (CD < l), ran- 358 A. MULDAL, H. L. GIBBS AND R. J. ROBERTSON TABLE l(a). The number of times Tree Swallows chose nest boxes before the Tree Swallows arrived between empty spirals and spirals occupied by bluebirds. in the spring (1977, 1978-one pair; 1979, For each spiral category, the lefthand column gives the 1980-three pairs; 1981-four pairs). Blue- number chosen (0) and the righthand column gives the expected frequency (E) based on the availability of each birds aggressivelycompete with Tree Swallows categoryat each stageof the settling sequence(see text for for nest sites when these are in short supply details). l(b). The number of times swallows chose be- (Kuerzi 194 1). Although boxeswere abundant, tween empty spirals and spirals occupiedby conspecifics. (on average, only 3% were already occupied by bluebirds when the swallowsarrived in the (a) spring), it was important to ascertain first Empty Bluebird Year 0 E 0 E whether swallowsavoided nesting in the same spiralsas bluebirds, or whether they “ignored” 1971 5 5.35 0.65 bluebirds. 1978 11 8.90 :, 2.10 1979 4 5.62 3 1.38 As settling progressed,the number of empty 1980 9 7.69 2 3.31 spirals and spirals containing swallows 1981 6 6.75 4 3.25 changed.Each time a new pair (or pairs) settled Total 35 34.31 10 10.69 in the spirals, we computed a posteriorithe (b) number of spirals available that were either Empty StWllOW empty or occupied by Tree Swallows or blue- Year 0 E 0 E birds, and the number of empty and occupied 1977 10 5.44 2 6.56 spirals that were actually chosen by swallows. 1978 10 5.45 1 5.55 The expected number of times that empty or 1979 7 4.14 3.86 occupied spirals would be chosen randomly 1980 4.00 :, 4.00 1981 : 3.66 1 3.34 was calculated by summing the probability of Total 41 22.69 5 23.3 1 each category of spiral (i.e., empty, occupied by swallows, or by bluebirds) that was chosen at each stage in the settling sequence.Devia- domly (CD = l), or in a clumped fashion tions of the swallows’ observed choices from (CD > 1).