Academic journal article The American Midland Naturalist

A Natural Experiment: Heterospecific Cross-Fostering of House Wrens (Troglodytes Aedon) by Tree Swallows (Tachycineta Bicolor)

Academic journal article The American Midland Naturalist

A Natural Experiment: Heterospecific Cross-Fostering of House Wrens (Troglodytes Aedon) by Tree Swallows (Tachycineta Bicolor)

Article excerpt

ABSTRACT.-

We report a natural cross-fostering experiment of two House Wren (Troglodytes aedon) eggs and nestlings by Tree Swallows (Tachycineta bicolor). We document an increase in duration of the House Wren incubation period by 1-2 d, which closely corresponds to the mean incubation period of their hosts. We also documented nestling growth rates of both the host Tree Swallows and the alloparented House Wrens. The host parents apparently fed the House Wren nestlings; however, the wrens exhibited slow growth compared with conspecific nestlings in neighboring nests. The wren nestlings eventually died at 6 and 13 d of age. We hypothesize that the lower incubation temperatures, and later, interspecific sibling competition that the House Wren nestlings were exposed to in the Tree Swallow nest may have prolonged the incubation period and slowed the growth rates of the House Wren nestlings.

INTRODUCTION

Embryonic and post-embryonic development is influenced by numerous intrinsic and extrinsic factors with selection favoring nest conditions and parental investment strategies that maximize fitness (Starck and Ricklefs, 1998; Deeming, 2002). Conditions and adult attendance strategies that eggs and nestlings are exposed to often vary between species, and may influence offspring developmental rates (Deeming, 2002). For instance, birds with lower attendance rates often have longer incubation periods than those with higher attendance rates (Boersma and Wheelwright, 1979; Martin et al, 2007). Though uncommon, studies of developmental patterns of heterospecific cross-fostered birds may be particularly helpful in elucidating differences in reproductive life history strategies among similar species, and the plasticity of developmental traits when reared under different conditions (Slagsvold, 2004). Here we report a natural heterospecific cross-fostering experiment, which resulted from the usurpation of a House Wren (Troglodytes aedon) nest, with two recently laid eggs, by a pair of Tree Swallows ( Tachycineta bicolor) . This situation provided an opportunity to study incubation period and nestling growth of House Wrens under the care of Tree Swallows.

METHODS

The study site consisted of open fields and second growth deciduous forests at Lux Arbor Reserve, Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan, USA. Boxes were placed to attract the study species and were arranged on poles without predator guards. We monitored active nests every 3 d until the nest was complete or laying had begun, at which point nests were checked daily until clutch completion (or the nest failed). Eggs were individually marked with a non-toxic felt tip marker, weighed (±0.1 g; Acculab PocketPro 60 g Electronic Balance; Salter Brecknell Electronic Pocket Balance) and measured (length and breadth; ±0.01 mm; Mituyo Digimatic electronic calipers). On the day of expected hatch, eggs were checked for signs of pipping. Nests were then checked daily until all viable eggs hatched or the nest failed. Incubation period (±1 d) for each nestling was classified as the duration of time (d) from clutch completion to hatch. As with the eggs, nestlings were individually marked using a non-toxic marker. We measured individual nestling growth by recording morphometric measurements of mass (±0.1 g); tarsus (±0.01 mm); bill (from the distal point of nares to the tip ±0.01 mm); and unflattened wing chord (±1 mm). In addition to the measurements, we noted the external development of the nestlings (Austin-Bythell, 2006) and digitally photographed the nestlings until age 14 d (hatch = d 1) when we ceased daily measurement.

OBSERVATIONS

On 15 Jun. 2004 two House Wren eggs were found while monitoring an active nest box that was known to be occupied by a pair of House Wrens. The observer noted that two Tree Swallows had entered and exited the box twice during the check. A subsequent nest check on 16 Jun. determined that no further house wren eggs had been laid. …

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