Sarah J. K Frey, Christopher C. Rimmer, Kent P. McFarland, and Stephane Menu research on the reliability of morphometric (measurement if size and shape) data in determining the sexuality of Bicknell’s Thrushes (type of bird). Bicknell’s Thrush is scientifically classified as Catharus bicknelli.
The study focuses on using primary formulae to differentiate between C. bicknelli and the Grey-cheeked Thrushes (Catharus minimus). It is interesting to note that the two species cannot be easily differentiated during the non-breeding season because they have the same monomorphic characteristics (sexes are similar in size and plumage). Efforts to understand the biology of Bicknell’s Thrushes have been hampered by the similarity in monomorphic characteristics (Frey et al. 409).
Morphometric data from C. bicknelli and C. minimus, specifically primary formulae, can be used to identify the two species and determine the sexuality of C. bicknelli. The research focusses on sexually identifying the male and female Bicknell’s Thrushes, and differentiating the two Cathurus species (Frey et al. 409).
Sampling and Variables
The samples used were Bicknell’s and Gray-cheeked Thrushes. The birds were captured with a mist net and banded (tagged at the birds’ feet for research purposes) at known breeding habitats in USA and Canada between May 1993 and September 2007. In the US (N = 422), researchers collected samples in seven sites spread across Vermont, New York, and New Hampshire.
In Canada (N = 199), all the twelve sampling sites were situated in Quebec. The ornithologists captured 58 C. minimus from two other sites in the US between in 2007 and 56 more at ten sites in Newfoundland, Canada over a four-year period. The variables under study were tail length, culmen length, wing chord, tarsus length, mass, and sex (Frey et al. 409).
The researchers examined the sexes of the samples by studying the presence of a cloacal protuberance (storage for sperm in males) or a brood patch (featherless skin in females used to heat eggs during incubation). The researchers differentiated second year (SY) and after second year (ASY) C. bicknelli by the shape of the rectrix feathers and the presence of buffy tips on the covert feathers in some SY birds.
The ornithologists sampled Bicknell’s Thrushes over the years because the study was based on long-term mark and recapture data (Frey et al. 409). The main morphometric techniques were Analysis of variance (ANOVA) and Discriminant Function Analysis (DFA). The study sampled data using the following primary (p) formulae: (p8-p1), (p8-p1), and (p8-p6). The researchers computed the variable statistics using SAS statistical software (Frey et al. 410).
Sex Discrimination of Bicknell’s Thrushes
Variables and differences in mean were calculated using a three-way ANOVA for SY and ASY birds. Variations in mean and the correlation between sex and age for C. bicknelli were computed using a two-way multivariate analysis of variance (MANOVA). The researchers used a backward stepwise DFA (stepDFA) to differentiate sexes in Bicknell’s Thrushes. A linear DFA (linDFA) was used to distinguish the sexes of C. bicknelli. The study ran a linDFA to determine the reliability of the p8-p1 formulae in providing increased discrimination between male and female C. bicknelli (Frey et al. 411).
The ornithologists used a two one-way MANOVA to differentiate the two species using the variables provided. A stepDFA was used to determine whether the primary formulae (p8-p1) was useful in differentiating the species. The researchers used a linDFA to discriminate between sexes in Bicknell’s Thrushes using the same steps employed in identifying the birds (Frey et al. 411).
Sex Discrimination of Bicknell’s Thrushes
Overall, ASY and male Bicknell’s Thrushes were larger than SYs and females (Frey et al. 411). Analyses of the variables showed correlation between tail length and wing chord in SY and ASY birds. There was also a close relationship between tarsus length and wing chord for ASY birds.
There were variations in primary formulae between the sexes (Frey et al. 412). The study revealed differences in sexes between SY and ASY females. The linDFA correctly identified 67.4% of female SY birds and 96.5% of male SY birds using tarsus length and wing chord. The linDFA correctly identified 90.1% of male ASY birds and 69.4% of female SY birds. The findings varied based on the primary formulae employed (Frey et al. 413).
In general, C. minimus were larger than C. bicknelli. C. minimus from Alaska had longer wing chords than those from Canada (Frey et al. 414). The most promising variables for identifying the two species were tail length and wing chord. The linDFA method identified over 98.6% of Bicknell’s Thrushes and 87.5% Gray-cheeked Thrushes. The best discriminants between the two species were tail length, wing chord, and the p8-p1 primary formulae (Frey et al. 415).
The analysis shows that morphometric data from C. bicknelli and C. minimus, specifically primary formulae, can be used to identify the two species and determine the sexuality of the C. bicknelli. The results show that using discriminant functions, wing chord measurements and the p8-p1 formulae, the ability to tell between the two Cathurus species increased.
The ability to tell between the male and female C. Bicknelli also increased (Frey et al. 419). Despite the morphometric differences, there exists a difficulty in identifying and determining the sexuality of the two species due to variability in findings and morphometric overlap.
Frey, Sarah J. K., Christopher C. Rimmer, Kent P. McFarland and Stephane Menu. “Identification and Sex Determination of Bicknell’s Thrushes Using Morphometric Data”. Journal of Field Ornithology 79.4 (2008): 408-420. Vermont Institute of Natural Sciences.