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A natural experiment: heterospecific cross-fostering of House Wrens (Troglodytes aedon) by tree swallows (Tachycineta bicolor).


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 bicolar). This situation provided an opportunity to study incubation period and nestling growth of House Wrens under the care of Tree Swallows.


The study site consisted of open fields and second growth deciduous forests at Lux Arbor Reserve, Kellogg Biological Station, Michigan State University, Hickory Comers, 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 ([+ or -]0.1 g; Acculab PocketPro 60 g Electronic Balance; Salter Brecknell Electronic Pocket Balance) and measured (length and breadth; [+ or -]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 ([+ or -]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 ([+ or -]0.1 g); tarsus ([+ or -]0.01 mm); bill (from the distal point of nares to the tip [+ or -]0.01 mm); and unflattened wing chord ([+ or -] 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.


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. When the box was opened that day an adult Tree Swallow was flushed from the nest, and the pair circled within 10 m throughout the visit. As the House Wrens laid no further eggs and the Tree Swallow pair appeared to be attending the box, we concluded that the House Wren nest had been usurped and daily monitoring ceased. On the next scheduled visit, 20 Jun. 2004, two Tree Swallow eggs were found in the nest with the House Wren eggs. As with the House Wren eggs, these were individually marked and measured. The female Tree Swallow (ASY) completed her clutch of four eggs on 22 Jun. Incubation is presumed to have started on the day of clutch completion when we first noted the eggs to be warm. Onset of incubation in Tree Swallows varies among individuals with most starting incubation on the penultimate or ultimate egg (Zach, 1982; Robertson et al., 1992; Ardia et al., 2006). The swallow eggs began pipping on 6 Jul. No pipping was observed for the House Wren eggs on 6 Jul.; however, two House Wren nestlings were observed at 0810 on 7 Jul. Swallow nestlings hatched by 1330 on 7 Jul. and 0928 on 8 Jul.


Growth rates were calculated using the logistic growth equation [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in nonlinear regression (PROC NLIN, SAS v9.1), where A is a value fitted to the asymptotic or adult mass, k is the growth constant, t is age (d) and i is the inflection point (A/2) (Ricklefs, 1967). Since tarsus growth is more linear than mass growth, the asymptotic values for both mass and tarsus lengths were overestimated; hence, we fit these curves to the mean adult value (Austin et al., in prep).

To calculate the growth rate for the alloparented House Wrens, we forced the asymptote through the adult mass for House Wrens to make estimates more comparable to conspecifics at the same site. If the asymptote isn't standardized to the adult mass, the growth curve of the alloparented House Wrens that is fit in non-linear regression (in SAS) is overestimated with a K-value of 0.459 [d.sup.-1] and an asymptotic value of 6.58 g. This indicates the plateau in mass of the alloparented wren nestlings, but this value is substantially lower than that of typical wrens, 11.13 g. This low asymptote, and relatively high growth rate, is an artifact of the growth estimation method and is unrealistic given the condition of the chicks. This necessitated the use of a fixed asymptote at mean adult mass for House Wrens (10.9 g; Dunning, 1993) to fit the growth curve.



The incubation period was 15-16 d for the House Wren eggs and 16-17 d for the Tree Swallow eggs; host clutch completion date was used to define the start of incubation for the House Wren eggs since no other adults were observed attending the nest. The incubation period of the Tree Swallows was slightly longer than is typical (average 14 to 15 d; range 11-19 d) (this study; Robertson et al., 1992); however this period was approximately 1-2 d longer for House Wren eggs than the average 14 d (this study).

While we never observed direct feeding of the alloparented House Wrens by the Tree Swallow pair, we never noted House Wrens attending the box during nest checks. We feel confident in the assumption that Tree Swallows were responsible for the sole care of the House Wren nestlings. To further support this claim, the female Tree Swallow was detected brooding all of the nestlings on five nest checks between 7-10 Jul., both adults were observed bringing food to the nest box and when the box was approached by the observers both adults vigilantly guarded the nest.

One House Wren nestling survived 6 d and the other 13 d before each died from apparent starvation. Throughout the nestling period, both wren nestlings were observed gaping at the observers, were visibly thin and had retarded feather development compared with conspecifics at the same site. The House Wren nestlings decreased in weight on day 2 with growth on subsequent checks increased until 2 to 3 d before each nestling's death when each House Wren chick experienced plateaus or declines in body mass (Fig. 1). The wing, bill and tarsus continued to grow even after the mass plateaus. Growth rate for these birds was calculated as 0.282 (n = 2; 95% confidence interval = 0.226-0.338) and 0.285 (n = 2; 95% CI = 0.253-0.318) for mass and tarsus, respectively. Nestlings at the same site with normal growth had a rate of 0.513 (n = 111; 95% CI = 0.492-0.534) for mass growth and 0.409 (n = 83; 95% CI = 0.391-0.427) for tarsus growth. Additionally, feather growth for the wren nestlings was delayed, lagging several days behind developmentally normal House Wrens of similar age (Austin-Bythell, 2006). In normally developing birds the pin feathers were <5 mm at 4 d of age, but in alloparented wrens pin feathers did not begin to break through the skin until day 5. Additionally, primary feathers typically start to break through the pin on day 8 while in the surviving wren nestling feathers did not break through until at least 11 d of age (Austin-Bythell, 2006).

In contrast, the swallow nestlings appeared normal and had a similar pattern of development to other Tree Swallow nestlings at this site (Fig. 1). The Tree Swallow nestlings from this brood had a mass growth rate of 0.411 (n = 4; 95% CI = 0.372-0.451) and a tarsus growth rate of 0.447 (n = 33; 95% CI = 0.389-0.504). Tree Swallow nestlings from the same site with normal growth patterns had a mass growth rate of 0.438 (n = 33; 95% CI: 0.401-0.474) and a tarsus growth rate of 0.460 (n = 33; 95% CI: 0.422-0.497). The host nestling growth rate appeared unaffected by inclusion of the House Wren nestlings. All Tree Swallow nestlings from this nest fledged successfully at approximately 22-23 d, which falls within the documented age range of 15-25 d though it is slightly longer than is typical (Robertson et al., 1992).


Heterospecific cross-fostering, either experimental or natural, may provide researchers with an opportunity to study development of birds under different conditions and reproductive life history strategies provided for by the host species (Slagsvold, 2004). Thus, the influence of interspecific differences in nest condition, parental investment strategies and diet on suites of developmental life history characteristics may be identified and assessed. The results from this natural cross-fostering event indicate that the alloparented House Wrens had prolonged incubation periods and slower patterns of growth than is typical for this species when reared under normal conditions. Given that our observations are derived from results of a single natural experiment compared with typical incubation and growth patterns from unmanipulated nests, we suggest several hypotheses to account for the observed differences and recommend subsequent experimentation to evaluate the proposed mechanisms for development differences.

Differences in attendance patterns likely do not account for the change in incubation period since House Wrens spend an average of 12.1 min incubating per bout (Johnson, 1998) and Tree Swallows spend an average of 11 min per bout (Kuerzi, 1941); however, data are limited on attendance patterns for these species. The average incubation temperature for House Wrens is 35.1 C (Tieleman et al., 2004), and the average Tree Swallow temperature is typically 34.1 C (Ardia and Clotfelter, 2006). Though small, this apparent difference in incubation temperature may be responsible for the increase in the House Wren incubation period. The wren eggs were not buried in the nesting material nor were they situated below the tree swallow eggs. Thus, an alternative explanation is that the House Wrens may have received less warmth from the incubating Tree Swallow because the eggs are slightly smaller than Tree Swallow eggs and could have experienced less contact with the brood patch.

Differences in diet and feeding behavior may have contributed to the abnormal growth of the alloparented House Wrens. Though both species are insectivores, they feed their young very differently with House Wrens typically feeding one prey item to one nestling per visit (Johnson, 1998) and Tree Swallows feeding their nestlings boluses of food comprised of a number of small insects (Robertson et al., 1992). There is also likely a difference in the size and identity of prey items with swallows feeding greater quantities of smaller aerial insects at each visit than wrens (Johnson, 1998; Robertson et al., 1992).

Heterospecific sibling competition may have also contributed to the slow growth and subsequent starvation of the alloparented House Wrens. Tree swallows, being larger in size, may simply have out-competed the wrens for access to food. For instance, in a study of heterospecific cross-fostering between Pied Flycatchers (Ficedula hypoleuca) and two species of tits, Slagsvold (2004) found that Pied Flycatcher nestlings fostered in tit nests (with host nestlings) had lower survival than those cross-fostered in nests either without host nestlings or those reared under natural conditions. This suggests that competition between heterospecific siblings was high or that parents preferentially fed their own nestlings over the cross-fostered birds (Slagsvold, 2004). Additionally, nestlings may have been preferentially fed due to differences in begging calls or gape flange size/coloration between Tree Swallow and House Wren nestlings (Gill, 2007). This may have occurred in the nest we observed, and based on the vigorous begging response of the Tree Swallows nestlings to the observer early in the nestling period (before day 7), this seems likely; however, data are limited. Observers did not note Tree Swallow nestlings sitting in the nest entrance, as commonly occurs later in the nestling period, while the House Wren nestlings were still alive. This suggests that the Tree Swallow nestlings were not excluding the wren nestlings from acquiring food before parents entered the nest box, but rather through increased Tree Swallow nestling size and the vigor of their begging.

Observational events such as this are rare, but provide an opportunity to initiate investigations of alloparental care and differences in development caused by heterospecific cross-fostering under natural conditions. Future work is needed to extend these results, and to investigate causal effects of the increased incubation period and low growth rates of House Wrens reared by Tree Swallows.

Acknowledgments.--The National Science Foundation provided funding (IRCEB 0212587). We are grateful to Kellogg Biological Station, Michigan State University for allowing us to conduct research at their facilities and for providing logistical support. We would also like to thank Martin Wikelski for encouraging us to follow this nest through to its natural completion.




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Oak Creek Laboratory of Biology, Oregon State University, Department of Fisheries and Wildlife, 104 Nash Hall, Corvallis 97333



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Author:Austin, Suzanne H.; Robinson, Tara Rodden; Robinson, W. Douglas; Chartier, Neil A.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Oct 1, 2009
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