Nest feathering responses by Tree Swallows (Tachycineta bicolor) to experimental warming

Abstract

Among many functions, bird nests protect eggs, developing young, and incubating adults from inclement weather. In Tree Swallows (Tachycineta bicolor), prior to and while females are incubating, males compete with rivals for feathers that they use to line nests. The thermal benefits hypothesis proposes that males add feathers to improve heat retention of nests. We tested this hypothesis on Tree Swallows nesting near Wolfville, Nova Scotia, Canada in 2013, 2014, and 2015, and in St. Denis, Saskatchewan in 2013. In 2013, at both locations we experimentally heated nests. After young had fledged, we counted and measured feathers in each nest. The hypothesis was not supported. The only significant result was that St. Denis (52.2°N) had fewer feathers than the Annapolis Valley (45.1°N) site, which is contrary to expectation. Our findings challenge the prevailing hypothesis for why male Tree Swallows feather nests.

Zusammenfassung

Die Ausstattung des Nestes mit Federn als Reaktion auf experimentelle Erwärmung bei Sumpfschwalben (Tachycineta bicolor)

Neben vielen anderen Funktionen schützen Vogelnester Gelege und die sich entwickelnden Küken sowie die brütenden Altvögel vor schlechten Witterungsbedingungen. Vor und während der Bebrütung durch die Weibchen konkurrieren die Männchen der Sumpfschwalben (Tachycineta bicolor) mit Rivalen um Federn, die sie zur Auskleidung ihrer Nester benutzten. Nach der Thermal Benefits-Hypothese fügen Männchen den Nestern Federn hinzu, um das Wärmerückhaltevermögen der Nester zu verbessern. Wir testeten diese Hypothese an brütenden Sumpfschwalben nahe Wolfville, Nova Scotia, Kanada, in den Jahren 2013, 2014, 2015 und in St. Denis, Saskatchewan, Kanada, in 2013. In 2013 erwärmten wir in beiden Untersuchungsgebieten die Nester. Nach dem Ausfliegen der Jungen haben wir in jedem Nest die Federn gezählt und sie vermessen. Die Hypothese wurde nicht gestützt. Das einzige signifikante Ergebnis war, dass die Nester in St. Denis (52.2°N) weniger Federn hatten als in dem Gebiet im Annapolis Valley (45.1°N), was der Erwartung widersprach. Unsere Ergebnisse stellen die vorherrschende Hypothese in Frage, warum Sumpfschwalben Männchen ihre Nester mit Federn ausstatten.

Appendices

Appendix 1: The recruitment probability hypothesis

The recruitment probability hypothesis proposes that males add more feathers to nests they judge to be of higher value (metrics described in Hainstock et al. 2010). Broods of such nests may be associated with higher quality females that invest more in individual eggs, that lay larger clutches, or that have more breeding experience raising clutches (Stutchbury and Robertson 1988), and males may add extra feathers to signal to females their future paternal care, or for heat retention (but see above). Progeny produced by these females are assumed to have a greater potential to contribute to a male’s Darwinian fitness. Males may assess female quality based on her age class, which in Tree Swallows can be evaluated using plumage characteristics, clutch initiation date [date that a first egg is laid, which is earlier for older females (Stutchbury and Robertson 1988) and is strongly associated with recruitment (Shutler et al. 2006)], and clutch size [which is strongly associated with lifetime reproductive success (Shutler et al. 2006)]. Although in Lesser Black-backed Gulls (Larus fuscus), larger clutch size is associated with lower quality eggs (Nager et al. 2000), in three tit species (Blue, Great, and Coal tits, Parus ater), clutch size determination of females is based on their own ability to feed their young and has nothing to do with mate quality, with lower quality females producing smaller clutches (Slagsvold and Lifjeld 1990). We are unaware of a study that has assessed whether brood value influences feathering.

In Nova Scotia in 2014, trios of nests initiated on the same day were randomly assigned to reduction (two eggs removed), control (two eggs picked up and replaced), or addition (two eggs added) treatments. Eggs from reduce clutches were placed in addition clutches. Modal clutch size in this population was five. We predicted that if we experimentally decreased and increased clutch size that males would add fewer and more feathers, respectively, because of changes in brood value. Similarly, we predicted that males would add fewer feathers to nests of younger females.

We used a GLM to test whether number of feathers and feather lengths were related to female age, clutch initiation date, and clutch size. Only 22 nests fledged from the clutch manipulation experiment, so that statistical comparisons among treatments had little power. Therefore, we simply considered feather number and feather lengths simultaneously for both manipulated and non-manipulated nests from all 3 years. Female age, clutch initiation date, and clutch size were not associated with feathering (Table 2). Results were qualitatively similar when experimental clutch manipulations were excluded.

Table 2 Results of GLM tests of the female quality hypothesis

We therefore found no support for the recruitment probability hypothesis, although our experimental power was low, with only eight of 11 manipulated nests successfully fledging. It is also possible that males may assess recruitment probability through metrics other than those we tested. Female Tree Swallows select males that have brighter plumage for extrapair copulations (Bitton et al. 2007) and it is possible that males use plumage brightness to assess female quality and hence brood value. Although we found no correlation between female age and number or length of feathers added by males, we did not evaluate plumage brightness within age classes. The mating system of Tree Swallows may also weaken selection for males to impress partners with feathering. First, Tree Swallows have one of the highest documented rates of extra-pair paternity in birds (Dunn et al. 1994). Second, Tree Swallows over most of their range have only a single clutch per year, and in fewer than 18% of cases do the same adults pair in successive years (Shutler and Clark 2003; Shutler et al. 2006; Winkler et al. 2011). In any case, our tests should be replicated with larger samples.

Appendix 2

See Table 3.

Table 3 Success rates and treatments for Tree Swallow (Tachycineta bicolor) nests initiated in the Annapolis Valley, Nova Scotia, 2013–2015