by Derek Engelbrecht
The harsh nasal chatter of Arrow-marked Babblers is as distinct as it is a familiar sound in the mesic woodlands of southern, central and eastern Africa. They are highly social cooperative breeders living in groups ranging from 3 to 13 individuals, defending their territory throughout the year. Although aspects of their breeding biology are known, there seems to be limited information about the growth and development of nestlings. This note describes the growth and development of a clutch of two Arrow-marked Babblers. The use of plumage and growth parameters is of value for studies where nestling age estimates are required for daily survival estimates. On 19 October 2010, I found an Arrow-marked Babbler nest containing two eggs in the Polokwane Game Reserve. The nest was placed in a medium-sized Vachellia tortilis, about 1.7 m off the ground and close to the tree’s main stem.
The two eggs measured 24.1 mm x 18.4 mm (4.24 g) and 24.8 mm x 19.1 mm (4.59 g), respectively. The nest was visited on alternate days to determine the hatching date. At 13:10 on 27 October 2010, the nest contained a recently hatched nestling and an unpipped egg. The next day, the other egg hatched, suggesting incubation at this nest commenced after laying the first egg, resulting in an asynchronous hatching pattern. The growth and development of nestlings were recorded mostly on alternate days during the nestling period. All measurements were recorded within 90 minutes after sunrise. To distinguish individual nestlings, they were marked on the thigh with a nontoxic marker pen and fitted with a metal ring when they were eight days old. Nestling development was described with regard to:
• plumage development
• increase in mass (g) measured with a portable digital electronic scale,
• increase in tarsus length (mm),
• length of the wing chord (mm) as described by de Beer et al. (2001).
Nestling development
Growth patterns of the two nestlings for selected parameters are presented below. It is interesting to note that upon fledging, the tarsi of the two nestlings were almost fully grown but the wings were not nearly fully grown yet. This is a strategy employed by many open, cup-nesting species to invest heavily in the growth of the legs to enable the nestlings to fledge early and be mobile, to reduce the risk of a predator locating the nest and depredating the entire brood.
Hatching (Day 0)
The newly hatched nestling was naked and blind. The skin was a pale flesh colour. On hatching day, the two nestlings weighed 5.6 g and 4.6 g, respectively. The bill was yellowish, and the gape flanges a slightly paler, yellow colour. The inside of the mouth and the tongue was a brighter yellow colour. Although there were no tongue spots or spots at the bill tips, the posterior central part of the tongue showed a slightly darker discolouration. The claws were pale horn coloured.
Day1
Eyes still closed. The individual pterylae are visible below the skin.
Day2
The eyes are open slits. Some primaries and secondaries erupted through the skin. Tail neosoptiles are visible. The bill starts to darken but the gape flanges remain the same colour.
Day3
The eyes are still open as slits but more so than on day 2. All the feather tracks have erupted except the capital and crural pterylae.
Day 4
All the feather tracts in pin, including some on the capital and crural tracts, but only just though. The eyes are almost fully open, but the nestlings prefer to keep it open like slits.
Day 5
The eyes are fully open, and all tracks are in pin.
Day 6
The first feathers on the capital, crural, femoral, dorsal and ventral pterylae in brush. Nestlings are very vocal, calling a babbler-like call.
Day 7
Most feathers on the capital (except crown), crural, femoral, dorsal and ventral pterylae in brush. Most secondaries and one or two primaries in brush, but only just.
Day 8
All pterylae in brush. All primaries and secondaries in brush.
Day 9
All tracts have most feathers in brush, but crown only starting to go in brush. Most secondary greater coverts and the first pri- mary greater coverts in brush.
Day 10
All feathers in brush. Primaries are between 25% and 50% in brush.
Day 12
All in brush, primaries approximately 66% in brush.
The nestlings fledged asynchronously when they were 13 days old. This is considerably shorter than the 18 to 21 days reported by Monadjem et al. (1995). According to Monadjem et al. (1995), nestlings of larger groups tend to fledge at a younger age (9 group members, 18 day nestling period) than smaller groups (3 group members, 21 days). The short nestling period in this study, therefore, seems unusually short. Despite the early fledging observed in this study, at least one of the fledglings survived as adults were seen carrying food 25 m from the nest three days after the younger of the two nestlings fledged. It would be interesting to investigate the proximal causes of variation in the nestling period of Arrow-marked Babblers. I acknowledge that, despite my well-designed field protocol to minimize disturbance at the nest during data collection, my activity may have contributed to earlier than usual fledging. An alternative explanation may involve partial predation of the oldest chick followed by premature fledging of the younger chick a day later. Whether the unusually short nestling period recorded here was natural or induced, it highlights the need for a more detailed study of these fascinating birds.
Monadjem A, Owen-Smith N, Kemp AC. 1994. Position of nest, incubation period and nestling period of the Arrowmarked Babbler. Ostrich 65: 341.