Feather moult and bird appearance are correlated with global warming over the last 200 years

Abstract

Global warming alters various avian phenological processes, including advanced reproduction and migration schedules. In birds, individual appearance is largely determined by plumage, influencing, for example, bird attractiveness, social status and camouflage. Juveniles of most passerine species replace their nest-grown plumage during the first months of life, a process that is called post-juvenile feather moult. Using data from ten natural history collections, we show that the extent of the post-juvenile moult has increased significantly over the last 212 years (1805-2016), a trend that is positively correlated with the temperature of the environment. Therefore, it seems that birds replaced more feathers under warmer conditions, causing juveniles to appear more similar to adult birds. Moreover, in several species, we describe a male-female switch in the extent of moult, with females currently replacing more feathers than males compared to the past. These results demonstrate different biological responses to climate warming by different phenotypes.

Fig. 1

The relationship between global mean temperature anomalies (GMTA) and feather moult extent. a The extent of post-juvenile moult of 19 passerine species, shown separately for each species (thin lines ± 95% confidence intervals, CIs) and the general trend (bold line ± CIs). The trend indicates a significant increase of moult extent with increasing GMTA in 16 species (∆AICc ≥ 2.10; Supplementary Table 3). The two circled insets depict the extent of wing moult in the beginning and the end of the examined period (1805–2016). The boxed inset illustrates the general trend of moult extent across the study period (years). b Male and female moult extent of 10 sexually dichromatic passerine species throughout the examined period, shown separately for each species (thin lines ± CIs) and the general trend (bold line ± CIs). In four species, the results indicate a stronger response to warming climates in females than in males (∆AICc ≥ 3.80), leading to an overall more extensive moult of females than that of males since ~1990. Thus, in these species, females replaced more feathers than males under warmer conditions. c Two examples demonstrating how the increasing extent of moult changed the appearance of a first year male Collared Flycatcher Ficedula albicollis (top two illustrations) and Eurasian Blackbird Turdus merula (bottom two illustrations) before 1900 (left) compared to 2015 (right). Since the birds moulted more feathers in recent years, their present-day plumage appears more similar to that of adult birds

Fig. 2

Passerine moult extent and phenological changes associated with global warming. a Global mean temperature anomalies (GMTA) trend since 1805 and bird feather moult extent. The moult index represents the relative extent of moult that was found in the study, averaged per year among all study species, such that the lowest relative extent of moult = 0 and the highest relative extent of moult = 1. Light grey and dark grey shaded areas represent two periods that are referred to in b. b Phenological changes in avian annual routine events over the last two centuries. Upper two rows (light grey = earlier years, dark grey = recent years): residents and short-distance migratory species – advancement of spring arrival and breeding, delayed autumn migration and shortened migration distance causing a longer moulting period and resulting in a more extensive moult. Middle two rows: Long-distance migratory species with pre-migration moult – advancement of spring arrival and breeding causing a longer moulting period, also resulting in a more extensive moult. Lower two rows: Long-distance migratory species with post-migration moulting period – it is suggested that advancement of the autumn migration combined with earlier arrival to the wintering areas and richer food resources in the northern Afrotropical Sahel zone after the summer rains allow these birds to moult more extensively. The changes in the timing of annual routine events are largely based on data from different published studies^–,,

Fig. 3

The relationship between plumage ornamentation differences between male and female birds. Plumage ornamentation score differences between male and female birds is calculated as (male_{ornamentation} − female_{ornamentation}). We illustrate the interaction in the models that tested the effects of global mean temperature anomalies (GMTA) and bird sex on the area of moulted feathers in the post-juvenile moult in relation to plumage ornamentation score differences among sexual-dichromatic bird species tested in the study (n = 10). In species with a larger difference in plumage ornamentation between the sexes, the response of females to GMTA was high compared to that of males, as shown here by their overall lower coefficients of the GMTA and sex interaction, with male relative to female values. The plumage ornamentation score is provided by Dale et al.. The method used for quantifying plumage colouration for male and female of each species includes red, green and blue (RGB) measurements in three dorsal (nape, crown, forehead) and three ventral (throat, upper breast, lower breast) body patches. For each plumage patch, RGB values for both sexes in all the ten species were collected