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Study confirms existence of dwarf hyacinth macaws

Research also defined the growth stages of this bird from the Pantanal, which was once an endangered species

The first flight of a hyacinth macaw as it leaves its artificial nest installed in a tree

Bruno Carvalho

A casual observer of a hyacinth macaw in flight will likely not have had the chance to observe one at the chick stage, when hatchlings spend three and a half months under the watchful care of their parents before they are ready to leave the nest and gain the skies. A study led by Brazilian researchers in the Pantanal and, at a smaller scale, in the Cerrado (wooded savanna biome), mapped out the key stages of development in the wild of around 400 nestlings of Anodorhynchus hyacinthinus, the scientific name of the parrot species. For 30 years, from 1991 to 2021, Neiva Guedes—a biologist at Anhanguera-UNIDERP University in Campo Grande, Mato Grosso do Sul, midwestern Brazil—and her colleagues monitored 473 natural nests and 415 artificial nests (made by the researchers) with freshly emerged hatchlings.

Monitoring began when egg laying was detected and ended when the chicks left the nest. Among the findings from the long-term study, which the researchers reported in a paper in the September edition of Scientific Reports, was the confirmed occurrence of dwarfs whose weight and total length was approximately a fifth of that of normal-sized birds. Dwarfs also had 70% smaller tails.

Smaller-sized individuals had previously been observed during the study but not systematically analyzed. “We identified 31 dwarf individuals among the hatchlings, approximately 8% of the total sample, but only the 15 dwarfs that fledged were analyzed,” explains Guedes. The reason for the occurrence of dwarfs in the species remains unknown. As in other bird species, several factors can lead to unequal hatchling development, including varying temperatures and rainfall levels, the quantity, quality and availability of food, differences between sexes, clutch size, and disease.

A better understanding of the species’ lifecycle from birth can aid conservation efforts. The hyacinth macaw is a species that is nonmigratory, monogamous, and slow to develop, with a low reproduction rate. In each fertile period, from July to February, the female lays an average of two eggs. However, only one hatchling will typically survive. A normal-sized adult can measure up to 1 meter (m) in length from tip of beak to tip of tail, and can weigh up to 1.3 kilograms (kg). Developing hatchlings can weigh up to 1.7 kg—excess weight is lost when they start to fly.

Edson DinizThe population of hyacinth macaws in the Pantanal is currently estimated at around 6,000 individualsEdson Diniz

Known for its deep blue plumage, Anodorhynchus hyacinthinus is the world’s largest psittacine species. It is native to Brazil’s tropical wetlands and is sometimes mistaken for its smaller-sized cousin, Spix’s macaw (Cyanopsitta spixii), found only in Brazil’s drier northeastern state of Bahia. The psittacine family includes parrots, parakeets, macaws, and cockatoos, most of which are found in tropical and subtropical regions. In the 1980s there were only 2,500 surviving specimen of the hyacinth macaw, then considered a threatened species. And while the population has since grown to an estimated 6,000 individuals, it is still regarded as endangered, although its status has now been downgraded to vulnerable. The observed population growth is largely thanks to three decades of efforts at Instituto Arara-Azul, a nongovernmental organization founded and now chaired by Guedes. Some fear, however, that the recent droughts and forest fires in the Pantanal could drive the species back to the verge of extinction.

Guedes and her team started their fieldwork three decades ago when they observed a pair of macaws defending their chosen nesting location. Ninety-five percent of the nests are built in hollows in the manduvi tree (Sterculia apetala), a large Pantanal species with a reddish-brown trunk and whose seeds are also a source of food for macaws. There is strong competition among breeding hyacinth macaw pairs and other species for these hollows because only trees older than 60 years produce cavities large enough to be used by the macaws. In their study, the researchers measured and weighed a total of 837 nestlings. But with more than half of them eventually dying from different natural causes, only the 396 chicks that fledged were included in their statistical analysis.

On average, the 381 fledged normal-sized birds left their nests after 107 days, weighing 1.1 kg and measuring 66.7 centimeters (cm) in total length. The 15 dwarf birds left the nest on average after 126 days, weighing 938 g and measuring 33.9 cm in length. Within the normal-sized group, 294 first-hatched chicks and 87 second-hatched chicks were measured. Among the chicks whose sex was determined in the study, 226 were female and 148 were male. There were no significant size differences between hatchlings of different sexes, with only tail length observed to be 24% larger in males than in females.

When a clutch produced two hatchlings, the first-hatched chick gained weight at a 42% higher rate than the second chick. “Knowing that the second-hatched chick is slower to develop is an important input for environmental management programs, allowing them to allocate more resources to second hatchlings, says Gabriela Vigo-Trauco, a Peruvian biologist who has completed a postdoctoral internship at Texas A&M University and is a director at the Macaw Society, a scientific project that has researched macaws and parrots in the lowlands of southeastern Peru since 1999.

Lucas RochaA dwarf (left) and normal-sized chick, both about 100 days oldLucas Rocha

A subsample of 42 eggs that produced 30 normal-sized hatchlings near the researchers’ base facility in the Caiman Ecological Refuge, in the municipality of Miranda, were measured on a weekly basis. The more frequent measurements allowed the researchers to map out the chicks’ development stages with greater precision. According to the paper, chick development encompassed three discrete phases: nestling (0–25 days), during which chicks were completely dependent on their parents for maintaining body temperature, and weight gain was slow; chick (26–77 days), a phase in which mass gain was fast until maximum weight was reached; and youth (78–107 days), in which weight was maintained until day number 90–95, followed by weight loss concomitantly with the first attempts at flying and until permanently leaving the nest.

“We used four different mathematical models to describe chick development,” explains Guedes. “Although they exhibit a different growth pattern, on leaving the nest dwarf macaws will mate and reproduce normally with other individuals.” Body mass, body length, and tail length were the variables entered in the equations to develop growth curves for the species.

Luís Fábio Silveira, an ornithologist at the Zoology Museum of the University of São Paulo (MZ-USP), praises the study as statistically robust and one of the few of its kind in Brazil to provide as much detail from data compiled over three decades of research. In addition to its important contributions to understanding the biology of hyacinth macaws, the research offers information that can be useful in comparative studies with other large psittacine species, such as Lear’s macaw (Anodorhynchus leari), a threatened species endemic to northeastern Bahia.

“Growth-curve information is important in ensuring that sufficient food is available in the hyacinth macaw’s habitat. If the population of dwarf birds starts to increase, this could be indicative that natural resources are depleting,” notes Silveira, who was not involved in Guedes’s study. “This research can also help us to more accurately estimate the age of hyacinth macaw chicks confiscated by environmental authorities, supporting efforts against the illegal trade and trafficking of birds.” Not to mention the contributions from the study to policymaking and environmental education to help preserve the hyacinth macaw itself.

Scientific article
GUEDES, N. M. R. et al. Growth model analysis of wild hyacinth macaw (Anodorhynchus hyacinthinus) nestlings based on long‑term monitoring in the Brazilian Pantanal. Scientific Reports. Sept. 13, 2022.