Sex-biases in distribution and resource use at different spatial scales in a migratory shorebird

Abstract

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex-biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex-biased impacts, we use range-wide tracking of individually color-ringed Icelandic black-tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non-breeding range to within-site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large-scale sex differences in distribution during the non-breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small-scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within-site distribution and resource use means that local-scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.

Keywords: Foraging; migratory range; non-breeding; segregation; sex dimorphism; waders.

Figure 1

Proportions of wintering male (white) and female (gray) Icelandic black-tailed godwits in the seven main winter regions. Chart size is proportional to the number of marked individuals recorded in each site with numbers reported within each chart. Note that expected sex ratios are not equal (i.e., 1:1) and vary with the capture season and location of these individuals – see Methods for details. The inset shows the 18 sites at which godwit foraging behavior was recorded, and the six sites (3, 6, 8, 9, 13, and 18) at which prey were sampled, on the intertidal mudflats (light gray) of the Tagus estuary, West Portugal (dark gray indicates the area below mean low water).

Figure 2

Densities (mean ± SE ind./m²) of Scrobicularia plana (dark gray bars) and Hediste diversicolor (light gray bars) at six godwit foraging locations on the Tagus estuary during each month of the winter 2006–2007. Site location is given by number (see Fig. 1 for location and Table 1 for flock diet composition of each site). Panels are displayed from top to bottom according to Hediste diversicolor density.

Figure 3

The mean (± SE) proportion of three different prey species in the diet of female (gray bars) and male (white bars) black-tailed godwits in flocks foraging primarily on (A) Scrobicularia plana or (B) Hediste diversicolor.

Figure 4

Proportion of re-sightings of each individually color-ringed (A) female (n = 29) and (B) male (n = 50) godwit on sites dominated by Hediste diversicolor during the winter of 2007–2008 (sites 3 and 6 on Fig. 2).

Figure 5

Size distribution of different size classes (white – small and medium, gray – large, and black – very large) of (A) Hediste diversicolor or (B) Scrobicularia plana consumed by individually color-ringed black-tailed godwits with different bill lengths. Numbers of individual godwits in each category are shown within the bars and bill length categories also indicate sex: males <89 mm and females >90 mm. Different letters indicate significant differences (P < 0.05) from Fisher exact tests of independence between each bill length category.

Figure 6

Differences in mean (± SE) handling time (gray bars) and extraction time (white bars) for black-tailed godwits foraging on four prey size classes of (A) Scrobicularia plana and (B) Hediste diversicolor; and (C) the estimated profitability (or handling efficiency, kJ/s¹) of each size class of Scrobicularia plana (dark gray bars) and Hediste diversicolor (light gray bars). Numbers above bars indicate sample sizes and different letters indicate significant differences (P < 0.05) from post-hoc tests.