Inheritance of Plumage Morphs in Little Eagles Hieraaetus Morphnoides in Northern New South Wales
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Australian Field Ornithology 2020, 37, 124–128 http://dx.doi.org/10.20938/afo37124128 Inheritance of plumage morphs in Little Eagles Hieraaetus morphnoides in northern New South Wales Candice Larkin* and S.J.S. Debus Zoology, University of New England, Armidale NSW 2351, Australia *Corresponding author. Email: [email protected] Abstract. Polymorphism and inheritance of plumage colour in the Little Eagle Hieraaetus morphnoides were studied in northern New South Wales, by tallying records of parental morph combinations in breeding pairs and the morphs of their offspring in the 1980s, 2000s and 2017–2019 (n = 41 pair combinations). The average ratio in breeding adults was 4.9 light: 1 dark, with dark males outnumbering dark females (1.8:1). Light × light pairs always produced light young (n = 32). Dark male × light female pairs variously produced light and dark offspring of both sexes (n = 12). Light male × dark female pairs produced dark young of either sex (n = 4) and one light young of unknown sex. One dark × dark pair produced a dark young. We conclude a Mendelian inheritance pattern with the dark morph recessive. From a small sample of mixed pairs with a dark male, the ratio of offspring morphs did not differ significantly from that expected by the Hardy–Weinberg equation, but mixed pairs with a light male produced 4 dark offspring to one light, versus the expected Hardy–Weinberg ratio of 1.95 light: 1 dark. This outcome suggests a non-random transmission of the dark allele by heterozygous light males, and a similar pattern of inheritance to that in the related Booted Eagle H. pennatus. Introduction Colour polymorphism is common in raptors, notably the family Accipitridae (hawks and eagles) (e.g. Ferguson- Lees & Christie 2005; Bosch et al. 2019). A key question concerns the function of polymorphism and why it is evolutionarily stable (e.g. Galeotti et al. 2003; Roulin 2004). There may be a link with niche and habitat breadth, use of open or semi-open habitats, and a mammal-rich diet, with the morphs having differential advantages in different habitats or lighting conditions (Galeotti et al. 2003; Roulin & Wink 2003). Among Australian raptors, the Brown Falcon Falco berigora has been considered to be polymorphic (e.g. Marchant & Higgins 1993). However, much of its plumage variation and so-called ‘morphs’, at least in south-eastern Australia, are related to age and gender (McDonald 2003), and the situation requires further resolution. Otherwise, the Figure 1. Adult light-morph Little Eagle in flight. Photo: Grey Goshawk Accipiter novaehollandiae is polymorphic, David Whelan with grey and white morphs. Mixed pairs of that species produce grey fledglings (Hollands 2003; SJSD pers. obs.) or white fledglings (R. Bilney pers. comm.), the latter presumably involving a heterozygous grey parent, and white pairs produce white offspring (Cupper & Cupper 1981), all of which suggests a simple dominant/recessive mechanism. The Little Eagle Hieraaetus morphnoides, like the other four species in the genus as currently constituted (Clark 2012; Lerner et al. 2017), is polymorphic, with discrete light and dark morphs (e.g. Ferguson-Lees & Christie 2005; Gjershaug et al. 2009). The defining character of the two morphs is the colour of the greater underwing-coverts, producing a solid dark underwing in the dark morph, which is also browner ventrally (Figures 1 and 2) (see e.g. Debus 1989, 2017; Marchant & Higgins 1993). An alleged rufous morph, represented by a single museum specimen and no field sightings or reports, is best regarded as an aberrant, Figure 2. Adult dark-morph Little Eagle in flight. Photo: exceptionally heavily pigmented light morph (Debus 1989; David Whelan Inheritance of plumage morphs in Little Eagle 125 Marchant & Higgins 1993). The Little Eagle forages in or each of those studies. In 1980, nine breeding pairs were over a range of habitats from grassland to forest, mainly monitored, four of these also monitored in 1981, and two woodland or open woodland, and in southern Australia monitored opportunistically through the mid-to-late 1980s. preys on a range of vertebrates though mostly mammals; In 2006–2009, seven breeding pairs were monitored successful broods are usually of one fledgling (e.g. in 2006, four of these in 2007, four in 2008 (including a Marchant & Higgins 1993; Debus 2017). The species is ‘new’ pair), and two in 2009 (including a ‘new’ pair). In strongly sexually size-dimorphic (e.g. Marchant & Higgins 2017–2019, 12, nine and seven breeding pairs were 1993), meaning that members of a pair can be sexed monitored in each year, respectively, as new pairs were by size (when together) and breeding behaviour, and found while some previously known pairs continued to be fledglings can be sexed (with high likelihood) when with monitored (though many did not breed in the drought of the parent(s) (see also Bosch et al. 2019). 2019), for an overall total of 22 pairs. In each time period, the Eagles were not marked, so the identity of randomly Debus (1989) presented preliminary findings from the sighted adults (away from nests) was uncertain and 1980s on the ratio of morphs in the Little Eagle and the not included in calculations for breeding pairs. Over the offspring morph of parental morph combinations, and decades, some pairs (territories) closest to Armidale city speculated on the genetics (possible sex-linkage and have been lost to urban expansion (e.g. Larkin et al. 2020), homozygosity/heterozygosity). Debus (2011) followed up but the study expanded to include additional pairs farther with quantification of offspring morphs from parental morph away. combinations in the same area (Northern Tablelands of New South Wales), from field studies in 2006–2009. Here The putative gender of offspring was assigned from visual we combine and expand those data with findings from assessment (through binoculars and telescope) of sexual a larger sample of pairs and their offspring in the same size-dimorphism of fledglings against their parent(s), based area in 2017–2019 (Larkin et al. 2020; CL & SJSD unpubl. on SJSD’s prior handling of adult and juvenile Little Eagles data). Meanwhile, a detailed analysis of morph ratios and of both sexes, although in some cases offspring gender inheritance patterns in the closely related Booted Eagle was not determined. To minimise disturbance to nests Hieraaetus pennatus (Bosch et al. 2019) provides some of this threatened species, nest-trees were not climbed, perspective on morphs and their inheritance in the Little nor nestlings handled or measured. Parentage was also Eagle. The morphs are lifelong plumage types, not age- putative (i.e. assumed no extra-pair copulations; no DNA related phases, and within morphs there is little colour investigation conducted), although extra-pair offspring in change from juvenile to adult, juveniles being ‘redder’ on raptors are rare (Roulin et al. 2004). the head and underbody than the respective adults (e.g. The Hardy–Weinberg equilibrium equation was used to Debus 1989). calculate the theoretical carrier (heterozygous) frequency, In our study we investigated the offspring morphs of based on the proportion of the recessive morph in the different parental morph combinations, in order to elucidate population (following e.g. Amar et al. 2013; Bosch et al. the mechanism of inheritance. We further hypothesise that 2019): the morphs may have differential survival or productivity p + q = 1, and p2 + 2pq + q2 = 1 in different environments as an area for potential future study (cf. relevant findings reported for the Booted Eagle where p is the frequency (% as a decimal) of the dominant by Bosch 2019 and Bosch et al. 2019). allele (here the light morph), q is the frequency of the recessive allele (here dark), and 2pq is the frequency of the heterozygous condition (here phenotypically light) Study area and methods in the population. The hypothesised homozygous and heterozygous proportions in the light morph in the sample The area sampled was centred on Armidale (30°30′S, population as a whole were then used in an online Hardy– 151°40′E) on the Northern Tablelands of New South Wales, Weinberg calculator (https://wpcalc.com/en/equilibrium- extending 60–80 km north-west to south, but mostly within hardy-weinberg/) to compare the observed versus expected a 20-km radius of the city. The study area is described frequency of offspring morph ratios in the combinations of by Debus & Ley (2009) and Larkin et al. (2020), and a light × dark parents in Table 1. contextual map is provided by Debus (2008). Locally, Little Eagles inhabit remnant eucalypt open forest and woodland in undulating country at ~1000 m above sea level, and Results their foraging extends into more open habitats (scattered woodland and adjoining pasture). The climate is temperate Plumage morphs in breeding pairs subhumid, with rainfall slightly summer-dominant. The number of pairs and their morph combinations In the 1980s, there were seven light × light pairs, one dark were tallied from studies of breeding pairs spanning male × light female pair and one light male × dark female three time periods: the 1980s (Debus 1989, 1991), 2000s pair known (n = 9 pairs). In the 2000s, there were seven (Debus 2011) and 2017–2019 (Larkin et al. 2020; SJSD light × light pairs, one dark male × light female pair, one & CL unpubl. data), not double-counting instances of the light male × dark female pair and one dark × dark pair same sex/morph combination (e.g. light × light pairs) in known (n = 10 pairs). In 2017–2019, there were 14 light × two territories in 2006–2009 and 2017–2019 where the light pairs, six dark male × light female pairs and two light same individuals might have been involved. Similarly, male × dark female pairs known (n = 22 pairs). The overall the numbers of progeny by morph and gender were ratio in the breeding population was 4.9 light: 1 dark, with tallied for these ‘pair-events’ where some pairs and their dark males outnumbering dark females by 1.8:1.