The delayed effect of climatic conditions on pre-fledging nestling haematocrit in a suboscine species in Patagonia

Haematocrit, the proportion of blood comprising erythrocytes, is often
used as a proxy of individual condition. Nestling haematocrit is
influenced by several factors, but ambient temperature is generally agreed
as a key driver. It is unclear which day(s) in embryonic or nestling
development are most influential in determining pre-fledgling haematocrit.
This is important, because if we are able to identify what day(s)
nestlings are physiologically most vulnerable to climatic conditions, this
may inform future conservation management and help mitigate the effects.
We investigated the effect of ambient temperature, precipitation, body
size, brood size, age, food abundance and habitat on nestling haematocrit
in Aphrastura spinicauda (thorn-tailed Rayaditos). We collected this data
from two climatically different locations in Chile: Northern Patagonia
(Pucón) and Sub-Antarctic Patagonia (Navarino Island), located ~1800 km
further south. We aimed to identify the key drivers of nestling
haematocrit and find when nestlings are physiologically most vulnerable to
climatic conditions. We confirm that ambient temperature is the key driver
of nestling haematocrit, with increasing ambient temperatures nestling
haematocrit decreased. However, precipitation also affected haematocrit in
the late incubation stage. At low and high precipitation, nestling
haematocrit was low and is optimal in light rainfall. Our results show a
delayed effect of ambient temperature and precipitation in late incubation
and on the day of hatching which can determine pre-fledgling haematocrit,
which may cause lower post-fledgling fitness. We found that climatic
conditions on the day haematocrit was collected were not important in
determining haematocrit. We found that haematocrit was higher in the
sub-Antarctic area and that higher temperatures and precipitation on the
day of hatching will result in lower nestling haematocrit and poorer
pre-fledgling condition. As atmospheric temperatures rise, nestling
fitness is at risk. This is a threat to many bird species, especially
those in locations vulnerable to climate breakdown.