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Effects of Weather on Breeding Ducks in North Dakota

Duck Arrival Dates

The arrival date—the day on which an individual of a species was first observed on the study area—was subject to some variability. An aberrant value could be recorded if an unusually early migrant was seen a week or more before others of that species. Such observations were uncommon among our data; in most years the arrival of the first single bird was followed by a wave of birds within a few days.

Mallard

Mallards arrived on the median date of 30 March at Salyer and 20 March at Woodworth (Table 2). Annual variation in arrival dates was considerable, however, ranging from 7 March to 10 April at Salyer and from 15 March to 6 April at Woodworth.

The arrival of mallards in spring was strongly influenced by temperatures during the usual arrival period. Arrival tended to be later when temperatures were low during 12 March-8 April (Fig. la). Arrival dates were delayed an average of 1.8 days (P<0.0001) for each Celsius degree difference in the mean temperature. A more detailed examination showed that mean temperatures during 12-25 March accounted for most of the variation in arrival dates.

Arrival dates at Salyer and at Woodworth tended to be similar when mean temperatures were similar (Fig. la). The difference adjusted for mean temperatures was 1.8 days earlier at Woodworth, which was not significant (P=0.30). Thus the 10-day difference in median arrival dates between the two areas was largely due to the earlier warming at Woodworth than at Salyer.

Gadwall

The median dates of gadwall arrival were 12-13 April (range, 30 March-2 May) at Salyer and 7 April (range, 20 March-28 April) at Woodworth (Table 2). The arrival of gadwalls was also influenced by low temperatures during the usual arrival period (Fig. lb). Gadwalls arrived an average of 1.46 days later for each Celsius degree change in mean temperature during 12 March-8 April (P<0.0001). Temperatures during the latter part of that period were particularly influential.

Unlike the mallard, the gadwall seemed to arrive significantly (P<0.01) earlier at Woodworth than at Salyer under similar temperature regimes (Fig. lb). The estimated difference in arrival dates between the two areas, adjusted for mean temperature, was 6.0 days.

Blue-winged Teal

The median arrival date of blue-winged teal (Table 2) was 15-16 April at Salyer (range, 1-25 April) and 11-12 April at Woodworth (range, 5-22 April). Blue-winged teal were influenced in their arrival by low temperatures (Fig. lc), particularly during 26 March-15 April. Arrival dates were delayed 1.07 days for each Celsius degree difference in mean temperature during the period (P<0.0001). Under similar temperatures, blue-winged teal seemed to arrive somewhat earlier at Woodworth than at Salyer; the adjusted difference was 2.5 days earlier at Woodworth (P = 0.12).

Redhead

Median arrival dates of redheads (Table 2) were 8 April at Salyer (range, 26 March-21 April), and 4-5 April at Woodworth (range, 25 March-21 April). Redheads tended to be delayed by low temperatures (Fig. 1d), particularly during 12 March-8 April. They were delayed 2.0 days for each Celsius degree difference in mean temperature (P<0.0001). Under similar temperature conditions, redheads arrived at Woodworth somewhat earlier (2.4 days; P=0.14) than at Salyer.

Table 2. Summary statistics of breeding activities of four duck species at the Salyer (S) and Woodworth (W) study areas. Dates are Julian.
Activity	Species and study area
	Mallard		Gadwall		Blue-winged teal		Redhead
	S	W	S	W	S	W	S	W
Arrival (n^a)	28	13	29	13	28	12	29	10
Median	89	79	102.5	97	105.5	101.5	98	94.5
Mean	86.7	83.6	102.2	94.8	105.4	102.5	98.3	94.3
SD^b	7.7	8.8	7.9	9.9	5.8	4.9	7.6	8.5
Initiation (n)	25	-	25	-	26	-	11	-
Median	109	-	137	-	130	-	123	-
Mean	108.4	-	132.4	-	128.9	-	126.8	-
SD	7.8	-	7.9	-	7.1	-	13.8	-
Peak hatching (n)	18	-	19	10	19	12	18	-
Median	176	-	190	180.5	185.5	180	193	-
Mean	176.5	-	189.8	183.6	186.3	178.3	191.1	-
SD	10.6	-	6.3	9.6	6.9	5.6	10.1	-
Span (n)	18	-	19	10	19	12	10	-
Mean	46.6	-	32.9	30.6	34.7	38.1	37.7	-
SD	5.9	-	4.8	6.2	4.2	7.3	7.9	-
^an=number of years. ^bSD=standard deviation.

GIF-Arrival date in relation to average temperature

General

Below-normal temperatures during or just before the typical arrival period tended to delay arrivals for each of the four species. The earliest arriving species, the mallard, was particularly influenced by temperatures during 12-25 March. The next earliest, the redhead, was apparently affected by temperatures during 12 March-8 April. The third species to arrive, the gadwall, had arrival dates most strongly related to mean temperatures during 26 March-8 April. The latest of the four species, the blue-winged teal, had arrival dates correlated with temperatures during 26 March-15 April.

In addition to effects of mean temperatures, differences in arrival date were observed between Salyer and Woodworth. Under similar weather regimes, birds of each species could be expected earlier at Woodworth. The difference was 1.8 days for mallards, 2.4 days for redheads, 2.5 days for blue-winged teal, and 6.0 days for gadwalls, only the last being clearly different from zero.

These findings corroborate the common notion that late, cold springs result in delayed arrivals of migrating ducks. Sowls (1955) suggested that later-arriving species were less affected by temperatures than the early-arriving mallard and pintail, but this contention was not supported by our data. Arrival date is determined by weather patterns throughout the migrational corridor during the entire period of migration; temperature at the terminus can only account for a portion of the variation.

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