SPECIES

Black Falcon Falco subniger Scientific name definitions

Stephen Debus
Version: 2.0 — Published March 17, 2023

Breeding

Introduction

Black Falcon is monogamous and breeds annually in favorable locations and seasons, with the pair bond apparently lasting several consecutive years in resident populations. Adopted nests are stick bowls or platforms, located in mature living or dead trees in woodland, in isolated trees, or occasionally on artificial structures. Nests may be occupied for several or many years, if they remain intact. The breeding cycle from egg laying to fledging lasts almost three months, and the post-fledging dependency period another two months, meaning the entire cycle takes almost five months (1, 3). For resident populations, territorial defense is most intense during the breeding season, especially during the pre-laying and incubation phases against conspecifics and corvids, and against large eagles in the nestling phase (19, 3).

Phenology

Pair Formation

Some pairs are thought to stay together all year, while others may separate after breeding (1). Pairs are said to be established about a month before egg laying, and to be paired in early June (45, 1). One pair resided and roosted together in a temporary summer (nonbreeding) territory (5). In the temperate southeastern agricultural belt, a brief aerial meeting and interaction by a male and female falcon in March, during otherwise solo foraging, suggested pair reaffirmation; soaring together and copulation by a different pair in February occurred near a nest later built by corvids, and used by the falcons in July the following year (19). In established pairs, displays, courtship feeding, and copulation occur from May onwards in preparation for egg laying in July (10, 19). In one territory, the female formed a pair with a new male a month after she was widowed and abandoned her first clutch; the pair courted, copulated, and tried to attend the nest, but were displaced by a strongly defending Australian Hobby (Falco longipennis) that had claimed the nest in the interim. Apparently the same pair of Black Falcons revisited and copulated near the nest in the following year, but were displaced by corvids that were building a nest in the same tree (19).

Nest Building

No nest is constructed. In southern Australia, available stick nests are selected and guarded from autumn (April–May) onwards, such that they are ready for egg laying in subsequent months (8, 10, 19). Nests are occasionally usurped from another raptor species (88). Nests are not maintained through the breeding cycle or between seasons, and, when nests deteriorate, the falcons select another (5; SJSD).

First Brood

In eastern Australia, egg laying occurs from late June or July through August (7, 38, 9, 8, 10, 19, 11). A few other records of eggs, nestlings, or fledglings across the falcon's range (3) support such a pattern, with inferred laying extending to September. Australia-wide, eggs in historical collections were collected from May to November, mostly from July–September with a strong peak in August; there is no significant latitudinal trend in laying date (114). Late dates for eggs may represent repeat attempts after an initial failure, given that replacement clutches can be laid (45), and falcons resume pre-laying behavior and nest occupation after the loss of a first clutch (38). However, one pair seemed to give up for the rest of the season after apparently losing a second clutch (38), suggesting there is a specific seasonal window within which laying can occur or young can be reared.

Nest Site

Microhabitat

Depending on locality and available tree types and sizes, adopted stick nests are sited 4‒28 m above ground, in mature trees 10–30 m tall and having a trunk diameter of 60‒167 cm at ca. 1.4 m above ground, or sometimes near the top of 30 m steel power pylons or on rural windmill platforms in treeless landscapes. Nest trees may be in woodland or isolated trees on plains, floodplains, footslopes, or near watercourses or wetlands, or in agricultural landscapes, near open fields (1, 5, 38, 3).

Site Characteristics

Black Falcons select vacant stick nests in the tallest living trees available, typically in emergent eucalypts in the lower parts of open, flat to undulating landscapes. Nests may be exposed to the sun and conspicuous. Nest trees typically have bare or dead horizontal branches near the nest, used by the adults for perching, copulating, and prey transfers, and have open access on at least one side, so the adults can fly directly to the rim; they also typically have dead or dead-topped tree(s) within ca. 100 m, used by the adults and fledglings as perches and food-transfer sites. In living trees, nests are situated 1–6 m below the top of the canopy and 0.5–3.5 m from the edge of the canopy (1, 22, 3).

Nest

Construction Process

Not applicable. When selecting and claiming a nest, the female Black Falcon shuffles, scratches, turns around, and digs in it with its bill, testing, sitting in and shaping it, and pulling at sticks with its bill (38). Adopted stick nests are altered little, except that the incubating female may nibble bark from sticks on the rim (1).

Structure and Composition

Adopted nests are typically built by crows or ravens (Corvus spp.), or accipitrid raptors, and consist of a stick bowl or platform with an egg cavity able to accommodate the body size of the incubating and brooding falcon. Some of the original lining by the previous occupants, e.g., fibrous bark, wool, or dried green foliage respectively, may persist initially and is responsible for some early claims that the Black Falcon supposedly lines its nest.

Dimensions

External dimensions of typical adopted corvid nests are ca. 60 × 40 cm, with an egg cavity ca. 15 cm deep. External dimensions of adopted accipitrid nests, e.g., constructed by Black-breasted Kite (Hamirostra melanosternon), Black Kite (Milvus migrans), Whistling Kite (Haliastur sphenurus), and Little Eagle (Hieraaetus morphnoides), range up to ca. 120 × 55 cm, but the falcons also occasionally use vacant Wedge-tailed Eagle (Aquila audax) nests of ca. 180 × 80 cm or more (62, 1, 22, 110, 88, 3). The egg cavity of an adopted Whistling Kite nest was 30 cm wide × 7 cm deep (41).

Maintenance and Reuse of Nests

No nest maintenance is conducted, although nests may be guarded for up to three months before egg laying (8, 10, 19). Nests become trodden flat by the advanced nestlings and fledglings (108, 98; SJSD). Nests are reused annually, for up to six years, as long as they remain serviceable (1, 22, 5, 38). Conversely, some nests are blown down between seasons (7, 19), forcing the falcons to find another nest.

Eggs

Shape

Eggs are oval or rounded oval in shape (1).

Size

Mean size 54 × 41 mm (range 47–60 × 35–44 mm) (1). Egg volume averages 45 cm3 (range 33–52 cm3) (114).

Mass

Egg mass is not recorded.

Eggshell Thickness

Mean thickness was ca. 1.9 mm (range 1.6–2.8 mm) before and after 1946, with no significant effect on eggshell thickness from the introduction and widespread agricultural use of DDT in Australia (115).

Color and Surface Texture

Eggs are close-grained, not glossy, buff or pinkish-buff, densely and evenly freckled a darker shade and reddish-brown (1).

Clutch Size

One to five eggs, usually two to four (mean 3.3 ± 0.9, mode 4, n = 105 clutches); the mean clutch-size increases with latitude (114).

Egg Laying

No precise information. One and then three eggs appeared in a nest over a period of six days, indicating a laying interval of two days (3). Black Falcon is single-brooded (1), although it can lay a replacement clutch within the same season if the first clutch is lost (45).

Incubation

Incubation Period

The incubation period has been stated as ca. 33 d (22), and estimated as 34 d and 34 ± 1 d by observational studies using behavioral cues (98, 19).

Parental Behavior

Incubation is performed by both sexes, but mainly by the female, which relies on the male for food (1, 3). Typically, the male relieves the female while she feeds on prey delivered by him, and the returning female displaces him from the nest. The male’s contribution to diurnal incubation is 1–5% of daylight, in usually short stints of 5–16 min, but up to 2 h when a female abandoned a clutch and the male took over incubation before deserting as well (19). Females incubate for 77–92% of daylight, in stints often averaging 0.5–1 h, though sometimes extending up to > 2.5 or > 3.5 h, with eggs left uncovered for 7–8% of daylight, during part of which time either parent may perch on or beside the nest (19).

Hatching

Hatching is almost synchronous, over 1–2 d (45, 22). The presence of hatchlings is indicated by the female parent's posture when settling down on chicks to brood, sitting high in the nest bowl with much shuffling (9), contrasting with the incubation posture when the parent is low and flat in the nest (SJSD).

Young Birds

Condition at Hatching

Chicks are semi-altricial and nidicolous, hatching in white down and with eyes closed (1). On day 2, they can hold their heads up to be fed by the female (9).

Growth and Development

Chicks are downy for the first three weeks, with remiges, rectrices, scapulars, and facial feathers emerging in week 3; from week 4 feathers gradually appear dorsally (including the head) and ventrally (week 5), until in week 6 they are mostly feathered with some down on the crown and underparts. Down persists on the underwings until week 6 or 7 at fledging, when the primaries and tail are still incompletely grown (7, 9, 10, 19, 11).

Chicks are active, sitting up and peering over the nest rim at 1 week; they can stand, move around the nest, flap their wings, defecate over the rim, and feed themselves on prey in the nest in week 3; in week 6 they can jump and flap on the nest and to the nest-side branch, until "branching" in the tree in week 6 and fledging at 6 weeks old (7, 9, 10, 19 , 11).

Sex Ratios and Sex Allocation

The sex ratio in hatchlings is little known, but is said to be almost equal at hatching (1). There is no information on sex allocation. The sex ratio in fledglings appears to be close to parity (see Behavior: Sexual Behavior).

Parental Care

Brooding

The female is mostly responsible for brooding the chicks, although some males share the brooding of small chicks while the female is off dealing with his delivered prey (1, 3). Females brood almost continuously during the first week, but daytime brooding declines through weeks 2‒3, to only in cold, wet weather by the end of week 3. One female brooded for 86% of daylight during week 1, and another only 8% from late in week 2 through week 3. Female brooding stints in week 1 averaged 75 min, but at times extended to ca. 4.5 h. In week 1, one male brooded for 1% of daylight, in stints of 4–6 min, and the chicks were unattended for 8% of daylight (19). Females also shade nestlings from hot sun (22).

Feeding

Mostly the female feeds the chicks, on prey supplied by the male, although some males occasionally feed chicks bill to bill in the female’s absence (1, 3). When feeding small downy chicks, the female utters a feeding vocalization (single deep croak; see Vocalizations and Vocal Behavior: Vocal Array), tears small pieces, and places them in the gaping bill of the chick, which reaches upward. When feeding older nestlings, pieces of prey are held in the bill for the chicks to reach out and take (1). A female sometimes feeds her brood members in turn (45, 7, 9). Maternal feeding sessions commonly take 10–20 min, depending on the size and condition of delivered prey, but may extend to 27 min for large broods (45, 7, 9). A male fed one large brood of downy chicks for 18 min (9).

Females feed chicks at any time of day according to the presence of cached or delivered food, with retrieval from caches during lulls in the male's deliveries (45, 7, 9, 10). Females feed nestlings bill to bill until the fledging stage, even though feathered nestlings can feed themselves on prey in the nest during parental absences (98, 45, 1, 7, 9, 11). Females forage and deliver prey during the chicks' feathering stage or in bad weather. If the male has poor success in hunting, the female will leave downy chicks and forage (98, 45, 1, 19). Prey is typically plucked and sometimes partly eaten before being fed to the nestlings, but it may sometimes be delivered whole and unplucked (45, 7, 8, 10). Males deliver prey to the female at a transfer perch or directly to the nest, at any time of day (98, 45, 7, 8, 9, 10, 19).

Parents deliver between 0.2 and 0.7 prey items per hour to nestlings, depending on brood size, prey abundance, and weather (7, 9, 19).

Nest Sanitation

The female parent sometimes discards prey skeletal remains away from the nest, and some pellets are ejected out of the nest (7, 9, 10). The outside of nests becomes covered in the nestlings' excreta (SJSD).

Cooperative Breeding

No reports.

Brood Parasitism by Other Species

No reports.

Fledgling Stage

Departure from the Nest

Fledglings jump and flap, or climb, to branches in the nest tree (“branching” behavior), and return to the nest, for up to five days before their first true flights from the nest tree (9). The nestling period has been stated as ca. 42 d (22). Young were said to fledge (fly from the nest tree) at 42‒49 d old (1), paraphrasing "between 6 and 7 weeks" (98), but more accurate values are 38 d for a male and 40 and 42 d for two females in one brood (97); 40 d (both young), and 38 and 42 d, for two broods of two; and 42–43 d for a brood of four (9, 10). After they fledge, they spend the first day or two, until all in the brood have fledged, within 40‒150 m of the nest, moving between trees in low flapping flight, and either return to the nest for food or are fed in surrounding trees (7, 9, 10, 11).

Growth

Young fledge at adult body size, but with visibly short wings and tail until ca. 8–10 d out of the nest (7, 9, 10). By early in week 3 post-fledging, their primaries and rectrices appear as adult in proportions (7). Growth of primaries and rectrices appears complete by ca. 2 weeks after fledging, or by ca. 65 d old, on the basis of the outer primaries still being ensheathed in “blood quills” at ca. 6 d, and primaries and rectrices still incompletely emerged at 8–10 d, post-fledging (9).

Association with Parents and Other Young

Fledglings associate with siblings, and interact aerially with one another and with a parent at times of prey delivery, and at other times, e.g., in social play (7, 18, 9, 10, 11, 88). New fledglings roost on the nest; advanced fledglings roost with the female parent at night in the nest tree (7, 9). Once the juveniles can fly well, they harass incoming, food-bearing parents for food, and in week 6 start to accompany foraging parents (7, 9).

Ability to Get Around, Feed, and Care for Self

Fledglings range up to ca. 200 m from the nest in the first few days post-fledging, and initially make low, direct flapping flights between trees; late in week 1 juveniles can soar, and range increasing distances (to 600 m) from the nest (7, 9). Juveniles start to range more widely, to 3–4 km from the nest, late in week 2 (7, 9). New fledglings can feed themselves on the nest, on branches, on logs, or on the ground on prey delivered by a parent. By week 2 they can find and eat prey in the parents' cache (7, 9, 10, 11). The female parent sometimes feeds fledglings bill to bill until week 2 after fledging (7, 9).

Immature Stage

Juveniles become independent of parental feeding and disperse from the natal territory 7–8 weeks after fledging (9), and apparently have dispersed by 10 weeks or 2 months (10, 11). Seemingly shorter post-fledging periods may have involved the death of fledglings (3). The juvenile initially feeds in the nest area to week 3 post-fledging, and intercepts its incoming, food-bearing parents aerially from late in week 1, although some food is still transferred in the nest area until week 5 (7, 9). The juvenile starts practicing hunting maneuvers in week 3, and makes determined though unsuccessful attacks on birds in week 6 (7, 9). It is fully dependent on parental feeding until week 3 or 4, and at least partly dependent until week 8 (9).

Recommended Citation

Debus, S. (2023). Black Falcon (Falco subniger), version 2.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.blafal1.02