Behavioural, mensural, and ecological correlates of polygyny in the eastern meadowlark (Sturnella magna)
Abstract
During 1982 and 1983 I studied male attributes and attributes
of the territory of male Eastern Meadowlarks (Sturnella magna) in
order to determine whether there was a correlation between any of
the attributes investigated and the number of females attracted
by a male. Seventeen males, nine of which were polygynous and
eight monogamous, were studied in 1982 and sixteen males.of which .. seven were polygynous and nine ~onogamous, were studied in 1983.
The study was conducted in Short Hills Park, 10 km southwest of
St. Catharines, Ontario and was designed to compare two
hypotheses: the "sexy son" hypothesis (Weatherhead and
Robertson,1977) and the polygyny threshold model (Verner and
Willson,1966, Orians, 1969). Male attributes investigated were
male size and song behaviour. Six measures of male size were
taken: weight, flattened and natural wing chord length, culmen
length, bill depth and length of the tarsometatarsus. In 1983
song repertoire size and song versatility measures were
investigated. Attributes of the territory studied were: territory
size, density of plant stems, percentage plant cover and measures
of vegetation structure. In 1983 Arthropods were collected from
each territory and sorted according to taxonomic group and size.
During 1983, territory attributes were sampled twice, once early
and once later in the nesting season. Analysis of data involved
univariate comparisons between monogamous and polygynous males
using T-tests and multivariate comparisons were made using
discriminant function analysis (DFA) and principle components analysis (PCA).No correlations were found between the number of females
attracted with, .ny measure of male size or with me, .sures of song
versatili or size of song repertoire. Also no correlation was
found between terri size and the number of females nesting on a terri . Some attributes of the male's terri id
distinguish between monog,mous and po s males of thistudy.
Analysis of Arthropod numbers showed that e~ .eran
counts were significantly great~r on polygynous territories,
a1 the total numb~rs of Arthropods collected showed no
s fico .nt differences between territories of monogamous and
po males. DFA chose ear teran and Hymenopteran
counts as multivariate discriminators; both variables we' e more
vegetation revealed that there were no univariate differences
between the two groups of males fOT 1982 stem densities, but
~ spp. and Solidago spp. were chosen DFA as multivariate
discriminators. The total number of plant stems and of Vicia spp.
stems were s ficantly
the early 1983 ing
on monogamous territories for
however DFA found no multivariate
discriminators" Variables concerned with the overall aspects of
vegetation structure showed significant differences between
territories of monogamous and polygynous males. DFA of the 1982
sampling of vegetation structure showed significantly greater mat
depth and vegetation height on polygynous territories, a finding
which was not supported, however, by peA. For the early 1983
sampling period, plant height was greater on polygynous territories. Multivariate analysis identified greater green
cover on polygynous territories, greater ground cover on
monogamous territories, and greater depth of mat material on
monogamous territories as discriminators between territories of
monogamous and polygynous males. A DFA on the major variables of
the study showed no significant difference between the
territories of monogamous and polygynous male Meadowlarks.
Of the correlations found, some were for non-prey Arthr~ods,
for cover plants with very small samples sizes, or for variables
which were greater for monogamous males during one sampling
period and polygynous males during the next. While multivariate
discriminators were found, peA showed no grouping of monogamous
or polygynous males according to any of the variables
investigated.
On the basis of the univariate and multivariate analysis of
major variables, I concluded that there were no correlations
between the number of females attracted with male attributes and
no unambiguous correlation with attributes of the territory. My
study does not unequivocally support either the "sexy son" or the
polygyny threshold hypothesis.