SPECIES

Saddle-billed Stork Ephippiorhynchus senegalensis Scientific name definitions

Jonah Gula
Version: 2.0 — Published June 25, 2021

Breeding

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The Saddle-billed Stork generally times its breeding so chicks fledge during the dry season when foraging is optimal. Reproduction therefore seems to be dependent on rainfall, which means they may not necessarily breed every year. For example, in a severe drought year not one hatch-year stork was observed in Liuwa Plain National Park, Zambia (JG), an area that is usually very productive (48, D. Smit, personal communication). More detailed research is needed on the entire breeding cycle as most information is fragmentary at best.

Phenology

A thorough study of the breeding cycle is very much needed. Time of breeding in a calendar year varies greatly across the entire distribution of the species because it is largely dependent on seasonal rain. Sporadic records of each stage of the breeding cycle and variation in seasonality across the range have limited the ability to precisely delineate regional patterns. However, it appears to generally time breeding so chicks fledge at the height of the dry season when aquatic prey are concentrated in shrinking water sources (90, 4). Thus, it is possible, although unstudied, that juvenile survival may be related to the timing of breeding in a given year. Fledged juveniles are often noted in the literature either without any indication of suspected age, or unreliable estimates of age, because aging criteria are largely unknown (see Fledging Stage and Immature Stage), making it difficult to ascertain when nesting may have occurred.

Most published literature only provides breeding months without reference to what stages of the cycle fall within the time period, or if the entire cycle does. Months in which breeding or nests without stage specified can be found in Table 1.

Pair Formation

Because pairs are thought to mate for many years, possibly even life, and courtship displays are minimal between long-term pairs, nothing is known about pair formation (10).

Nest Building

Observations of nest building or repair are scarce and do not provide much in the way of a pattern. Months in which storks have been reported carrying nest material or building can be found in Table 1.

Egg Laying

In Southern Africa, egg laying or eggs in the nest have been recorded from the middle of the rainy season to the middle of the dry season. The pattern in East Africa is less straightforward due to two rainy seasons in some areas. The few records of eggs in West Africa are from the end of the rainy season. Months in which egg laying has been reported can be found in Table 1.

Broods

Nearly all records of chicks in nests, regardless of size, are during the dry season for the respective region. Months in which chicks have been recorded in the nest can be found in Table 1.

Despite numerous references to "recently-fledged" young, inferences about fledging phenology are weak due to a lack of information about development. However, known fledging months have been reported for several countries and can be found in Table 1.

Nest Site

Selection Process

As a solitary nester, this stork builds its own nest or uses old nests of other large birds such as Tawny Eagles (Aquila rapax; 4) or Wahlberg's Eagles (Hieraaetus wahlbergi; 12). It never nests in a colony with conspecifics or other species. Factors influencing site selection have not been studied beyond this.

Site Characteristics

Nests are built on the tops of trees – never within the canopy – from as low as 4.5 m to as high as 24 m. It has also been observed nesting directly on broken trunks of dead trees. Documented nest trees include Acacia spp., Adansonia digitata, Albizzia harveyi, Ceiba pentandra, Combretum imberbe, Copaifera mopane, Diospyros mespiliformis, Eridendron anfractuosum, Euphorbia spp., Faidherbia albida, Ficus spp., Garcinia livingstonei, Khaya senegalensis, Kigelia spp., Manilkara mochisa​​​​​​, Milicia excelsa, Philenoptera violacea, Phoenix reclinata, Schotia brachypetala, Syzygium spp., and Vitex doniana (12, 91, G. Douglas and P. Hancock, personal communication). They are usually located in open woodlands, on top of anthills, or isolated in swamps, floodplains, and pans – although only near water, not in it. Although cliff nesting has been purported, it has never been confirmed and is likely a case of misidentification (12). Although nesting is usually away from human habitations, unlike some colonial species, Kahl (92) found an active nest along a highway and amongst human dwellings. However, it is usually considered shy at the nest and prone to nest abandonment when disturbed (12, 92).

Nest

Construction Process

In a captive pair, the male carried materials to the nest more frequently (4.7 times per hour) than the female (2 times per hour). During this process, the male also spent more time on the nest than the female. He also regurgitated water onto the nest twice per hour and stomped on the nest occasionally (3).

Structure and Composition

Nests are large, flat platforms made mostly of sticks – resembling that of large eagles and vultures – and lined with grass, reeds, sedges, and mud (93, 12). One nest observed in Zambia was apparently made entirely of grass (94).

Dimensions

The diameter of the entire nest can measure 1–2 m and it is 0.5 m thick, with a slightly concave center where the eggs and chicks reside.

Maintenance or Reuse of Nests

Nest sites are used year after year by storks, but Secretarybirds (Sagittarius serpentarius) have been known to usurp unoccupied nests (4). In addition to using old nests of eagles (4, 12), pairs have also been observed using old nests of Hamerkop (Scopus umbretta; 91) and Secretarybird (95).

Eggs

Size

Measurements vary from 75.6–86.3 x 55.3–62.0 mm, averaging 81.3 x 57.8 mm (n = 18; 12, 96).

Mass

Mean mass (unknown developmental stage) reported as 146 g (96).

Eggshell Thickness

Schönwetter (96) reported the average eggshell weight to be 18 g (14.5–19.9 g, n = 6). Shell thickness was 0.7 mm. Factors influencing variation in egg size are unstudied as most data from eggs are historic.

Color and Surface Texture

Eggs are dull white, coarsely textured, and glossy.

Clutch Size

Pitman (12) suggested that clutch size seems to be limited to three in the north of the range (i.e., north of the equator), while four is common in Zambia and Zimbabwe. He also noted that observations of juveniles after they have left the nest do not necessarily inform how large the initial clutch was because some eggs or young birds may have been lost already. Mean clutch size has been reported as 2.8 (range: 1–5, SD: unreported; n = 54), and most commonly 2–3 (2). A brood of five is considered uncommon, and Pitman (12) questioned whether juveniles of multiple broods may join together. Evidence of this doesn't exist, so incidences of five juveniles may be indicative of unusually productive years.

Egg Laying

Eggs in captivity have been laid two days apart (3).

Incubation

Incubation Period

The incubation period is not accurately recorded. Pitman (12) argued against a period of 30–35 days – which is typical of smaller storks – due to the larger size of Saddle-billed Stork eggs. Eggs in captivity, however, have been incubated for 30–38 days (3, 97).

Parental Behavior

Both sexes alternate incubating the eggs for periods ranging from 1 h 25 min to 5 h 50 min, and sometimes they are both present on the nest (2). The incubating parent sits very low in the bowl of the nest so as to remain undetected from the ground (12). In a captive study, the female spent more than twice as much time incubating as the male, which did the majority of the nest-building (3). During the incubation period, parents will regurgitate water on the nest and eggs (10). Extended observations of nest behaviors are not available from the wild, and information about incubation would be especially useful, as data from captivity may be biased due to better nutrition, etc.

Hatching

Preliminary Events and Vocalizations

Captive chicks have been heard squeaking within the egg prior to and during hatching (M. Herry, personal communication).

Parental Assistance and Disposal of Eggshells

After chicks have hatched, either parent may consume the eggshells (2).

Young Birds

Condition at Hatching

Newly-hatched chicks are covered in white down, have very short bills, and weigh less than 200 g (3). The dorsal base of the bill is flattened where the "saddle" will grow.

Growth and Development

Young are altricial. Data from captive storks make up the majority of what is known about development in the nest, yet empirical information is mostly limited to behavior of young. At 5 d old, a pair of chicks born in captivity averaged 218 g; at 18 d they were 1.06 kg, and at 78 d they were 4.07 kg. Activity during the first month was limited to resting and begging for food. Standing and walking was first observed at 33 d, after which activity also increased to include preening (3). While the short bill does grow rapidly in size, it does not grow as quickly and in proportion to the rest of the body; the bill does not reach adult proportions in the nest. At 3 weeks old the bill is only 8.9 cm (12).

Parental Care

Brooding

Both parents brood their chicks closely for the first 10 d after hatching (4). In captivity, a female was observed brooding more than the male during the first 5 d, after which time they only stood over the chicks while at the nest. Only one adult was present on the nest at a given time except when exchanging duties (3).

Feeding

Both parents regurgitate food onto the floor of the nest for the chicks or directly into their open bills. During the warmest period of the day they also regurgitate water onto the chicks to cool them off (10). Provisioning in the same manner continues after young have fledged (86, 98), and, in captivity, even after young have begun feeding on their own (3). Fledged young are frequently left alone while parents forage. When parents stop bringing food for young in not known.

Cooperative Breeding

Does not breed cooperatively.

Brood Parasitism by Other Species

No information, but likely does not occur.

Fledgling Stage

Departure from the Nest

Time to fledging is a basic knowledge gap for this stork. However, young of the closely-related Black-necked Stork (Ephippiorhynchus asiaticus) leave the nest at about 3 months (K. S. G. Sundar, personal communication), so it is likely similar in the Saddle-billed Stork. At the Dallas Zoo, which has successfully bred more Saddle-billed Storks than other captive facilities, average fledge date has been 70 d (range: 58–82 d; M. Herry, personal communication). These are probably much earlier than in the wild due to nutritional status and potential for keepers to accidentally cause young to leave the nest (as was likely in one case in Dallas). Pitman (12) certainly underestimated the nestling period at 46–48 d, especially considering no captive chicks have even left the nest that early. Kahl (10) reported young still in a nest at 90 d, and suggested they fledge at 100–115 d. Brown et al. (2) suggested the entire breeding cycle is possibly longer than that of the Marabou Stork (Leptoptilos crumenifer), which fledges at 135 d (99), but it is unclear on what this idea is based.

An observation from the wild of young nearly as large as their parents (10) agrees with data from the Dallas Zoo in which nearly all young (except one accidentally bumped off the nest by a keeper) weighed 5 kg at fledging – roughly the same as an adult. It seems young are unable to fly upon leaving the nest and are only capable to short bursts of flight for some time thereafter, possibly even for several months (12, 98, 3). However, all of this needs much more study in the wild.

Association with Parents or Other Young

The little that is known about this stage of life is that it is usually synchronized with the height of the dry season and therefore the easiest foraging when aquatic prey are concentrated (90, 4). It is unclear how long young remain dependent on their parents or in their natal area, or when they begin foraging on their own. Pitman (12) mistakenly took several observations of recently-fledged young to mean they were independent at an early age, as parents often leave young unattended while foraging (JG). Knowledge of independence/separation from parents may be particularly confounded by this parental behavior. Pitman also noted young from the same brood have been observed together when their bills begin to show color, although the timing of this is unknown and may depend on nutrition, sex, or other factors, as demonstrated by much variation in captivity (see Appearance).

Immature Stage

This stage of the Saddle-billed Stork's life is completely undescribed, especially because timing of independence is unknown. A dispersing immature stork observed in southern Burkina Faso, approximately 175 km from the nearest known breeding site (see Movements and Migration), had faint red coloration on the base of its bill (B. Portier, personal communication). However, because the timing of coloration changes are unknown, it is unclear how old this disperser would have been. It is possible that upon independence siblings remain together as they leave their natal territory. During this period they may pass through territories of unrelated adults that are aggressive toward the dispersers (JG). Research on morphological changes, specifically coloration development, would improve the knowledge on dispersal through the use of online photo databases to age birds, as would a telemetry study.

Recommended Citation

Gula, J. (2021). Saddle-billed Stork (Ephippiorhynchus senegalensis), version 2.0. In Birds of the World (S. M. Billerman and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.sabsto1.02