Figure
1.
Map showing location of study area
| Citation: |
Richard Evan Feldman, Antonio Celis-Murillo, Jill L. Deppe, Michael P. Ward. 2021: Stopover behavior of Red-eyed Vireos (Vireo olivaceus) during fall migration on the coast of the Yucatan Peninsula. Avian Research, 12(1): 66. DOI: 10.1186/s40657-021-00299-w
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For migrating birds, stopover requires spending time and energy that otherwise could be allocated to flying. Thus, birds optimally refuel their subsequent migratory flight by reducing stopover duration or foraging activity in food-rich environments. In coastal habitats, birds may forego refueling and take short stopovers irrespective of local food availability. Given the paucity of studies exploring how migrants adjust stopover behavior in response to temporal variation in food availability, especially in the Neotropics, we fixed radio tags to 51 Red-eyed Vireos (Vireo olivaceous) over two years at two sites on the coast of Mexico's Yucatan Peninsula.
We applied VHF radio tags during the fall of 2016 and 2017, and tracked birds using automatic and manual receiving units. We estimated stopover duration and activity levels (one site only) for between six and fifteen birds, depending on site and year. We measured fruit availability weekly along the net lanes where we captured birds. We used a generalized linear model to estimate the relationships between stopover duration/activity level and fruit density, bird body mass and year. We interpreted relationships for the model with the lowest AICc value.
We found that approximately half of the birds departed on the same day they were captured. For the birds that stayed longer, we could not discern whether they did so because they were light, or fruit density was high. On the other hand, lighter birds were more active than heavier birds but only in one of the two years.
Given our results, it is unlikely that Red-eyed Vireos refuel along the Yucatan coast. However, they still likely need to recuperate from crossing the Gulf of Mexico, which may necessitate foraging more often if in poor body condition. If the birds then move inland then stopover should be thought of as a large-scale phenomenon, where habitats with different functions may be spread out over a broad landscape.
The Spot-billed Pelican (Pelecanus philippensis), a near threatened bird, one of eight pelican species in the world, can be found only in South and Southeast Asia over an area between 129000 and 181000 km2 with strongholds in India, Sri Lanka, southern Cambodia and coastal areas of Sumatra. In India, it is presently distributed in southern and north-eastern India with concentrations in Andhra Pradesh, Tamil Nadu, Karnataka and Assam states (BirdLife International, 2001, 2011). Although observations of a natural history of breeding of the Spot-billed Pelican have been reported from various sites in India (Campbell, 1902; Rhenius, 1907; Neelakandan, 1949; Gee, 1960; Ganguli, 1964; Johnson, 1971; Venkatraman, 1996; Taher, 2007), few specific attempts have been made to explore the breeding biology of this pelican in detail in India except by Nagulu (1983) and Rao (1986). In general, there has been no quantified account of breeding success, a key variable in demographic studies, of Spot-billed Pelican in India. Hence, an attempt was made to document the breeding biology of the Spot-billed Pelican from September 2007 to September 2010 in the Karaivetti Bird Sanctuary, Tamil Nadu, India.
The study was carried out at the Karaivetti Lake, situated at 10°58′01″N and 79°11′07″E and covering an area of about 4.54 km2 in the Ariyalur District (Fig. 1). It was declared a bird sanctuary by the Forest Department in 1997. Rainfall ranges between 800–2000 mm and the temperature varies from 14℃ to 33℃. This freshwater lake is fed by the Pullambadi and Kattalai canals. It is the largest waterbody in the district and attracts a great number of birds every year. Vegetation, both natural and planted, in and at the edges of the lake, consists of Acacia nilotica, Prosopis chilensis, Azadirachta indica and Tamarindus indica. The Acacia nilotica plantation is the major nesting site for the pelicans. Recently, the lake has been designated as an Important Bird Area (IBA site code: IN-TN-13) in India by the Indian Bird Conservation Network (Islam and Rahmani, 2004) but despite this, no attempt has been made so far to study this wetland or its dependant avifaunal species.
Spot-billed Pelican nests were found in clusters on trees in the lake. I observed the behavior of the Spot-billed Pelican from a distance with binoculars and a spotting scope, permitting detailed observations without noticeably disturbing the pelicans. In total, 418 hours were spent studying the pelicans from September 2007 to September 2010. Completely visible nests along with their contents (eggs/chicks), built in the outermost trees in clusters found inside the lake, were chosen for a detailed study of the breeding biology of the Spot-billed Pelican during the study period. All eggs laid, eggs hatched and those failed to hatch, were recorded in order to calculate nesting success. In general, incubation can start as soon as the first egg is laid, in which case eggs will hatch asynchronously (one or two days apart); when the clutch is completed, eggs hatch synchronously, or at some point in between. In the case of the pelican, incubation starts as soon as the first egg is laid. For altricial species, the fledging period is typically the time spent in the nest, although birds may be dependent on their parents for a much longer period out of the nest (in some cases several weeks). In the present study, the fledging period of the Spot-billed Pelican was considered the time between the hatching of eggs and the independence of the chicks (leave the parents). In total, 68 nests were monitored for the present study: 14 nests in September 2007–June 2008, 20 nests in September 2008–June 09 and 34 nests in September 2009–June 2010. Besides, information on breeding was also collected from other breeding sites in Tamil Nadu.
The greater part of the Spot-billed Pelican colony arrives at the Karaivetti Lake in late November while a very few arrive in late December. After a few days from their arrival, the pelican starts breeding activities when there is sufficient water in the lake. However, among the five breeding sites in Tamil Nadu, i.e., the bird sanctuaries of Vedanthangal, Karikili, Koonthagulam, Karaivetti and Melaselvanur & Kelaselvanur, breeding starts much earlier (late September) in the Vedanthangal and Karikili bird sanctuaries, where pelicans arrive much earlier than in the other three sites. In general then, the breeding season of the Spot-billed Pelican starts in September and ends in May for the entire state of Tamil Nadu. However, the season may at times extend up to June when the arrival of the pelicans is delayed (Table 1). Sharma and Raghavaiah (2002) reported that breeding of the Spot-billed Pelican is dependent on the monsoon and not on the amount of rainfall. It should be noted that the onset of the monsoon largely determines the arrival of pelicans in Tamil Nadu. The breeding season of the pelicans observed in the present study is similar to the observations by Kannan (2007), Talukdar (1994; 1995a; 1995b), Neelakandan (1949) and Lamba (1963) in other parts of India, Legge (1880) and Baker (1929) in Sri Lanka and Oates (1878) at Pegu in Myanmar (Burma).
| Breeding activities | Months | |||||||||||
| Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug. | Sep. | Oct. | Nov. | Dec. | |
| Construction of nests |  |
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| Nest with Eggs | ||||||||||||
| Nest with chicks | ||||||||||||
| Overall breeding activities | ||||||||||||
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The Spot-billed Pelican is a gregarious bird, roosting and nesting on trees in large numbers at the breeding sites. However, among the 149 species of birds that the Karaivetti Lake harbors, the Spot-billed Pelicans are the last arrivals and thus are forced to occupy the remaining trees for either nesting or roosting in the lake. Largely thin-canopied trees, present at the periphery of clusters of trees (Acacia nilotica) present in the lake, are often ignored by other bird species and therefore these trees are only occupied by pelicans. Every year, the Karaivetti Lake harbors both non-breeding (not mature enough to breed) and breeding Spot-billed Pelicans. Although pelicans arrive in large numbers, only a small part of pelicans actually breed (of the 114 individual birds, only 36 bred in 2007–2008, 100 of 265 in 2008–2009 and 112 of 295 in 2009–2010) since pelicans reach maturity only after three years.
The male pelican develops a bright yellow skin around its pinkish red eyes. The breeding adults are brilliantly white with darker grey primaries. Both sexes develop a grey and white mixed crest during the initial period of breeding, which gradually is shed as the breeding advances. The bill of both sexes becomes brilliant yellow and with noticeable dark brownish spots during the breeding season.
The Spot-billed Pelican, soon after its arrival, spends three to five days to settle and during these days, the birds find their nesting trees for breeding activities. The Spot-billed Pelican pairs after a few display events in many nesting trees in the lake. However, their display is not as attractive and impressive as that of other waterbirds. Pelicans perform their display activities only at the top of trees and not in the air or in water, in contrast to other pelican species. Brown and Urban (1969) reported group display in Great White Pelicans (P. onocrotalus), while Schaller (1964) reported strut walking in American White Pelicans (P. erythrorhynchos). However, no such displays were observed in the Spot-billed Pelican. In the Spot-billed Pelican, the bill and gular pouch are on prominent display. The male initially distends his reddish bright gular pouch and wobbles vigorously. After a few seconds, the male swings his head up and down and sideways (Fig. 2a, 2b). During this display, the pelican also throws his bill a few times over the back down to the tail (Fig. 2c). After a few seconds, the bird claps his bill loudly while swaying the head up and down. Schreiber (1977) reported a similar behavior by the Brown Pelican, (P. occidentalis). However, not all these displays are performed exclusively to attract the opposite sex. Head sways and bill claps are often performed when any intruder nears the pelican or even when in a disturbed state. Hence, head sways and bill claps are signals of threat to other males or predators besides acting as courtship signals to attract eventual partners. After head sways and bill claps, both sexes of the pelican bow to each other as an act of acknowledgement (Fig. 2d). After this event, a few males take flight and do some circling in the air above the nest for a few seconds. The partner at the nest performs the head sways, bill claps and bow activities when the male returns to the nest from his flight. Mating occurs at the nest just after the courtship displays. The male holds the neck of his partner between his mandibles, mounts her back and keeps his wings spread and outstretched while mating (Fig. 3). Once the mating is over, the male releases the neck of the female from his mandibles and dismounts.
The Spot-billed Pelican start nest building largely after courtship. It is initiated by the collection of nest material, mainly small branches. Mating was also noticed during the collection of nest material and in some cases even after nest construction. Both sexes share nest building activities. However, the male largely collects the nest material and the female receives and aligns the material for nesting. Nest construction is undertaken by both members of the pair as reported elsewhere (Nagulu, 1983; Nagulu and Rao, 1990). Nest construction is largely done during the morning hours (07:00–10:00 hours, n = 68). Nest material is largely collected from the ground and occasionally from plants and older nests. Nest material consists primarily of small twigs and branches as reported by Oates (1878). Nests are placed very close to each other in such a way that an out-stretched wing can touch the nest of a nearby pelican. The nests are constructed on the top and outer canopy and a minimum of 5 to a maximum of 7 nests are found in a single nest plant. No nest was found in the inner canopy of a tree since this prevents easy landing and takeoff of pelicans due to their size. Up to 6–12 nests have been recorded in a single tree in Assam (Talukdar, 1995a) and up to seven nests in a series at Kokkare Bellur (Subramanya and Manu, 1996). Nest construction largely ceases in early December; however a few nested in February 2008 due to their late arrival. Although nests look loosely arranged, they are strong enough to hold two adult birds. Fresh vegetation is often seen lining the nest.
Individual birds tend to nest in the same place in the canopy each year, although new locations within the canopy and new trees are sometimes chosen (Subramanya and Manu, 1996). Although nests are found in the same place every year, identification was not possible and it is therefore not known if the same individual birds are involved, but nest site fidelity may not be ruled out.
In India, the Spot-billed Pelican often nest beside the Painted Stork (Mycteria leucocephala) (Abraham, 1973) or the Lesser Adjutant (Leptoptilos javanicus) (Talukdar, 1995b). Oates (1878) mentioned that nests in trees in the Sittang colony in Myanmar were often created by adjutants (Leptoptilus dubius). In Cambodia, the colonies are again mixed with adjutants and other storks (Parr et al., 1996). In the Karaivetti Lake, the Painted Stork and Asian Openbill-stork (Anastomus oscitans) shared the nest trees with the Spot-billed Pelican
The Spot-billed Pelican lays its first egg 5–10 days after nest construction, with clutch sizes varying from two to three eggs (Table 2). In the Sittang colony, the clutch invariably contained three eggs (Oates, 1878), although there was presumably some undetected variation. Four-egg clutches have been recorded in Assam (Talukdar, 1995b). In Nelapattu, India, the mean clutch size was 2.4 (n = 13; Nagulu, 1983), with clutches of three eggs completed in about seven days. Clutch sizes of 2–4 have been reported elsewhere (Whistler, 1949; Ali and Ripley, 1978; Nagulu, 1983; Rao, 1986; del Hoyo et al., 1992). Eggs are laid at a minimum of two- to three-day intervals (mean = 2.04, SD = 0.4) at the Karaivetti Lake. The pelicans are not very aggressive against predators and thus they do leave the nest when any bird of prey approaches their nests.
| Parameter | Breeding period | |||
| 2007–2008 | 2008–2009 | 2009–2010 | Overall | |
| Clutch size | ||||
| Mean | 2.42 | 2.95 | 2.64 | 2.69 |
| Mode | 3 | 3 | 3 | 3 |
| Range | 2–3 | 2–3 | 2–3 | 2–3 |
| Incubation period | ||||
| Mean | 30.82 | 30.5 | 30.9 | 30.8 |
| Range | 25–36 | 29–32 | 30–32 | 25–36 |
| SD and SE | 1.98, 0.36 | 1.02, 0.13 | 0.87, 0.09 | 1.18, 0.08 |
| Fledging period | ||||
| Mean | 95.06 | 94.05 | 92.91 | 93.71 |
| Range | 91–98 | 90–99 | 90–102 | 90–102 |
| SD and SE | 2.21, 0.57 | 2.85, 0.60 | 2.74, 0.47 | 2.77, 0.33 |
| Survival rate | ||||
| % of successful nest a | 100 | 100 | 100 | 100 |
| % of eggs hatched | 85.29 | 94.91 | 100 | 95.43 |
| Nest success | ||||
| Rate (%) b | 79.41 | 91.52 | 94.4 | 90.28 |
| Fledging/total nests | 1.9 | 2.7 | 2.5 | 2.3 |
| a nests that hatched at least one young. b total fledglings/total eggs. | ||||
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The Spot-billed Pelican starts incubation as soon as the first egg is laid (Nagulu, 1983). Both sexes share the incubation; however, incubation by females predominates. Both sexes contribute to "domestic duties" during nesting (Lamba, 1963). The incubation period varied from a minimum of 25 days to a maximum of 36 days (Table 2). During incubation, the adult bird puffs its wings to fully cover the entire nest and performs no activities. Nearly similar incubation periods (30–36 days) have been reported elsewhere (Nagulu, 1983; Rao, 1986; del Hoyo et al., 1992).
Hatching of the first egg starts after an average of 30 days. Newly hatched birds are altricial, i.e., born naked with red flesh (Fig. 4). The fledging period continues from a minimum of 90 days to a maximum of 102 days (Table 2). Fledging periods of around four months have been reported for the Spot-billed Pelican by Nagulu (1983), del Hoyo et al. (1992) and Talukdar (1995). The young birds start moving, perching and standing only after 30 days. During this act, chicks often fall from the nest, but no adults show any interest to rescue them from the water. Few are able to regain the nest and die in the water. During the study period, nine chicks lost their life due to such occurrences (2 in 2007–2008, 2 in 2008–2009 and 5 in 2009–2010). Adults largely try to flee when any intruder nears the nest rather than showing any aggression. Adults remain in the nest up to a maximum of 15 days after hatching; after that, they remain outside the nest to safeguard the young for about a further maximum of 19 days. After 30 days of hatching, adults are rarely seen in the nest. Adults visit the nest only during delivery of food to the young. When the young are 30–36 days old, they do not remain in the nest and dismantle it by pulling out the sticks from the nest. The young remain perching on the branches till they finally take off. At the end of the breeding season, it is very difficult to locate even the rudiments of nests on the tree and one finds on the ground, under the nesting tree, heaps of fallen white sticks.
The Spot-billed Pelican feeds its young in three ways (Fig. 5). After a few days of hatching, the adults give a pre-digested liquid diet to their chicks. Adults drip this predigested liquid through the mandibles and right to the mouth of the young (Fig. 5a). After 7–10 days, the adult, by standing in front of the young, vigorously shakes its neck and regurgitates the food onto the nest (Fig. 5b). The young pick up this food and eat it bit by bit by seriously tossing or jerking their mandibles. After 30 days, the young are very much capable to draw the food straight from the gullet of the parent by inserting their mandibles up to the neck into the adults' mouth (Fig. 5c). Up to 30 days, adults feed their young largely 3–7 times a day and gradually reduce the frequency.
No studies have been reported on nest success of the Spot-billed Pelican. However, breeding success was reported to be constrained largely by floods, cyclones and felling of nesting trees (Nagulu, 1983; Santharam, 1993; Narasimhulu, 1995; Talukdar, 1995b). At Kokkare Bellur, India, an average of 1.6 nestlings fledged per nest in 1995. In the present study, a mean of 2.3 nestlings fledged per nest at the Karaivetti Lake (Table 2). Nagulu and Rao (1990) judged that the pelicans nesting at Nelapattu had a breeding success of 73.5%. Of the 68 nests studied, the Spot-billed Pelican had a nest success of 90.28% during September 2007 to September 2010 at the Karaivetti Lake (Table 2).
The Spot-billed Pelican breeds during November and April at the Karaivetti Lake. The availability of suitable nesting trees, nest material, colonies surrounded by water for safety reasons, good food supplies within the lake and at nearby lakes, fewer human disturbances and poaching activities could be the key reasons for the successful nesting activities of the Spot-billed Pelican in recent years at the Karaivetti Lake.
I thank the Academy of Higher Education, National College, Trichy -1 for academic support throughout the study. I also thank the Department of Science and Technology (SERC-Fast track Young scientist Scheme), India for funding the study and the Tamil Nadu Forest Department for granting me the required permission. I also thank IDEAWILD for its logistical support. I thank Mr. V.Kannan, Ms. N.Sheeba and Mr. P.Senthilkumar for sharing their knowledge.
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Figures(4) / Tables(2)
| Breeding activities | Months | |||||||||||
| Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug. | Sep. | Oct. | Nov. | Dec. | |
| Construction of nests | |
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| Nest with Eggs | ||||||||||||
| Nest with chicks | ||||||||||||
| Overall breeding activities | ||||||||||||
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| Parameter | Breeding period | |||
| 2007–2008 | 2008–2009 | 2009–2010 | Overall | |
| Clutch size | ||||
| Mean | 2.42 | 2.95 | 2.64 | 2.69 |
| Mode | 3 | 3 | 3 | 3 |
| Range | 2–3 | 2–3 | 2–3 | 2–3 |
| Incubation period | ||||
| Mean | 30.82 | 30.5 | 30.9 | 30.8 |
| Range | 25–36 | 29–32 | 30–32 | 25–36 |
| SD and SE | 1.98, 0.36 | 1.02, 0.13 | 0.87, 0.09 | 1.18, 0.08 |
| Fledging period | ||||
| Mean | 95.06 | 94.05 | 92.91 | 93.71 |
| Range | 91–98 | 90–99 | 90–102 | 90–102 |
| SD and SE | 2.21, 0.57 | 2.85, 0.60 | 2.74, 0.47 | 2.77, 0.33 |
| Survival rate | ||||
| % of successful nest a | 100 | 100 | 100 | 100 |
| % of eggs hatched | 85.29 | 94.91 | 100 | 95.43 |
| Nest success | ||||
| Rate (%) b | 79.41 | 91.52 | 94.4 | 90.28 |
| Fledging/total nests | 1.9 | 2.7 | 2.5 | 2.3 |
| a nests that hatched at least one young. b total fledglings/total eggs. | ||||
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