Hatching date did not differ between controls and pairs that were fed from hatching to fledging (F = 0.07, P = 0.79; Table 1), nor did hatching date differ according to burrow type (F = 1.04, P = 0.31); however, hatching date differed among years (F = 6.26, P < 0.001; Table 1). Mean hatching date was later in 1996 compared to all years (Tukey tests, P ≤ 0.02) except 1997 (P = 0.29), and later in 1997 than 1998 (P = 0.05; Table 1). When hatching dates were compared among all three experimental groups in 1993 and 1996, there was no effect of treatment (F = 0.70, P = 0.50; Table 1), nor was there a year-by-treatment interaction (F = 0.50, P = 0.61), but there was a significant effect of year (F = 16.5, P < 0.001; Table 1).
Table 1. Mean ± SE (sample size) hatching dates (June 1 = 1) for broods of food-supplemented and control burrowing owl pairs over 5 years in Saskatchewan, Canada
|Unfed controls||6.7 ± 2.8 (6)||8.0 ± 1.6 (11)||17.0 ± 3.7 (6)||10.3 ± 1.9 (10)||7.8 ± 1.5 (8)|
|Fed (hatching to fledging)||8.4 ± 1.0 (5)||9.3 ± 1.5 (10)||13.8 ± 2.9 (9)||11.8 ± 1.8 (12)||5.9 ± 1.1 (21)|
|Fed (laying to fledging)||–||10.3 ± 1.8 (14)||17.3 ± 1.5 (16)||–||–|
Number of hatchlings did not differ among years (F = 1.11, P = 0.36), nor between controls and pairs fed during the nestling period (F = 0.06, P = 0.81), and there was no significant year-by-treatment interaction (F = 0.61, P = 0.66; Fig. 2A). When owls were also fed during the laying, incubation, and nestling stages (1993 and 1996), number of hatchlings did not differ among the three experimental groups (F = 0.49, P = 0.61) or between years (F = 1.01, P = 0.32), and there was no year-by-treatment interaction (F = 0.04, P = 0.97; Fig. 2A).
Figure 2. Mean (±SE) number of hatchlings (A), number of fledglings (B), and percentage of hatchlings fledged (C) per successful nest, for burrowing owl pairs in three experimental treatments. Pairs supplemented with food from hatching until fledging are shown in relation to control pairs in five study years. In 1993 and 1996, a third treatment of pairs supplemented with food from egg laying until fledging occurred. Sample sizes are presented at the base of each bar. Number of fledglings is given for pairs in both natural and artificial burrows, but number of hatchlings and percentage of hatchlings fledged could only be determined for pairs in artificial burrows.
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Quantity and quality of fledglings
Pairs that were fed from the time of hatching until fledging produced on average 2.4 more young compared with controls (F = 53.9, P < 0.001; Fig. 2B), with no effect of burrow type (F = 0.59, P = 0.44). Mean number of fledglings produced per nest significantly differed among years (F = 15.9, P < 0.001; Fig. 2B). In 1997, fledgling production was higher compared to the other 4 years (P ≤ 0.02 for all pairwise comparisons); however, there was also a significant interaction between year and treatment (F = 8.5, P < 0.001; Fig. 2B). Feeding treatments in 1993, 1996, and 1998 all resulted in pairs fledging more offspring than unfed pairs (pairwise comparisons, P < 0.01). However, feeding treatments resulted in no difference in offspring production in 1992 or 1997 (pairwise comparisons, P > 0.70), although in 1992 fed pairs produced, on average, one more fledgling than unfed pairs (Fig. 2B).
In 1993 and 1996, when some pairs were also fed from egg laying through fledging, treatment (unfed, fed from hatch until fledging, and fed from laying until fledging) significantly influenced the number of fledglings per pair (F = 33.76, P < 0.001; Fig. 2B). Fledgling production for pairs fed through the egg laying, incubation, and nestling periods was no higher than for pairs fed during the nestling period alone (P = 1.00), but pairs fed from hatching to fledging and those fed from egg laying through fledging fledged 3.6 and 3.8 more young, respectively, than did unfed pairs (Tukey test, P < 0.001; Fig. 2B). There was no difference in fledgling output between the 2 years (F = 1.04, P = 0.31), but there was a significant year-by-treatment interaction (F = 4.73, P = 0.01). In 1993, pairs in both feeding treatments produced more fledglings than pairs not fed (Tukey pairwise comparisons, P < 0.01; Fig. 2B). Pairs that were fed from laying to fledging in 1996, produced more fledglings than pairs that were unfed (Tukey pairwise comparisons, P < 0.05). Although pairs that were fed from hatching to fledging produced 2.2 more fledglings than unfed pairs, this was not statistically significant (P = 0.098; Fig. 2B). Analyses examining variation in the percentage of hatchlings fledged (Fig. 2C) in response to treatment and burrow type mirrored those examining the number of fledglings produced and so are not presented.
Unfed owlets died in the first half of the nestling period in 1992, 1997, and 1998, but mortality was unrelated to age in 1993 and 1996 (Fig. 3). Of all deaths (N = 176), 169 (96%) were attributed to food shortage. Ninety-six of these 169 nestlings had been weighed within 5 days of their death. Fifteen percent of these showed normal patterns of mass gain, but 85% experienced mass loss, or a reduced rate of mass gain, before death. Of the 169 nestlings that apparently died from food shortage, 8% were found emaciated but otherwise intact, 18% were partially eaten, and 73% were completely consumed by their siblings or parents.
Figure 3. Survival of burrowing owl nestlings, from age 0 to 41 days (age 0 = individual's hatch day), in each experimental group (artificial burrows only). The numbers of hatchlings in “Unfed” and “Fed (hatching to fledging)” groups, respectively, were 33 and 37 (1992), 89 and 82 (1993), 47 and 46 (1996), 53 and 77 (1997), and 31 and 164 (1998). The number of hatchlings in the “Fed (laying to fledging)” treatment was 119 in 1993, and 130 in 1996.
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We had difficulty consistently recapturing individuals from some broods; therefore sample sizes used in analyses requiring growth measurements are lower than those in other analyses. Mean fledgling mass per brood was unaffected by year or supplemental feeding, whether two experimental groups over 3 years were examined or three experimental groups over 2 years (Tables 2 and 3). In all 3 years, owlets were structurally smaller (i.e., lower PC1 values) in unfed broods compared with broods receiving extra food from hatching until fledging (Tables 2 and 3). Likewise, when all three treatments in 1993 and 1996 were analyzed, supplemental feeding had a significant influence on fledgling structural size (Tables 2 and 3).
Table 2. Mean (±SE) burrowing owl fledgling mass and structural size (PC1; see text for description) per brood, in relation to year (1992, 1993, and 1996) and feeding treatment in Saskatchewan, Canada
|Treatment||Mass (g)||Size (PC1)||# Broods|
|Unfed controls||125.7 ± 6.0||−0.92 ± 0.56||4|
|Fed (hatching to fledging)||138.5 ± 3.0||0.37 ± 0.18||5|
|Unfed controls||137.5 ± 5.7||−0.60 ± 0.60||9|
|Fed (hatching to fledging)||132.2 ± 3.4||−0.27 ± 0.25||10|
|Fed (laying to fledging)||135.2 ± 1.9||0.02 ± 0.16||14|
|Unfed controls||126.5 ± 3.6||−0.70 ± 0.44||6|
|Fed (hatching to fledging)||137.4 ± 7.9||0.43 ± 0.34||5|
|Fed (laying to fledging)||133.0 ± 1.6||0.21 ± 0.21||15|
Table 3. Two-way ANOVA tables for the effects of various supplemental feeding treatments and study year on burrowing owl average brood fledgling mass (g) and structural size
| ||Mass (g)||Size (PC1)|
| F || P || F || P |
|Fed from hatching to fledging|
|Treatment × Year||2.01||0.14a||0.70||0.50|
|Fed from hatch to fledging|
|Fed from egg laying until fledging|
|Treatment × Year||1.92||0.15c||0.57||0.57|