Growth model analysis of wild hyacinth macaw (Anodorhynchus hyacinthinus) nestlings based on long-term monitoring in the Brazilian Pantanal

Abstract

Studies on the breeding of vulnerable and endangered bird species are hindered by low numbers of individuals, inaccessible location of nests, unfavourable environmental conditions, and complex behavioural patterns. In addition, intraspecific variation may emerge only following long-term, systematic observations of little-known patterns and processes. Here, data collected over 30 years were used to determine growth model of hyacinth macaw (Anodorhynchus hyacinthinus) chicks in the Pantanal biome of Brazil. During this period, the speed of growth and body mass of chicks varied widely. Four growth models were tested: logistic, Gompertz, Richards, and cubic polynomial. They were fitted using three biometric measurements: body mass, total length, and tail length. The best-fitting growth curves were identified using Akaike's information criterion. The best models were the cubic polynomial for body mass, Richards for total length, and Gompertz for tail length. We confirmed the occurrence of dwarf individuals, whose body mass, total length, and tail length were 20%, 22%, and 70% smaller, respectively, than in the overall population. The dwarfs remain small in size after having fledged and are easily identified as adults. We discuss the importance of long-term studies to identify windows of opportunity for further research that will help in the conservation of endangered macaw species.

Figure 1

From 1991 to 2021, natural nests (n = 473, in blue) and installed artificial nests (n = 415, in red) were monitored in the Pantanal in the states of Mato Grosso do Sul and Mato Grosso, and more recently (last 15 years) in the Cerrado biome. This map was prepared independently with qgis v 3.10 software (www.qgis.org). Source of political boundaries: IBGE (www.ibge.gov.br).

Figure 2

Measurements of the tail length (A) and crop (B), and nestling mass for the hyacinth macaw in the Pantanal.

Figure 3

Development phases of the chicks: (A) nestling, 0–25 days, birds on picture are 17 and 18 days, respectively. Mass gain at this phase is slow, (B) chick, 26–77 days, birds on picture are 44 and 45 days, respectively, (C) chicks, birds on picture are 61 and 62 days, respectively. Geometric growth until maximum weight is attained, (D) juvenile, 78–107 days, birds on pictures are 104 and 105 days, respectively. Weight is maintained up to 90–95 days when weight loss begins with the first attempts of flying.

Figure 4

Growth curves fit of the four models tested in 412 nestlings of hyacinth macaws monitored over 30 years. The full black circles represent 381 individuals, and the empty squares represent the 15 dwarf individuals. (A) body mass, (B) total length, and (C) = tail length. Figures were produced using GraphPad Prism v. 8.

Figure 5

Fit of growth curves to 1st chick and 2nd chick, and to male and female hyacinth macaws. (A) = body mass; cubic polynomial model, (B) = total length; Gompertz model and (C) = tail length; Gompertz model. Figures were produced using GraphPad Prism v. 8.