Discussion
Our behavioural test with young male blackcaps captured on migration
showed that birds preferred anthocyanin- or fat-enriched food
alternatives depending on their physiological condition. The results
supported a scenario in which feeding decisions helped birds to cope
with the oxidative challenges they face as they recover from and prepare
to the intense aerobic exercise during endurance flight. In this
scenario, haemosporidian parasites had an impact on host physiology by
exacerbating oxidative challenges, which influenced on individual food
preferences supporting the idea that increasing the intake of dietary
antioxidants may be a self-medication mechanism to cope with elevated
risk of oxidative stress due to infection (Mancio-Silva et al. 2017;
Zuzarte-Luís and Mota 2018). This interpretation was supported by our
observation of infected birds having lower total antioxidant capacity,
and individuals with higher levels of oxidative damage to lipids having
more preference for anthocyanin-enriched food.
Co-infected blackcaps preferred anthocyanin-enriched food. Consumption
of antioxidants such as anthocyanins could be a means of self-medicating
if it reduced oxidative damage caused by the parasite or by activation
of the immune system during the infection (Beaulieu and Schaefer 2013;
de Roode et al. 2013; Muriel 2020). This effect of parasitism was only
significant in multiple infections, which could increase parasite
virulence or the cost of the host’s immune response (Pigeault et al.
2018; Garcia-Longoria et al. 2022). In this context, the consumption of
antioxidants could be detrimental to the energy intake essential for
migration performance (Beaulieu and Schaefer 2014), or it could
compromise the active search of alternative essential nutrients, such as
polyunsaturated fatty acids that may enhance physiological performance
during migration (Weber 2009).
We found a correlation between infection and the oxidative balance,
since infected birds showed lower values of plasma antioxidant capacity.
Antioxidant depletion associated with immune activation (Costantini and
Møller 2009) or production of free radicals as a result of parasite
exploitation (Percário et al. 2012) may induce an increase in ROS in
infected birds (Delhaye et al. 2016). Similar results of infections have
been found in other stressful situations such as during reproduction
(Badás et al. 2015), or in poor habitat (Messina et al. 2022). However,
we did not detect any correlation between infection status and other
biomarkers of oxidative status. These correlations could have been
concealed by a hormetic response, which is supported by a positive
correlation between tGSH and RBC-MDA. Faced with predictable oxidative
stress, migrating birds could up-regulate antioxidant defences before
suffering too much oxidative damage (Costantini 2010; McWilliams et al.
2021), thus buffering the relationships between oxidative parameters and
the sources of stress.
We did not find any relationship between parasitemia and biomarkers of
oxidative stress, although other studies have found it in other species
such as great tits Parus major (Isaksson et al. 2013; Delhaye et
al. 2016). However, blackcaps showed very low parasitemia in our study.
This result indicates that blackcaps during autumn migration have mostly
chronic infections with few parasites in the bloodstream, yet such
low-intensity infections may have an impact on oxidative status during
migration, which could contribute to the mortality cost of
haemosporidian infections observed in other migratory bird species
(Asghar et al. 2015).
We found a positive correlation between relative body mass and
preference for fat-enriched food, which we attribute to the fact that
body mass of migrating blackcaps captured on stopover is most dependent
on the time they have spent at the stopover site, as birds gain body
mass from arrival to departure (Langslow 1976; Arizaga et al. 2010).
Thus, leaner birds may prefer anthocyanin-enriched food if dietary
antioxidants help them cope with oxidative damage associated to the
aerobic effort of the preceding flight stage (Costantini 2008; Eikenaar
et al. 2020). Conversely, fatter birds closer to departure may focus on
putting on fats to fuel the next flight stage, thereby preferring
fat-enriched food. Preference for olive oil could be further increased
close to departure if birds benefit from acquiring so-called ”natural
doping” polyunsaturated fatty acids present in olive oil as these may
increase metabolic efficiency during endurance flight (Weber 2009). Our
interpretation of the relationship between body mass and food
preferences observed in blackcaps is supported by the fact that
blackcaps lose weight on arrival at stopover sites (Langslow 1976),
which may be indicative of a period of tissue repair during which birds
do not put on any fat. Furthermore, birds with less oxidative damage in
plasma (as measured by MDA) selected fats rather than anthocyanins,
supporting the idea that individuals with a good oxidative status could
focus on rebuilding fat stores or prioritise the intake of alternative
essential nutrients that may give them a physiological advantage during
endurance flight (Weber 2009; Costantini 2010; Beaulieu and Schaefer
2014).
Understanding the intricate relationships between body condition,
oxidative status and dietary preferences during migration has long been
at the centre of research on the physiological performance of migrating
birds (Weber 2009). Our results support the idea that haemosporidian
infections influence on these relationships by exacerbating the
oxidative challenges faced by birds during this critical life cycle
stage, thereby providing a physiological mechanism to explain the
negative impact of chronic avian malaria infections on survival observed
in migrating birds (Asghar et al. 2015), and emphasising the importance
of accounting for parasite infections in studies of animal performance
(Chrétien et al. 2023).
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