Hatching Asynchrony in Purple Martins: Testing an Assumption

1
Hatching Asynchrony in Purple Martins Testing an
Assumption 
Nichole Cudworth, Austin Mercadante, Mark
Stanback Department of Biology, Davidson
College, Davidson, NC
Abstract
To investigate the importance of relative size
on chick condition, we manipulated brood
composition in nests of purple martins (Progne
subis). Using data from 2000-2005, we first
compared the condition (residuals of a mass-wing
regression) of alpha (longest-winged) and omega
(shortest-winged) chicks in our experimental
broods (see below). As expected, omega chicks
were in poorer condition than were alpha chicks
in the same brood, demonstrating that size rank
does impact competition and condition. To
determine whether the relative size of other
chicks in the brood influences the condition of
alphas and omegas, we compared alpha condition in
broods containing 4 large and 1 small chick to
those in broods containing 4 small and 1 large
chick. We then did similar comparisons of omega
chicks in the same experimental broods. We found
no difference in the condition of alphas or
omegas in the two treatments. It appears that
the size of the other chicks has no significant
effect on the condition of alpha and even omega
chicks. This suggests that the common but
untested practice of measuring hatching
asynchrony using only alpha-omega differences is
justified, at least in purple martins.
We thank the Purple Martin Conservation
Association and the NSF REU program for funding,
the Sholar family for access to martins, and many
past students for assistance with data
collection. Photo by James R. Hill III
(www.purplemartin.org)
The condition of alpha chicks was similar in the
single- and multi-runt treatments.
The multi-runt treatment consisted of a single
large chick and 4 small chicks. The size
disparity between the largest and smallest chick
was similar in both treatments. We measured
chicks 2 weeks after manipulation and calculated
a condition index using the residuals from a 3rd
order polynomial regression of mass on wing
length. We used a paired t-test to compare the
condition of alpha chicks in single- and
multi-runt broods in the same year with similar
nest dates (pooling broods if numbers were
uneven). If competition for food (the functional
consequence of hatching asynchrony) is mediated
by the size disparity between largest and
smallest chicks (without regard to the other
chicks), treatment should have no significant
effect on the condition of either alpha or omega
chicks.
Introduction
Unlike mammals, birds can influence birthing
(hatching) intervals within litters (broods).
Because bird eggs do not begin to develop until
externally warmed, avian parents can determine
the pattern of hatching by when in the course of
egg-laying they initiate incubation. Because
chicks begin feeding as soon as they hatch,
asynchronous hatching leads directly to a
competitive size hierarchy within the brood - one
that can lead to the death of the later-hatched
young if food availability declines (the Brood
Reduction Hypothesis). To test this and other
hypotheses about asynchronous hatching,
experimenters have defined asynchrony temporally
(via time intervals between first- and
last-hatched chicks) rather than functionally
(via measuring competition). Moreover,
experimenters tend to ignore the hatching
time/size of other chicks within the brood. A
brood with one large and four small chicks is
considered equivalent to a brood with four large
and one small chick -- as long as the size
difference between the largest and smallest is
identical. In this study we explicitly test this
assumption by measuring the condition of largest
and smallest chicks in purple martin (Progne
subis) broods with the above composition. By
testing this assumption, our research will have
important implications for future experimental
work in this area.

Results
The condition of omega chicks was similar in the
single- and multi-runt treatments.
Conclusions

• Omega chicks were in significantly poorer
  condition than were alpha chicks in the same
  brood.
• The condition of alpha chicks was similar in
  single- and multi-runt broods.
• The condition of omega chicks was similar in
  single- and multi-runt broods.
• The relative sizes of purple martin chicks other
  than the alpha and omega do not significantly
  influence the competitive outcome. This
  justifies the common practice of measuring
  hatching asynchrony with respect to these two
  chicks.

Methods
We manipulated broods at a large purple martin
colony near Davidson, NC from 2000-2005. By
moving young chicks between nests, we created
broods of 5 chicks with two distinct competitive
hierarchies. In single runt nests, we placed 4
large chicks and a single small chick
(approximately one-half the mass of a larger
chick).
Omega chicks in experimental broods were in
significantly poorer condition than were alpha
chicks in the same brood.