American Oystercatcher Working Group


Author: Kim Peters (KP), South Carolina Cooperative Fish and Wildlife Research Unit, Clemson University, Clemson, SC and New Jersey Audubon Society, Cape May Point, NJ.


As with other species of oystercatchers (Hockey 1981, Hockey 1996b, Zwarts et al. 1996b), American Oystercatchers are sexually dimorphic, with adult females generally bigger and heavier than adult males, although with considerable range overlap (Appendix 4, Carson-Bremer 2010).  Similar gender differences have been observed in immature (2Y-3Y) birds in South Carolina and Georgia (Appendix 8, Carlson-Bremer et al. 2010).  In SC and GA, immature males weighed an average of 40g less than adults, and immature females weighed 50g less than adults, although with substantial range overlap (Appendix 8, Carlson-Bremer et al. 2010).  In H. ostralagus, yearlings also weighed on average 30g less than adults; however if standardized for body size, immature H. ostralagus weighed more than adults (Zwarts et al 1996a).  In general, female and immature H. ostralagus tend to store more fat in winter than males (Zwarts et al. 1996b).  Breeding birds in South Carolina lighter than those caught further north, and New Jersey breeders generally heavier than those from other Atlantic coast breeding sites (Appendix 4).

Birds caught during fall and winter in South Carolina heavier than local breeding birds (Appendix 4).  This pattern similarly observed in H. ostralagus, where average mass varied between mid-summer and mid-winter by nearly 100 g (Zwarts et al. 1996b), with overlap (450-650g summer, 500-725g winter).  Changes in H. ostralagus weight have been attributed primarily to fat accumulation and approximately 10 g of lean dry mass (Zwarts et al. 1996b).  H. leucopodus mass has also been shown to vary by at least 18% from January (post breeding season) to August (pre breeding season) (H. Sitters, personal comm.). In H. ostralagus, temporal weight changes were so pronounced that sexual dimorphism sometimes reversed (van de Pol et al. 2008).


Adult female wing, culmen, and tarsus lengths greater than male, but with significant overlap (Appendices 5-7), again reflecting patterns exhibited in other species (Hockey 1996b).  Similar relationships observed in immature (2Y-3Y) birds in South Carolina and Georgia, with the exception that female tarsus length did not differ significantly from males (Appendix 8, Carlson-Bremer et al. 2010).  In H. ostralegus, adult female bills longer than adult males by approximately 10 mm, and bill length was deemed a more reliable indicator of gender than mass (van de Pol et al. 2008).  In Virginia, South Carolina and Georgia, there was considerable overlap in H. palliatus weight, bill length, and wing length between males and females, so that these parameters represent unreliable indicators of sex (Appendices 5-7, Carlson-Bremer et al. 2010).  Birds captured in Massachusetts tended to have shorter bills than those at other sites along the Atlantic Coast (Appendix 6).  Tarsus and wing estimates also vary by banding site, likely due to differences in methodologies (KP, Appendix 5, Appendix 7).  South American individuals (H. p. durnfordi) have shorter bills (male, 70.9 mm (± 3.96 SD, n = 9); female, 78.8 (± 5.23, n = 6) than their North American counterparts, possibly due to increased abrasion from feeding on rocky shores (EN), and also shorter legs (Hayman et al. 1986).  In H. ostralegus, wing length decreased by 0.34/month on average from December -September due to feather wear (Zwarts et al 1996b) but this pattern was not observed in South Carolina birds (Appendix 5).

Bill tip measurements (i.e., 3 mm from tip) were obtained for South Carolina (Appendix 10).  Bill tip shape has been identified as an indicator of feeding tactic and wear, and in H. ostralagus “bill shape types” (i.e., pointed, chisel, blunt) vary by study, region and time (van de Pol et al. 2008, 2009) and are confounded with bill length (Zwarts et al 1996a). Terminology used to describe different types also varies by region and may account for some of the variation (van de Pol et al 2009).  In South Carolina, H. palliatus bill tip height tended to be at the high end of the range reported for H. ostralagus (Appendix 10, Zwarts et al. 1996a, Peters 2005, van de Pol et al. 2009), and bill tip width at the low end (Appendix 10, Peters 2005, van de Pol et al. 2009) and were primarily classified as ‘chisel’ shaped (KP).  This potentially reflects foraging strategy at the South Carolina overwintering site, at which oysters (Crassostrea virginica) and other bivalves constituted the primary food sources (Peters 2005).  H. ostralagus bill shape may also include intermediates (pointed-chisel, pointed-blunt, chisel-blunt), potentially attributable to mixing feeding techniques or technique switching (Swennen et al. 1983, van de Pol et al. 2009).  The proportion of blunt bills represented also varies seasonally in H. ostralegus, with numbers of blunt bills decreasing from March through August (van de Pol et al 2008).  In H. leucopodus, blunt bills outnumber unabraded bills by about 2:1 and are about 5 mm shorter. (H. Sitters personal comm).  H. ostralegus females more likely to have blunt bills, but this difference decreased between 1985 and 2005 (van de Pol et al. 2008).  H. ostralegus males also eat more shellfish than females, and males have higher bill tip (approx 7%) than females characterized with the same bill shape (van de Pol et al. 2008, 2009).  Differences were not noted between male and female bill shape in H. palliatus in Virginia, although these measurements characterized overall bill shape rather than bill tip (Appendix 9). Male and female confidence intervals for bill-tip height and width overlapped in South Carolina birds (Appendix 10).