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Brady, Ryan S. – Nest lining behavior, nest microclimate, and nest defense behavior of Burrowing Owls. 2004.

burrowing owlIn Chapter One, I examined patterns of dung use by burrowing owls nesting in artificial burrows and tested three hypotheses that attempt to explain the function of dung in burrowing owl nests: (1) Anti-predation, (2) Optimal Microclimate, and (3) Anti-ectoparasitism. I documented the types of dung used, its moisture content, and other nest-lining materials, if present. I also measured the volume of dung within nest chambers and at ground-level burrow entrances at multiple points in the nesting cycle. To test the Anti-predation Hypothesis, I monitored predation of actual burrowing owl nests and related this to the amount of dung at the nest. Moreover, I conducted an experiment using artificial nests to compare predation rates of nests with and without dung. For the Optimal Microclimate Hypothesis, I used data loggers placed within nest chambers during incubation and the early nestling period to measure temperature and relative humidity. I used a handheld digital monitor inserted through the burrow tunnel to record chamber carbon dioxide levels at various times in the nesting cycle. I then related all three microclimate variables to the volume of dung present in the nest chamber. Finally, to test the Anti-ectoparasitism Hypothesis, I indexed the number of fleas on adult female and nestling owls and related these to chamber dung volume.

All burrowing owl nests in my study contained at least some livestock dung. Owls began lining nests after pair formation and usually stopped no later than during incubation. Dung volume at tunnel entrances was not correlated with dung volume in nest chambers. Predation of actual nests was unrelated to total dung volume, and artificial nests lined with dung were depredated equally as often as those without dung. Chamber dung volume did not significantly affect mean temperature or relative humidity of nest chambers, but nests with more dung experienced smaller temperature ranges and lower carbon dioxide concentrations during incubation. Adult females had few to no fleas but ectoparasite loads of nestlings were unrelated to chamber dung volume. My results do not support the Anti-predation and Anti-ectoparasitism Hypotheses, but the hypothesis that livestock dung in burrowing owl nests creates a more favorable microclimate for adult owls and their developing young remains tenable.

In Chapter Two, I evaluated several microclimate variables for burrowing owls nesting in plastic artificial burrow systems (ABSs) and examined associated effects on hatching success, nestling growth, and productivity. I used data loggers mounted on the chamber wall to record mean temperature, temperature range, and relative humidity within nest burrows during three stages in the nesting cycle, including mid-incubation, 7- to 10-day-old nestlings, and 25- to 28-day old nestlings. I used a handheld, digital monitor and inserted its sensor into the nest chamber via the tunnel entrance of an unearthed ABS to measure carbon dioxide during mid-incubation and when nestlings were 7, 15, and 25 days old.

For temperature and relative humidity, I sampled 19 nests during mid-incubation, 18 nests during the early nestling period, and 12 nests during the late nestling period. Chamber mean temperature averaged 18.27 ± 0.77ºC during mid-incubation, 23.51 ± 0.46ºC during the early nestling period, and 21.50 ± 0.38ºC during the late nestling period. Temperature ranges for these periods were 2.01 ± 0.27ºC, 3.21 ± 0.33ºC, and 3.26 ± 0.29ºC, respectively, while chamber relative humidity averaged 78.2 ± 2.4%, 91.3 ± 1.6%, and 89.9 ± 2.3%. Mean chamber carbon dioxide concentration was 0.620 ± 0.129% during mid-incubation, 2.131 ± 0.345% during the early nestling period, 0.969 ± 0.204% during the mid-nestling period, and 0.867 ± 0.194% during the late nestling period. Mean temperatures, temperature ranges, and relative humidities were significantly lower during incubation than during the early or late nestling period. Carbon dioxide concentrations were significantly higher during the early nestling period than during all other periods. Chamber and tunnel dimensions had no significant effects on nest microclimate. Hatching success was not significantly affected by any of the four microclimate parameters, but nest chambers that experienced larger temperature ranges during the early nestling period had nestlings with lower mass at 25 days of age. Productivity was significantly higher for nests with lower carbon dioxide during the early nestling period. I found no evidence that ABSs provide microclimates different from those of natural burrow systems, which supports the continued use of artificial burrows as tools for researching and managing burrowing owls.

In Chapter Three, I examined nest defense by burrowing owls in response to a simulated terrestrial predator during the mid- to late nestling period. I deployed a stuffed badger mounted on a plastic sled 50 m from the focal owls’ nest burrow and dragged the badger past the nest by means of a monofilament line wound around a garden hose reel. During each trial, I recorded the following responses for each parent bird: (1) nearest approach distance, (2) presence/absence of threat displays, bobs, hovers, dives, strikes, and screams (collectively, the general defense index) and (3) number of alarm (chatter) calls. For nestlings, I recorded behaviors as in adults and also if some or all of the brood flushed as the stuffed badger approached the nest burrow. I then investigated effects of food availability, parent sex, nestling age, and brood size on the intensity of nest defense. I assessed food availability during two breeding seasons by small rodent sampling and explored potential mechanisms for differing defense intensity by comparing mean female mass, mean nestling mass, and brood size between years.

Thirty-six of 41 (88%) focal males and 24 of 33 (73%) focal females that were present during trials responded to the simulated predator, and both sexes exhibited similar response types, including approaching the predator, threat displaying, bobbing, hovering, diving, striking, screaming, and alarm calling. Young nestlings usually remained within nest or satellite burrows, but in 11 of 22 (50%) trials where nestlings were 25 days or older, all or part of the brood exhibited a flushing (escape) response. In only one case did a non-focal owl help defend a nest other than its own. Small rodent sampling indicated significantly greater prey availability in 2001 than 2002. Year and parent sex had a significant interaction effect on the general defense index as females, but not males, decreased their intensity of nest defense in 2002 when food resources were less abundant. The other indices of defense intensity, nearest approach distance and number of alarm calls, were unaffected by year or sex, however. I also detected no effects of brood size or nestling age on any of the three measures of nest defense intensity for either sex in either year. These results only partially support current parental investment theory by showing that female, but not male, burrowing owls adjust their nest defense behavior in response to future offspring survival prospects. This sexual difference may be related to (1) reduced energy reserves in females, (2) a male-biased sex ratio giving females higher probability of future reproduction, or (3) sexual differences in other, poorly known life-history characteristics.

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