Baubock et al. • CLIMATE CHANGE DOES NOT AFFECT PROTANDRY 
215 
TABLE 3. Relationship between sex ratios and climate variables for each species as measured by reverse stepwise 
regression. Climate variables tested were mean temperature in the year of migration (Mean T). mean temperature in the 
year prior to migration (Lagged mean T). NAO index, and ENSO index. Slopes are for femalcstmales per C. Only changes 
in the sex ratios of Mourning Warblers were significantly related to mean temperature in the year of migration (reverse 
stepwise regression, P = 0.002). after correcting for the number of tests {« - 7). The significant P values reported for 
Black-and-white Warblers and Baltimore Orioles disappear after correcting for multiple tests. 
Clini car. 
Black-and-white 
Warbler 
Mourning. Warbler 
Common 
Yellowihmai 
Lustcin Towhec 
American 
Goldfinch Ballimore Oriole American Redstart 
Mean T 
Mean T 
None 
None 
None Lagged mean T None 
Slope 
0.22 
0.77 
0.17 
R- 
0.14 
0.29 
0.21 
P 
0.035 
0.002 
0.010 
reflects male quality and females are able to 
choose the best males based on secondary sexual 
characteristics. The ‘rank advantage’ hypothesis 
(Morbey and Ydenberg 2001. Coppack and Pulido 
2009. Saino et al. 2010) suggests earlier arriving 
males may be able to secure better territories, 
which makes them more attractive to females. 
However, we did not find that extent of protandry 
was related to strength ot territoriality. Males of 
one of the two non-territorial species we studied, 
American Goldfinches, still arrived significantly 
earlier than females. Thus, it seems unlikely that 
territoriality and mate choice are the exclusive 
reasons for protandry. 
A last hypothesis is that earlier arriving males 
gain an advantage because they can breed earlier 
or more frequently; the 'mate opportunity’ 
hypothesis (Morbey and Ydenberg 2001, Coppack 
and Pulido 2009. Saino et al. 2010). However, this 
hypothesis docs not explain why males should 
arrive earlier than females. Females must also be 
arriving earlier for earlier arriving males to mate 
more frequently. /Ml the birds studied are serially 
monogamous (Poole 2005) and it is unlikely that 
earlier arriving males gain an advantage by being 
able to have more mates. 
It is not clear why our results contrast with those 
of Moller (2004) who found that protandry 
increased for Barn Swallows in Europe as a result 
of climate change. We found no increase in 
protandry in any of the species we examined. It is 
possible males may gain an advantage from arriving 
early relative to females, but females may also gain 
an advantage if they arrive early with respect to 
other females. Early arriving females may be able to 
choose among more potential mates, or perhaps take 
advantage of better environmental conditions, such 
as better nesting sites, or benefit from maintaining 
synchrony with their preferred food sources. The net 
effect would he that both males and females advance 
their arrival times as much as possible or necessary, 
leaving the original difference in migration times 
intact. Our Finding suggests that protandry may 
remain fairly consistent as the climate changes. 
Female/male ratios did not change over time for 
any species and only the fcmale/male ratios of 
Mourning Warblers were significantly related to a 
climate variable, mean temperature. The sex ratios 
for three species (Black-and-white Warbler, 
Mourning Warbler, and Baltimore Oriole; Ta¬ 
ble 3) that reflected a relationship to a climate 
variable, were associated with the temperature 
metrics of ihe current year or to the temperature 
metrics of the previous year. The relationship w'as 
positive, indicating more females in years with 
higher temperature, or in the year following warm 
years (Table 3). This is the opposite of the 
prediction of the ‘differential susceptibility’ 
hypothesis of protandry (Morbey and Ydenberg 
2001, Saino et al. 2010), in which one would 
expect males to have greater mortality in cold 
years, as they arrive earlier and are more exposed 
to poor conditions such as low temperatures and 
lack of food. Another possibility is that males and 
females migrate along different routes depending 
on the temperature. 
We found the extent of protandry is not 
changing in response to climate change, contrast¬ 
ing with previous findings in Europe (Mpller 
2004). We suggest researchers continue exploring 
the occurrence and magnitude of protandry, and 
the response of protandry to climate change to 
examine it our results are general or whether the 
eflects of climate change on protandry vary 
substantially across species and locations. 
Additional studies of birds on breeding areas 
could provide more information about the cause 
of protandry and effects of climate change. 
