Abstract Observations of breeding White-throated Needletails Hirundapus caudacutus using nestboxes in Japan showed that birds utilised the vertical internal walls of the nestbox for resting and roosting. Nestlings moved from the nest at the base of the nestbox at the age of about 30 days and spent the following 3.5 weeks before fledging clinging to the walls of the nestbox. Our observations suggest that one function of the ‘needles’ in the tail of this and related species is to support and anchor the bird while it clings to vertical surfaces. Nestlings had longer needles than adults, perhaps an adaptation to the prolonged period they spend clinging to the side of the nest walls prior to fledging.
The needletails and spine-tailed swifts are characterised by their unusual tail feathers, with the shaft being bare at its tip and stiffened along the whole length of the feather. However, details regarding these ‘needles’ and their function with respect to the ecology of these swifts are not well known. For example, in what situations do the swifts use these needles? Do they use them for breeding or while roosting? When and how do the needles emerge during a bird’s life?
We studied White-throated Needletails Hirundapus caudacutus in Japan, focusing on the morphological characteristics of the needles and their functions in association with the birds’ ecology and behaviour. White-throated Needletails breed in eastern Siberia, through eastern China and Japan (Chantler 1999; Ornithological Society of Japan 2012), and spend their non-breeding season in eastern Australia (Tarburton 2021; Yamaguchi et al. 2021), although resident populations can be found in the Himalayas and in Assam, India (Chantler 1999). The species is recorded as a rare vagrant to Europe, including Britain.
In Japan, White-throated Needletails use natural hollows in trees and artificial nestboxes for breeding (Yonekawa & Kawabe 1994; Yamaguchi et al. 2020). The nestboxes are useful for studying the species as they allow the birds to be captured for ringing and also enable monitoring of the breeding process, through direct observation or by using cameras. We therefore studied birds breeding in nestboxes to obtain information on how the needles of these swifts develop and when they are used.
We have studied the ecology of the White-throated Needletail using nestboxes (Tokachi Plain, eastern Hokkaido, Japan) since 2017 (Yamaguchi et al. 2020, 2021). The study area is an agricultural field of about 20 ha with scattered, broadleaved deciduous trees such as Japanese Emperor Oak Quercus dentata andJapanese Oak Q. mongolica var. crispula. Ten nestboxes were placed on mature trees each year, almost all of which were used during this study. The wooden nestboxes were square-shaped, in cross-section but the size was designed to correspond with that of natural tree hollows that were used for breeding (Yonekawa & Kawabe 1994). The nest entrances were approximately 2.5–3.0 m above the ground, circular and 18 cm in diameter. The internal depth of the box from its base to the bottom of the entrance was 1.0–1.7 m and the area of the base of the box was 30–40 cm x 30–40 cm. An inner wall made of wire mesh or sticks was attached inside the box (see Yamaguchi et al. (2020) for details).
In July and August 2022, we captured adult White-throated Needletails using mist-nets placed around the nests. Nestlings aged 15–17 days were taken from the nests for ringing. We examined the shape, size, and stiffness of the needled tails in 19 adults and 14 nestlings from three broods. The birds’ tails were photographed against 1 mm-squared paper for each individual, and the length of the needles of the central tail feathers was measured on the photo from the tip to the bare base to an accuracy of 0.1 mm. We compared adult and nestling measurements by t-test, after confirming that the measurements of nestlings were not significantly different among broods1. We attached a time-lapse camera (TREL 20J or Thanko RD-1006AT) to the inner ceiling of the nestboxes and monitored the interior every five or 15 minutes throughout the nesting period.
1 One-way ANOVA, F=0.612, P=0.560, df1=2, df2=11
Results and discussion
In all 19 adult White-throated Needletails, the feather shafts on all ten tail feathers were stiffened along their entire length, and the shafts extended beyond the feather tips, resulting in bare, pointed ‘needles’. The tips of these needles were loosely curved downward. The inner and outer vanes of the tail feathers were soft, similar to those of other groups of birds.
By the age of two weeks, nestlings had already developed stiffened tail-feather shafts that were bare at the tips, although the sheaths protecting the still-growing feathers were only broken close to the tip (plate 001). The tips of these needles were slightly curved, similar to those of adult birds. The central bare shafts in nestlings aged 15–17 days were significantly longer than in the adults2.
2 Juveniles: 5.41±0.14 SE mm, 4.5–6.4 mm, n=14; adults: 3.20±0.16 SE mm, 2.1–4.8 mm, n=19; t=10.28,P<0.001
Camera monitoring from inside the nestboxes showed that breeding adults often clung to the inner vertical wall while resting during the day and while roosting at night (plate 002). Adult birds roosted inside the nestboxes as soon as they started arriving at the breeding areas in spring and always clung on the wall of the box while roosting, except during incubation and brooding periods. In all instances where birds rested or roosted on the inside walls of the nestbox, the tips of the tail feathers were held firmly against the surface of the wall.
At around two weeks old, nestlings started moving from the nest at the bottom of the nestbox to the walls. They remained clinging to the wall, both day and night, from the age of about 30–55 days old, after which they fledged. The young birds sometimes moved vertically up or down the wall, particularly when they were being fed by the parents, but they did not move back down to the base of the nestbox. As the young birds got closer to fledging, they would exercise their wings while clinging to the walls of the nestbox. As in the adults, young birds always kept the tips of their tail feathers held firmly against the wall.
Yonekawa & Kawabe (1994) observed similar behaviour in four natural nests in hollow trees that were used for breeding, but this is the first time that this behaviour has been documented to such a degree. Furthermore, we observed the swifts resting on the inner wall of tree hollows other than those used for nesting (plate 003). We noticed this several times during periods of heavy rain and strong wind. Although these hollows were not as deep as the nesting hollows, the swifts pressed their tails firmly to the surface of the inner wall. Based on our observations, it follows that the function of the needle-like projections in the tail of the White-throated Needletail and related species is to support the birds while it clings to vertical surfaces, similar to the stiffened tails of woodpeckers, treecreepers and woodcreepers. In these three groups, however, the vanes of the feathers are stiffened towards the tips as well as the shaft, whereas only the feather shaft is hardened in needletailed swifts. This difference is probably associated with the duration of time spent clinging to vertical surface and variation in flight aerodynamics between needletailed swifts and tree climbers, such as woodpeckers. Needletailed swifts spend a considerable amount of time in the air and use on vertical surfaces only for resting and roosting, while tree climbers spend a lot more time on vertical surfaces through their life, including during feeding and other periods when they are actively moving up and around trees, and thus require more stiffened tail feathers. The soft vanes on the tail feathers of the needletailed swifts may play an important role in changing direction or achieving stability during flight.
In some species, such as the White-collared Swift Streptoprocne zonaris and the White-naped Swift S. semicollaris, such needles become apparent at the tips of the tail feathers after the surrounding feather webs have been worn away following repeated contact with hard surfaces while resting or roosting (Chantler & Driessens 1999). This is not the case in the White-throated Needletail, where the bare needles are present on the newly grown tail feathers of young birds.
The slightly curved tips of the needles in adult and nestling White-throated Needletails likely aid the bird further while clinging to a vertical surface. The longer needles in nestlings perhaps offer more efficient support while the young birds cling to the side of the nest for some three and a half weeks. More detailed observations and experiments are necessary to clarify reasons associated with these aspects of the needles.
Needletailed swifts belong to the tribe Chaeturini, and many species are known to inhabit forests and nest in hollow trees or crevices (Chantler 1999; Chantler & Driessens 1999). Several species, such as Chimney Swift Chaetura pelagica and Short-tailed Swift C. brachyura, inhabit urban areas and nest in chimneys and ventilation shafts, but they originally nested in natural tree hollows. The size and conditions of these hollows are not well known, but it is likely that these swifts also cling onto inner vertical surfaces in the same manner as the White-throated Needletail. The tail needles of these swifts probably evolved through the birds’ use of tree hollows and their behaviour of clinging to vertical surfaces.
Why, then, do other species of swift not have needled tails in spite of their perching/clinging behaviour on vertical surfaces? For example, Apus swifts, such as Common A. apus and Pallid Swifts A. pallidus, cling to rock surfaces but do not have needles on the tail. This is perhaps because the Apus swifts spend considerably more time in the air than the needletailed swifts (see Liechti et al. (2013) for Alpine Swifts Tachymarptis melbaand Hedenström et al. (2016) for Common Swifts). Needletailed swifts have been shown to roost in cavities even before breeding has commenced, while many of the Apus swifts roost on the wing. Furthermore, Apusswifts nest in cavities, crevices, and on rock shelves, where not much vertical space is available for adults and nestlings to cling to vertical surfaces. Many of the Apus swifts have moderately or deeply forked tails, which aids manoeuvrability in the air but are not optimally shaped for use as an anchor when clinging to a vertical surface. In contrast, all needletailed swifts have broad, rather square or slightly rounded tails, which are suitable for supporting central or inner needles to aid clinging to vertical surfaces. In comparison, woodpeckers, treecreepers and woodcreepers have specialised wedge-shaped tails. On the other hand, Apusswifts have more powerful feet and sharper claws than the needletailed swifts, which aid with clinging to vertical surfaces.
There is limited information regarding roosting behaviour of needletailed swifts from their wintering areas. White-throated Needletails are reported to roost on vertical trunks and the upper branches of trees at the edge of forest breaks or on ridgetops (Tarburton 2021), though no specific information is available on how they use their stiffened needled tails while roosting but it is unlikely to differ from on the breeding grounds.
Finally, do the needles have any function associated with flight? Chantler (1999) speculated upon the possible aerodynamic functions of needled tails, but no data is available regarding this at present. The ecological (habitat, nesting sites), behavioural (duration of stay on surfaces, flying patterns) and morphological (tail shape, feet strength) traits of swifts described here are interrelated, and sophisticated statistical analyses are necessary to elucidate such complicated relationships, while also considering phylogeny. It nonetheless appears clear that at least part of the function of the tail needles of White-throated Needletails is to support and anchor the bird when clinging to a vertical surface.
We would like to thank the site managers of the study area for giving us permission to conduct this study. This study was funded in part by JSPS KAKENHI Grant Number 20K20587 to N. M. Yamaguchi, S. Mori and H. Higuchi.
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Hiroyoshi Higuchi, Hiroshi Yonekawa, Sayaka Mori, Satoshi Konno, Miwa Konno, and Noriyuki M. Yamaguchi
Hiroyoshi Higuchi, Research and Education Center for Natural Sciences, Keio University, Hiyoshi 4-1-1, Yokohama 223-8521, Japan; e-mail [email protected]
Hiroshi Yonekawa, Eduence Field Production, Akituki 1 jou 2-1-6, Hokkaido 079-8401, Japan
Sayaka Mori, Department of Environmental Sciences, College of Agriculture, Food and Environment Sciences, Rakuno Gakuen University, Ebetsu, Hokkaido 069-8501, Japan
Satoshi and Miwa Konno, Nishi 15jo-Minami 37-1-14, Obihiro, Hokkaido, Japan
Noriyuki M. Yamaguchi, Graduate School of Fisheries and Environmental Sciences, Nagasaki University, Bunkyo-machi 1-14, Nagasaki 852-8521, Japan