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Hi Fritz and Gayle,
I didn't see an answer to your question so hopefully I'm not duplicating a response already sent.
The answer to your first question is that they begin moving as a result of photoperiod and hormonal changes in their bodies. However, the speed with which they move north is regulated by a number of factors. The availability of suitable weather conditions and the amount of stored fat are important and, for at least some species, there is a hypothesis that their advance north is governed by temperature ( represented on maps by isotherms = a line linking points of equal temperature). I imagine different species would follow different isotherms/temperatures north. The theory is that by advancing behind the appropriate isotherm, birds will find food available for them which is also timed to the appropriate temperature.
Regarding your second question, one of the concerns hypothesized about global warming is that spring warming will occur too quickly leading to several possible problems such as birds arriving too early on the breeding grounds and not finding food or finding their food source does not peak at the same time as they need it to feed their young. This is a very complicated system and it will only be with time that we'll be able to say that this is the case or not.
If you read the accounts of Purple Martin in Bent's Life History, written early in the last century, you'll see he describes severe die-offs at the northern edges of their ranges (i.e. Canada) when an extended cold, wet period is encountered late in the spring. If it becomes very common that extended cold, wet periods follow late winter/early spring warm periods a number of species may be adversely affected, particularly aerial insectivores.
If a nest fails, many species will re-nest, especially if the nest has failed early in the process. Others will not. Some species will have multiple broods in a season, but a number raise only one. There are some species which produce multiple broods in the southern parts of their breeding range but not the northern parts so I suppose these species might start to produce multiple broods further north if all other factors remain the same.
One other point is that with a mild winter such as we've just had, more species and individuals are able to over winter thereby avoiding the perilous migration journey. The advantage to overwintering or migrating early is that the individuals get first choice of the best breeding areas. The disadvantage, as I mentioned above, is that you risk harsher climatic conditions and a scarcity of food so they may die before getting to the breeding season.
The pros seem to be outweighing the cons at the moment as we have seen and are seeing a number of species reaching Nova Scotia, overwintering and then breeding in subsequent years. There are also species which are becoming much less common and for some of these it may represent a negative effect of the changing weather.
All the best,
Lance
From: naturens-owner@chebucto.ns.ca [mailto:naturens-owner@chebucto.ns.ca] On Behalf Of Fritz McEvoy
Sent: Sunday, April 22, 2012 11:26 AM
To: NS NATURE
Subject: EXTERNAL: [NatureNS] multiple breeding ?
Hi All,
All these reports of early avian arrivals in N.S. prompt me to ask a couple of questions.
First: How do the migrants know to leave their winter bases early for their breeding grounds; or do they leave at the same time as usual and just arrive early because they don't encounter any bad weather along the way?
Second: Will this great weather encourage birds to breed earlier than usual and if so are they likely to breed for a second or third time this year; even if they normally breed only once before leaving for their winter homes? Or will they just leave for their winter homes early?
All the best.
Fritz McEvoy
Sunrise Valley, CB
--Boundary_(ID_D8OgycMZDoESSaPY5ez8kQ)
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</o:shapelayout></xml><![endif]--></head><body lang=3DEN-US link=3Dbl=
ue vlink=3Dpurple><div class=3DWordSection1><p class=3DMsoNormal><spa=
n style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color:=
#1F497D'>Hi Fritz and Gayle,<o:p></o:p></span></p><p class=3DMsoNorma=
l><span style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";=
color:#1F497D'><o:p> </o:p></span></p><p class=3DMsoNormal><span=
style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color:#=
1F497D'>I didn’t see an answer to your question so hopefully I&=
#8217;m not duplicating a response already sent.<o:p></o:p></span></p=
><p class=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Ca=
libri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p cla=
ss=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Calibri",=
"sans-serif";color:#1F497D'>The answer to your first question is that=
they begin moving as a result of photoperiod and hormonal changes in=
their bodies. However, the speed with which they move north is regul=
ated by a number
of factors. The availability of suitable weather conditions and the =
amount of stored fat are important and, for at least some species, th=
ere is a hypothesis that their advance north is governed by temperatu=
re ( represented on maps by isotherms =3D a line linking points of eq=
ual temperature). I imagine different species would follow different =
isotherms/temperatures north. The theory is that by advancing behind =
the appropriate isotherm, birds will find food available for them whi=
ch is also timed to the appropriate temperature.<o:p></o:p></span></p=
><p class=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Ca=
libri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p cla=
ss=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Calibri",=
"sans-serif";color:#1F497D'>Regarding your second question, one of th=
e concerns hypothesized about global warming is that spring warming w=
ill occur too quickly leading to several possible problems such as bi=
rds arriving too early on the breeding grounds and not finding food
or finding their food source does not peak at the same time as they =
need it to feed their young. This is a very complicated system and it=
will only be with time that we’ll be able to say that this is =
the case or not. <o:p></o:p></span></p><p class=3DMsoNormal><span sty=
le=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F49=
7D'><o:p> </o:p></span></p><p class=3DMsoNormal><span style=3D'f=
ont-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If =
you read the accounts of Purple Martin in Bent’s Life History, =
written early in the last century, you’ll see he describes seve=
re die-offs at the northern edges of their ranges (i.e. Canada) when =
an extended cold, wet period is encountered late in the spring. If it=
becomes very common that extended cold, wet periods follow late wint=
er/early spring warm periods a number of species may be adversely aff=
ected, particularly aerial insectivores.<o:p></o:p></span></p><p clas=
s=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Calibri","
sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=3DMso=
Normal><span style=3D'font-size:11.0pt;font-family:"Calibri","sans-se=
rif";color:#1F497D'>If a nest fails, many species will re-nest, espec=
ially if the nest has failed early in the process. Others will not. S=
ome species will have multiple broods in a season, but a number raise=
only one. There are some species which produce multiple broods in th=
e southern parts of their breeding range but not the northern parts s=
o I suppose these species might start to produce multiple broods furt=
her north if all other factors remain the same.<o:p></o:p></span></p>=
<p class=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Cal=
ibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p clas=
s=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Calibri","=
sans-serif";color:#1F497D'>One other point is that with a mild winter=
such as we’ve just had, more species and individuals are able =
to over winter thereby avoiding the perilous migration journey. The a
dvantage to overwintering or migrating early is that the individuals =
get first choice of the best breeding areas. The disadvantage, as I m=
entioned above, is that you risk harsher climatic conditions and a sc=
arcity of food so they may die before getting to the breeding season.=
<o:p></o:p></span></p><p class=3DMsoNormal><span style=3D'font=
-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p>&=
nbsp;</o:p></span></p><p class=3DMsoNormal><span style=3D'font-size:1=
1.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The pros seem=
to be outweighing the cons at the moment as we have seen and are see=
ing a number of species reaching Nova Scotia, overwintering and then =
breeding in subsequent years. There are also species which are becomi=
ng much less common and for some of these it may represent a negative=
effect of the changing weather.<o:p></o:p></span></p><p class=3DMsoN=
ormal><span style=3D'font-size:11.0pt;font-family:"Calibri","sans-ser=
if";color:#1F497D'><o:p> </o:p></span></p><p class=3DMsoNormal><=
sp
an style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color=
:#1F497D'>All the best,<o:p></o:p></span></p><p class=3DMsoNormal><sp=
an style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color=
:#1F497D'><o:p> </o:p></span></p><p class=3DMsoNormal><span styl=
e=3D'font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497=
D'>Lance<o:p></o:p></span></p><p class=3DMsoNormal><span style=3D'fon=
t-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p>=
</o:p></span></p><p class=3DMsoNormal><span style=3D'font-size:=
11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> <=
/o:p></span></p><p class=3DMsoNormal><span style=3D'font-size:11.0pt;=
font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></=
span></p><div><div style=3D'border:none;border-top:solid #B5C4DF 1.0p=
t;padding:3.0pt 0in 0in 0in'><p class=3DMsoNormal><b><span style=3D'f=
ont-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><s=
pan style=3D'font-size:10.0pt;font-family:"Tahoma","sans-serif"'> nat=
urens-owner@chebuc
to.ns.ca [mailto:naturens-owner@chebucto.ns.ca] <b>On Behalf Of </b>F=
ritz McEvoy<br><b>Sent:</b> Sunday, April 22, 2012 11:26 AM<br><b>To:=
</b> NS NATURE<br><b>Subject:</b> EXTERNAL: [NatureNS] multiple breed=
ing ?<o:p></o:p></span></p></div></div><p class=3DMsoNormal><o:p>&nbs=
p;</o:p></p><div><p class=3DMsoNormal><span style=3D'font-size:10.0pt=
;font-family:"Tahoma","sans-serif"'>Hi All,<br> All these=
reports of early avian arrivals in N.S. prompt me to ask a couple of=
questions. <br> First: How do the migrants know&nbs=
p;to leave their winter bases early for their breeding grounds; or do=
they leave at the same time as usual and just arrive early because t=
hey don't encounter any bad weather along the way?<br> Se=
cond: Will this great weather encourage birds to breed earlier than u=
sual and if so are they likely to breed for a second or thi=
rd time this year; even if they normally breed only once before =
leaving for their winter homes? Or will they just leave fo
r their winter homes early?<br> All the b=
est.<br> &=
nbsp; &nbs=
p; Fritz McEvoy<br> &n=
bsp;  =
; Sunrise Valley, CB <o:p></o:p></s=
pan></p></div></div></body></html>
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