Hatching asynchrony occurs when birds begin to incubate their eggs before all of the eggs in the clutch have been laid.  When this occurs, the first laid eggs hatch before later laid eggs, resulting in a size hierarchy within the brood.  These size hierarchies have variable effects.  In some species, the last hatched offspring always perish, either from starvation (inability to compete for resources) or because the older offspring kill them (siblicide).  This phenomenon, observed in Pelicans (Pelecanus erythrorhynchos) (Evans and McMahon 1987, Evans 1990) and Boobies (Sula sp.) (Drummond and Chavelas 1989, Anderson 1990, 1995), is known as obligate brood reduction.  In other species, the younger (smaller) offspring may survive, but experience a much higher risk of mortality than do their older (larger) broodmates.  This phenomenon, which is common in many passerines (Strehl 1978, Clark and Wilson 1981, Richter 1984, Magrath 1990, Stouffer and Power 1991, Ricklefs 1995, Amundsen and Slagsvold 1996) and in parrots (Stoleson and Beissinger 1997, Krebs 1999), is known as facultative brood reduction.
      Outwardly, because of its deleterious effect on younger offspring, hatching asynchrony seems maladaptive.  Thus, hatching asynchrony is of interest in understanding the evolution of avian breeding strategies (Clark and Wilson 1981, Magrath 1990, Amundsen and Slagsvold 1996).  Fifteen different hypotheses have been presented to explain why hatching asynchrony occurs (Magrath 1990).  Several of these propose that parents and older broodmates might benefit from the mortality of younger brood mates (Clark and Wilson 1981, Drummond 1987, Amundsen and Slagsvold 1996, Wiebe 1996, Aparicio 1997).  These hypotheses suggest that hatching asynchrony is, in itself, an adaptation.  In addition, hypotheses have been presented suggesting that ecological or physiological constraints force some birds to hatch their broods asynchronously (Magrath 1990, Stoleson  and Beissinger 1999).  In this case, hatching asynchrony is not adaptive, but is the consequence of a trade-off between selective pressures that favor early incubation and selective pressures that favor synchronous hatching.
      The egg viability hypothesis proposes that synchronous hatching is otherwise adaptive, but, because the viability of unincubated eggs declines with time, birds are forced to commence incubation before all the eggs are laid.  Temperature is the most important factor determining the viability of unincubated eggs.  Embryos are most susceptible to exposure to high and moderate temperatures (Webb 1987).  At cool temperatures, freshly laid eggs remain in a state of stasis (O’Connor  1984); no development will occur in eggs maintained at temperatures below 27 oC (physiological zero) (Rol’nik 1970, White and Kinney 1974, Wilson 1991).  Thus, freshly laid eggs maintained at temperatures below physiological zero can remain viable for an extended period of time.  Development commences as soon as the eggs are warmed above physiological zero.  However, optimal development occurs at incubation temperatures, typically 36 oC and 38oC (Webb 1987, Rahn 1991).  Thus, in temperate climates, cool ambient temperatures allow the eggs to remain dormant until the onset of incubation (Drent 1975, Ewert 1992).  On the other hand, exposure to temperatures above physiological zero but below normal incubation temperature can cause abnormal development and embryo mortality (Wilson 1991, Deeming and Ferguson 1992).  As a result, in warm climates, birds may be constrained to commence incubation early in order to maintain egg viability and increase hatching success. 
      Only one study has been conducted to test the egg viability hypothesis in passerine birds (perching birds) (Veiga 1992, Veiga and Vinuela 1993).  However, among passerines, there is a  general correlation between hatching asynchrony and latitude (Clark and Wilson 1981, Hussell 1985), which suggests that egg viability might provide a strong selective pressure favoring hatching asynchrony.  For example, in House Sparrows (Passer domesticus), eggs left unincubated for 3 or more days and exposed to temperatures above physiological zero exhibited reduced hatching success compared to eggs that were incubated earlier (Veiga 1992, Veiga and Vinuela 1993). However, that study failed to show whether hatching asynchrony represented an adaptive trade-off between selective pressure for early incubation and selective pressure for total brood synchrony.
      The current study will test the egg viability hypothesis in the American Barn Swallow (Hirundo rustica erythrogaster).  In addition, egg manipulation experiments will be conducted to determine whether observed (unmanipulated) levels of hatching asynchrony represent an optimal trade-off between the need to incubate the eggs early and the need to synchronize hatching.  Barn Swallows in Texas represent an ideal species in which to conduct these studies.  First, Texas climate varies from relatively cool in the spring to extremely hot in the summer.  Thus, the importance of egg viability in favoring hatching asynchrony should vary as the season advances and ambient temperatures increase.  Secondly, observations of 361 nests at two colonies south of Commerce, Texas during the summers of 1998 and 1999, suggest that hatching asynchrony is typical, but variable in this species.  Most birds lay 5 eggs, but begin incubation after laying the 3rd or 4th eggs and hatching asynchrony is most pronounced near the end of the breeding season when ambient temperatures are the highest.
       The egg viability hypothesis makes some predictions that can be rather easily tested.  First, unincubated eggs should become less viable with time.  Second, if early incubation is a response to this loss of viability, then loss of viability should be detectable within the time frame normally required to complete a clutch.  To test these predictions, eggs will be removed from the nests of barn swallows immediately after laying and before any incubation takes place (prior to 10 AM).  The removed eggs will be replaced with artificial surrogate eggs in order to keep the parents attendant at the nest.  The real eggs will be sequestered into a holding box equipped with an electronic thermometer recording daily maximum and minimum temperatures.  Barn Swallows lay one egg each morning until the clutch is complete, and each of these eggs in experimental nests will be marked according to laying sequence and sequestered into the holding box.  Once egg laying has ended, the surrogate eggs in the nest will be removed and the real eggs will be placed back into the nest.  As a result of this manipulation, each egg will have been exposed to ambient temperatures for a different number of days (ranging from 0 to 4).  The egg viability hypothesis predicts that the first laid eggs should suffer reduced hatching success and that this reduced hatching success should be correlated with the extent to which the eggs are exposed to ambient temperatures above physiological zero.  The manipulation will allow hatching success to be measured and assessed relative to laying sequence and the level of exposure to ambient temperatures above physiological zero.  Hatching success will also be compared to that of control nests.
      The egg viability hypothesis suggests that hatching asynchrony is imposed by a physiological constraint and represents a potential cost to the parents in terms of lowered reproductive success.  Thus, it is also predicted that experimentally synchronized nests should have higher reproductive success than control nests, nests for which eggs have been sequestered, and completely asynchronous nests.  To test this prediction, two other manipulations must be conducted: completely asynchronous nests and completely synchronous nests.
Completely asynchronous nests will be created by fostering eggs onto parents that are already incubating complete clutches.  The eggs from focal nests will be removed each morning as they are laid and replaced with artificial surrogate eggs.  The real eggs will be placed into the nest of an incubating foster pair.  To maintain the appropriate clutch size, eggs will be removed from the foster parent’s nest and placed into a third (dump) nest (not to be included in the study).  As the eggs hatch, they will be placed back into the original parent’s nest, removing surrogate eggs appropriately.  The end result will be a brood of nestlings all of which hatched on different days (completely asynchronous brood). 
     To generate synchronous broods, unmanipulated nests will be monitored until the first nestlings hatch.  At that point, the remaining unhatched eggs will be exchanged with hatch-day nestlings adopted from other nests (donor nests – not to be included in analyses).  This will generate broods of young in which all of the nestlings hatched on the same day.  Under the egg viability hypothesis, these latter nests should experience the greatest reproductive success.  Reproductive success will be measured as the number of young surviving to fledging and as nestling mass on Day 12 (asymptotic mass).  Kruskal-Wallis multiway nonparametric ANOVA will be used to statistically evaluate the data.
 

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