Breeding

Pair Formation

Females settle on a male’s territory 1–2 d after her arrival (Carey 1982). The pair associates until the female begins incubation. Females either remain with the same male or nest with a different male for successive nests within a breeding season. Occasional pairs move together and renest as far as 1 km from their earlier nest site (Payne and Payne 1993a, Payne 1996).

Nest Building

Female chooses the nest site and builds the nest, nest-building takes as long as 8 d early in the season, and as little as 2 d later in summer (RBP).

First/Later Broods

In southern Michigan, first eggs (in 2,492 active nests over 9 yr) from 14 May through 13 August (Payne and Payne 1998). Some later nests were renestings or subsequent nestings of banded females that had nested earlier in the season; other late nests were the first nest of the season for the female. Latest breeding records, 18 Sep. 2005, two females caring for fledged broods in Ann Arbor; 21 Sept 2006, female with partly blue male, the female fed a vocal, begging long-tailed fledgling (RBP). In Ontario, egg dates of 128 nests from 26 May to 15 August, and one nest contained 3 young on 26 Sep. (Peck and James 1987). No intensive studies have been made to determine whether unbanded locally late-arriving females have already bred elsewhere (i.e., with a brood patch indicating a previous nesting attempt). Success of nests thorugh fledgling of young buntings is independent of the season; half the young fledged come from nests begun in July and August (Payne and Payne 1998, Fig. 7).

Female builds nest alone, visiting several sites in low, branching vegetation within 1 m of the ground and bringing nesting material from nearby. She chooses a variety of sites not limited to her first nest plant species, either the plant species in which she first nested successfully, or in her natal nest plant species (Payne and Payne 1989, RBP). Female often changes nesting sites; later nests are often in late-growing plants such as goldenrod (Solidago sp.).

Nests are built in fields and edges of woods, roadsides, railroad rights-of-way. Some early nests are built in understory shrubs in forest when the tree canopy is late in leafing out (as in black raspberries Rubus occidentalis growing under black locust Robinia pseudoacacia) or canopy is defoliated by caterpillars; many later nests are built in plants that grow only in later summer. Nests are built 0.3 to 1 m above ground, incorporating into the nest structure several vertical or oblique stems as in a crotch or fork of a branching herb or shrub as supports (Taber and Johnston 1968, RBP).

More than 40 nest plant species are recorded in Michigan; many nests are built in black raspberries and gray dogwood (Cornus racemosa), near the edge of a clone just under the leafy canopy. The canopy conceals the nest from above, and it protects the eggs and young from sun and rain. Nests also often in goldenrod, stinging nettles (Urtica dioica), and multiflora rose (Rosa multiflora), staghorn sumac (Rhus typhina), and juniper (Juniperus communis, J. virginiana). Herbs with stiff erect stems and side branches also used, including joe-pye weed (Eupatorium maculatum), cow-parsnip (Heracleum maximum); bracken fern (Pteridium aquilinum), spreading dogbane (Apocynum androsaemifolium) (infrequently, the side branches are thin and offer little support and the leaves little cover for a nest), cane blackberries (Rubus allegheniensis), (rarely, the prickly bramble canes are too stiff for buntings to bind together with nest material and the bramble canopy is too sparse to conceal a nest). Other shrubs and herbs with nests include wild gooseberry (Ribes cynosbatis), glaucous honeysuckle (Lonicera dioica), river-bank grape (Vitus riparia), white cockle or snowy campion (Silene pratensis), milkweed (Asclepias sp.), meadowsweet (Spiraea alba), giant ragweed (Ambrosia trifida), great burdock (Arctium lappa) and field thistle (Cirsium arvense). Occasionally nests are built within 0.5 m of the ground in bedstraw (Galium sp.) and poison-ivy (Toxicodendron radicans), and in prickly muliflora rose growing inside a rusty abandoned car. A few nests in tree saplings (elm Ulmus americana, prickly ash (Zanthoxylum americanum), sugar maple Acer saccharum, oaks Quercus spp., apple Pyrus malus); < 1% nests 10 m or higher in trees (Michigan: RBP). In West Virginia, nests in vines (greenbriar Smilax app., grape and Virginia creeper Parthenocissus quinquefolia) or in spreading shrubs (blackberry, mountain laurel Kalmia latifolia, spicebush lindera benzoin, azalea Rhodeodendron spp.) (Bell and Whitmore 2000).

Low herbs and shrubs with the branches more vertical than horizontal and a form less bush-like at nest level occasionally are nest sites in Michigan. Occasional nests built in common crop plants maize (Zea mays) and soybean (Glycine max), and these nests incorporated additional plants in support. No nests found in erect leafy or upright stems of abundant native horsetails, grasses, sedges or cat-tails Typha spp. Nests built in native flora and in invasive, non-native species including multiflora rose (Rosa multiflora), purple loosestrife (Lythrum salicaria), spotted knapweed (Cirsium maculosa), garlic mustard (Alliaria petiolata), autumn olive (Eleagnus umbellate; infrequent, the forking side branches are sparse), and privet (Lingustrum vulgare).

Choice of nest plant appears determined by canopy cover over the nest, branching form and rigidity of the plant to support nest. The most frequently used nest plants in s. Michigan, black raspberries and gray dogwood, have forking stems that support the nest just below a continuous canopy of leaves which conceal the nest from above. These two shrubs accounted for half the nests, yet nests were by no means easy to find, because these shrubs are common, and fewer than 5% had a bunting nest; the same in goldenrod and bracken fern. Together with the abundance of the most-used nest plants and the opportunistic use of many kinds of nest plants, the bunting nest sites do not form an easy search image.

Construction

Only female builds nest, sometimes accompanied by male who takes no part in nest building. Early in breeding season a nest may be constructed in 8–10 d, later in summer in 2 d. Female builds throughout daylight hours, and nestbuilding activity peaks in early morning.

Structure And Composition

Nest an open cup of soft leaves, coarse grasses, stems, and strips of bark. Nest building usually begins with green strips woven around 2-3 supporting stems, often with a bottom of open and decayed skeletons of broad leaves such as aspen (Populus spp.); loose material often hangs below. Nest materials are held in place by weaving and by wrapping with spider web, especially around the rim. Nest cup is lined with finer material: slender grasses, rootlets, thistle down, strips of grapevine bark and sometimes deer hair.

Dimensions

Michigan, early and late nests (n = 14 nests): outside diameter, range = 74–94 mm, mean = 82.93 ± 6.98 SD; inside diameter, range = 43–63 mm, mean = 51.93 ± 6.15 SD; outside depth, range = 58–100 mm, mean = 66.86 ± 11.92 SD; inside depth, range = 33–47 mm, mean = 41.15 ± 4.55 SD. Early nests often larger with more visual and insulative protection than later nests.

Microclimate

Nests hidden from above by leaves (within 10 cm of nest), but often visible from the side, perhaps allowing incubating or brooding female to view an approaching predator. Nest cover measured as 2-20% of sunlight reaching the nest (Lanyon 1981 . Nest concealment at nest height, measured by proportion of nest visible when observers flushed the female, is 70-75% (Burhans and Thompson 2001). Physical values of microsite and insulative value of nest otherwise not recorded.

Maintenance Or Reuse Of Nest, Alternate Nests

Once built and eggs laid, nest not actively maintained. Nests with growing young sometimes fall into disrepair, tip, or slip from the support with no attempt by the female to repair the nest. A nest is only used once, new nests are constructed for successive nesting attempts. One nest in 2,492 was built by constructing additional layers over an older nest. In field study, nests that were tipped by wind and rain, or observers, and still had eggs or nestlings often were repaired by the observer, using green grass, twine or fine wire to hold the supporting stems together; the nesting female accepted the repaired nests and continued with success to hatching and fledging (RBP).

Shape

Short oval to short subelliptical, variable within and among clutches.

Size

In 13 clutches (3–4 eggs), 42 eggs taken in 1878–1942 and now in collections of UMMZ: length (mm), range = 17.1–20.5, mean = 18.75 ± 0.88 SD. Width (mm), range = 13.4–14.8, mean = 14.15 ± 0.35 SD. Volume (mm³) = 0.512 x length x width², range = 1.61–2.19, mean = 1.92 ± 0.14 SD. Mean egg size varies among clutches for all dimensions (P < 0.001), though some clutches are particularly variable in egg size (equality of variances, volume, F = 1.86, P = 0.04). Proportion of total variance that is explained by variance among clutches is 61% for width, 50% for length, and 55% for volume (RBP).

Mass Of Fresh, Whole Egg (G)

Mean = 2.08 ± 0.15 SD (n = 42), calculated (volume x specific gravity of 1.08); mean = 2.09 ± 0.25 SD (n = 9), direct field measurements). Weight loss of eggs during incubation, 16% (Morgan 1976). Egg size proportional to weight of adult female, 2.08 g/14.38 g = 14.5% (RBP).

Eggshell Thickness (Mm; N = 4)

Range = 0.09–0.11, mean = 0.1, Michigan and Indiana, eggs collected 1878–1902 (UMMZ).

Color

Generally white with no markings. About 1% are marked with reddish-brown or purplish flecks or spots (McLean 1886, Barrows 1912). In Michigan in some nests, 2 or 3 eggs in a clutch are unmarked white and one is spotted, but no behavior or genetic evidence that these are laid by different females. All spotted eggs in three clutches of a color-banded female were similar in color, pattern, and size of spots (photographs, RBP). Surface texture, smooth and slightly glossy.

Egg Laying

Eggs laid after nest is constructed and lined. Eggs are laid on consecutive days in early morning within an hour after sunrise (George 1952, RBP). Female visits nest occasionally but is usually inattentive until clutch completed. If predator takes nest, female abandons and builds elsewhere. Loss of eggs in nests parasitized by cowbirds not compensated by laying of additional or replacement eggs (Figs. 8, 9).

Intraspecific Brood Parasitism

Occasional or rare. Fewer than 1% of broods examined had young that were genetically mismatched with attending female (allozymes of parents and offspring in a population in Michigan, Westneat 1987b; minisatellite fingerprinting in a population in New York, Westneat 1990). One nest in Michigan had 6 eggs, suggesting laying by 2 females; only one was seen to incubate, and in later years she had clutches of 3 and 4 (RBP). In Ontario one of 126 clutches had 6eggs, others had 1–4 eggs (Peck and James 1987).

Onset Of Broodiness And Incubation

Females inattentive at nest during laying period except for covering eggs in cool, rainy weather. Incubation begins with the last egg of a clutch.

Incubation Patch

Females have a ventral brood patch, males have none. The brood patch becomes defeathered when female builds first nest of the season; it becomes vascularized during laying, edematous during incubation, rapidly loses swelling during the feeding of the nestlings, and appears wrinkled by the time the nestlings are 5 days old. The defeathered ventrum remains bare until the female molts at end of breeding season.

Incubation Period

Average 12–13 d, sometimes 11 d in summer, 14 d in cool spring weather (Taber and Johnston 1968, Holcomb 1966, Morgan 1976, Payne 1982, Peck and James 1987, RBP).

Parental Behavior

Females incubate, males show no interest, though they sometimes chip near the nest when an observer is nearby. More often the males chip when the nest has chicks, especially when these are ready to leave the nest (RBP).

Incubation constancy during the daytime is 79%, attendance periods on a warm day varied from 22 to 106 min., attentive periods averaged 25 min., and inattentive sessions averaged 11 min. in all-day observations at a nest (Morgan 1976). Incubation is interrupted when the female leaves the nest to forage. Males do not incubate and usually do not associate with incubating females.

Hardiness Of Eggs

Not reported.

Preliminary Events And Vocalizations

Egg pipped from the inside by nestling. No vocalizations noted.

Shell Breaking And Emergence

Brood usually hatches on the same day, usually during the morning, occasionally over 2 days.

Parental Assistance And Disposal Of Eggshells

Young hatch with no direct parental assistance. Female eats the eggshells at nest (Morgan 1976).

Condition At Hatching

Altricial, naked except for sparse down, eyes closed, no visual response. Mass, linear measurements: see Young birds: growth and development. Amount and dis-tribution of feathers: downy neossoptiles 2–8 mm, the number varying among tracts (unilateral number where a bilateral tract): coronal 6, occipital 5, mid-dorsal 4, pelvic 4, scapular 7, femoral 5, rectrix 12, greater secondary covert 9, middle secondary covert 7 (Wetherbee 1957). Color and pattern of plumage: sparse “mouse grey” 5E3 (Kornerup and Wanscher 1967) natal down is present from hatching, and remains attached to the tips of the Juvenile plumage until worn away after the bird leaves the nest. Color of bare parts: skin pinkish orange, bill blackish, corners of mouth yellow, lining of mouth orange, no pattern, eyes closed, blackish through skin, egg tooth retained for 2 days (Bradley 1948, RBP). Degree of coordinated movement or ability to find food: young altricial, depend on adults for care. Responses, body attitude: respond to touch on the nest or body by raising head, stretching neck upward, opening mouth and gaping and begging; conspicuously vocal only when starving.

Growth And Development

On day of hatching, nestlings weigh about 2.1 g, increase to 3.3 g on day 1 after hatching, 4.5 g on day 2, and double their weight again (9.1 g) by day 4 or 5 (Morgan 1976). Tarsus length increases from 10 mm on day 3 (when larger nestlings can be banded but not color-banded), 11–12 mm on day 4, 15 mm on days 5 and 6, and 16–17 mm by day 8. Eyes open by day 5. Flight feathers show under the skin at day 4, pinfeathers appear on back at day 5, feathers emerge from the sheaths at day 6, young can flutter to ground at day 8, but sides remain bare (Bradley 1948). At fledgling, wing about 51mm (UMMZ, MNA).

Growth rate of nestling buntings varies with age, brood size, and whether the brood has a nestling cowbird. Nestlings of 2, 4 and 6 d of age in unparasitized broods gain mass more rapidly in broods of 2 than in broods of 3 or 4 (mean mass gain, 1.4, 1.2, and 1.2 g/d), but the rate of tarsus growth (1.4 mm/d) is independent of brood size. Nestlings in parasitized broods gain less mass per day, and the rate of tarsus growth does not differ from nestlings in the unparasitized broods. The difference in rate of mass gain between the fastest-and slowest-growing nestlings is larger in parasitized broods than in unparasitized broods. Mean mass of chicks at 2, 4, and 6 d in parasitized broods is 3.3 g, 5.8 g, and 9.3 g respectively; mean mass of chicks in unparasitized broods is higher by about 10-15% (Dearborn et al. 1998).

In behavior, nestlings remain inactive until parent arrives at nest with food, then stretch neck upward and gape widely. The eyes are closed until day 5 or 6, when nestlings respond to an observer at nest with a crouch rather than begging. Can grasp a stick with feet at days 6 and 7, balance unsteadily on a perch from days 7 to 9 (Holcomb 1966). On day 8, may depart nest if disturbed. Quiet except when female is at the nest and when nearly ready to fledge. Nestlings do not interfere with each other but beg competitively for food when parent is at nest; when one begs, the others do the same. Between feeding visits, a day or two before fledging, they preen themselves; allopreening not observed. Huddle together and may warm each other, conserving energy. No field observations on time from fledging to independence. Hand-reared birds begin pecking at food by day 28, appear capable of independence by day 30, nevertheless they beg actively and incessently for food through day 35 (RBP).

Development of thermoregulation not reported. Young departing a nest without being disturbed range in weight from 9 g (just out of nest; it was banded and returned to natal area the next year, RBP) to a more usual 12 g (Holcomb 1966, Yuri 2002). First day after fledging, young remain hidden and are difficult to flush (RBP). At 11 days of age a brood was able to fly 7 m or further (Bradley 1948).

Brooding

Females brood young on hatching day. A female observed for one day early in incubation spent 10 h, 27 min on the nest. She increased her time to 14 h, 3 min on the day the eggs hatched, not leaving to feed until 30 min after daylight. The next day she brooded 6 h, 9 min, 43.7% of the daylight period (Morgan 1976). At another nest, the female on hatching day was on the nest 72% of the time, next day 51% of the time, and the fourth day only 16% of the time. At day 8 the nest was observed all day; the female was on the nest all night but during the 15-hour day she brooded 1 h 9 min, 7.5% of the time (Bradley 1948). Elsewhere, female brooded or shaded the 3-day young 7.2% of the time and was at the nest twice that long (Weldon and Haddad 2005). No all-day observations from a blind for a complete nesting cycle, but extensive video recordings provide data for broods of different ages (Dearborn et al. 1998).

Only the female broods. Brooding periods range from 1 to 27 min; periods off the nest range from 3 to 27 and average 12 min (Morgan 1976). Brooding decreases with bunting age, 25 min/h at day 2, 15 min at day 4, and 5 min at day 6 (Dearborn et al. 1998). Females are more attentive during cold, wet weather (RBP).

Feeding

Females feed young from day of hatching (Fig. 10). Time of first feeding not described; females feed hatched young before all eggs hatch and some eggs hatch after young have been fed and brooded for a day.At most nests observed only females brought food to brood. After young leave the nest, females continue to feed them and males sometimes help in parental care.

Females bring food to the nest, one or several insects held in the bill, and insert the insects into mouth of the nestlings when they gape (photo in Allen 1933). Small nestlings are fed small insects and spiders, larger nestlings a wider range of sizes of food. No detailed studies of the food of nestlings reported. Foods brought to nestlings in Michigan include grasshoppers from the ground, smooth green caterpillars from the woodland canopy, dragonflies, spiders, spider egg cases, and berries. Other foods include spiders, mayflies and other caterpillars and moths (Taber and Johnston 1968).

Males do not feed females or bring food for them to pass to nestlings. Males give little parental care. In Indiana and Michigan male feeds young at 10% of nests near fledging time (Bradley 1948, Sutton 1959, Morgan 1976, Carey and Nolan 1979, Westneat 1988a, b, Payne 1989), and in Missouri the male occasionally feeds nestlings at 12% of nests (Dearborn et al. 1998). Fledged brood sometimes splits into two sets of fledglings, one fed by male, the other by female (RBP). In both single sets and double sets some males feed their young (Sutton, in Taber and Johnston 1968, RBP). Older adult males are more likely than first-year males to feed the fledged brood (Westneat 1987b, 1988a, b). Males may be unable to assess their parentage, as are female buntings, which feed any young in their nest including brood-parasitic cowbirds (Payne 1989, 1998, Payne and Payne 1998). Female whose mate feeds their fledglings from an early brood has a shorter period (mean, 8 days earlier) until she begins another nest (Westneat 1988b; male's relief in caring for fledged young may contribute to breeding success of the female on his territory.

Both male and female give chip notes near the nest when the female is on the nest and when the nest has nestlings; and then they defend the nest, scolding and flying over an intruder. Males increase their defense of nesting area after the young hatch and especially after they fledge (Westneat 1989, RBP). Both male and female scold, giving a rapid series of chips and flight display when fledglings from a neighboring territory move into the male's own territory and a human intruder appears (Westneat 1989, RBP).

Feeding rates increase with nestling age. At one nest, female fed average of 2.8 feeding trips/h when nestlings were 1 d old, 4 trips when 2 d, 5.2 trips when 3 d and 7.9 trips when 8 d of age (Bradley 1948). In another nest with small young, a female fed the brood 54 times in a day, at intervals ranging 16 min (Morgan 1976). In video recordings, a nestling at 2 d of age receives half as much food as a nestling at 4 d. Nestling begging rate increases with age, 34s/h at day 2, 50 s/h at day 4, and 57 s/h at day 6. In broods with a cowbird chick, the bunting chicks beg more intensively but receive less food than in broods without a cowbird chick (Dearborn et al. 1998 . In other recordings, females fed 3-day bunting broods 5.2 to 5.9 times an hour (Weldon and Haddad 2005).

Nest Sanitation

Nestlings defecate after being fed. Female eats fecal sac more often than she removes it during the first day or two after the young hatch; in later days she drops it ≥ 10 m(Bradley 1948, Morgan 1976). Young sometimes perch on the rim and defecate on the day of leaving the nest; these feces are not removed (RBP).

Not observed. Fledglings from one brood sometimes remain on territory when parents rear a second brood. No observations of young helping at the later nest. In cases when first-year birds return to establish territories near their natal territory, no observations of cooperative behavior between these birds and their older relatives. Neighboring males that share song themes do not cooperate (in contrast, they countersing and chase each other), yet song activity in neighborhoods may attract other males and females to mate.

Risk Of Cowbird Parasitism In Space And Time

Nests often parasitized by Brown-headed Cowbirds (Molothrus ater) throughout their range. Over the years, 40 of 165 (24.2%) Ontario nests were parasitized through the season (Peck and James 1987); likewise, 411 of 1,721 (19.5%) nests in Michigan (eight seasons, Payne and Payne 1998); 18 of 60 (30%) nests in West Virginia (two seasons; Bell and Whitmore 2000). Higher rates of parasitism reported in other studies (Robinson et al. 1995, Burhans et al. 2000) were based on short-season fieldwork that missed the second half of the breeding season. Proportion of nests parasitized decreases through late spring and summer. In Michigan, at Niles, in May 41% of bunting nests were parasitized. At the George Reserve, parasitism rates were lower but changed in the same way through the season, in May 23%, in June 26%, in July 9%, and none in August (Payne and Payne 1998). Forty years earlier in this population the rates of parasitism changed in the same way through the season, when from 1934 through 1946 four (44%) of nine bunting nests were parasitized by 10 Jul, and none of nine nests were parasitized after 10 Jul (Sutton 1959). Late bunting nests are not parasitized because the latest date of cowbird laying in any host species was 15 Jul, 3 wk before the latest bunting clutch. At Niles more than half the bunting nests were begun after the peak of cowbird season (Wood 1951, Payne 1965, 1976, Payne and Payne 1998). Usually one cowbird egg (sometimes two, rarely three or four) is laid in the parasitized nest ( Payne and Payne 1998, Burhans and Thompson 2000).

Cowbird Parasitism And Nest Sites

In Michigan, neither nests located in different habitats (edge, thicket, swamp, upland, upland woods, mesic woods) nor nests built in different plants (Rubus, Cornus, bracken, goldenrod, gooseberry, honeysuckle, multiflora rose, saplings, sumac) differ in proportion parasitized (Payne and Payne 1998). In West Virginia, nests with less tree canopy cover and more tall snags nearby are more likely to be parasitized than nests with more cover and fewer snags (Bell and Whitmore 2000).

Cowbird Parasitism And Time Of Laying

Cowbirds lay during the laying period at the bunting nest. Exceptionally, a cowbird lays at an inappropriate time, before the bunting has laid (in which case the nest is usually deserted), or after the bunting clutch is complete. In Michigan, a nest on 16 Jun 1989 had 4 bunting eggs; 5 d later had 3 day-old buntings and 1 cowbird egg; after another 5 d had 3 buntings with feathers developing from the sheaths and 1 cowbird that had just hatched; and 2 d later had 3 buntings ready to leave the nest and a dead cowbird nestling; the cowbird laid its egg halfway through incubation (RBP).

Behavior Interactions Of Buntings And Cowbirds

Breeding adult buntings are not seen to interact with adult cowbirds in natural conditions. They behave aggressively to a model female cowbird placed near their nest before incubation, though not significantly more than to control of a model sparrow (Robertson and Norman 1977, Burhans 2000). Female buntings accept cowbird eggs and incubate them along with their own in parasitized broods, as long as the clutch has one or more bunting egg; females desert nests when cowbird eggs are laid before their own, otherwise accept the spotted eggs into their own unspotted white clutch. A nest found with only a cowbird egg is likely to have been deserted. After a cowbird hatches, the female bunting feeds the nestling cowbird even though it differs from nestling buntings: the cowbird has pink mouth lining and white rictal flanges, and not the bunting colors of orange mouth and yellow flanges (Payne 1998).

Egg Removal And Clutch Size

Bunting clutch size is smaller in parastized nests. Cowbirds often remove a bunting egg from the nest before laying their own egg (Morgan 1976), and bunting clutches with cowbird eggs have fewer bunting eggs on average than unparasitized nests. In some nests, two bunting eggs disappear during days when cowbird eggs are laid. Nests with ≥ 1 cowbird eggs average 1.06 fewer bunting eggs (George Reserve) and 0.77 fewer bunting eggs (Niles) than unparasitized nests. In a few parasitized nests, no bunting eggs were removed, and a full set of 4 eggs remained even when 1, 2, or 3 cowbird eggs were laid (Payne and Payne 1998). In the two years with most complete observations, mean number of bunting eggs was lower in parasitized nests at Niles (unparasitized nests, n = 174, mean 3.13 ± 0.59 SD; parasitized nests, n = 97, meant 2.36 ± 0.75 SD) and at the George Reserve (unparasitized nests, n = 87, mean 3.41 ± 0.8 SD; parasitized nests, n =37, meant 2.35 ± 0.82 SD).

Nestling Competition

Proportion of nests that produce fledged buntings is lower in parasitized nests. In nests where at least one egg was laid, breeding success was determined, and nest was undisturbed by observers, 56.4% of 1,310 unparasitized nests and 19.5% of 411 parasitized nests were successful in fledging at least part of the bunting brood.

Cowbirds hatch a day or two before buntings, hatch at a larger size, and grow more rapidly. Number of buntings fledged per nest is lower in nests parasitized by a cowbird. In one area, in 45 nests that fledged a cowbird, mean number of buntings fledged 1.27 ± 0.86 SD; in 451 unparasitized nests, mean buntings fledged 2.71 ± 0.75 SD, about 1.5 fewer in the parasitized nests. In the other area, in 29 nests that had fledged a cowbird, 0.83 ± 0.89 SD buntings fledged; in unparasitized nests, 2.85 ± 0.74 SD buntings fledged. A cowbird fledged costs the breeding buntings on average 1.5-2.0 young buntings fledged from the nest (Payne and Payne 1989) (Fig.11).

Nesting success varies with intensity of parasitism and hatching success of cowbird eggs. Nests with one cowbird egg are more likely to fledge a bunting than nests with ≥ 2 cowbird eggs (22.1% of 339 singly-parasitized nests, 6.9% of 72 multiply-parasitized nests). In nests where a cowbird hatches, brood parasitism decreases chances of bunting fledging (33.2% of 184 parasitized nests successful vs. 56.4% of all 1,310 unparasitized nests). In parasitized nests where cowbird fails to fledge, success of buntings is low (8.4% of 335 parasitized nests, 56.4% of all 1,310 nests). Not all failed nests that are parasitized failed due to cowbird parasitism; predation and weather lead to many of these failures.

In nests where buntings fledge and a cowbird also survives to fledge, the brood produces fewer buntings (Payne and Payne 1998). Cowbird nestlings affect the growth and survival of bunting nestlings in several ways. The young cowbird outcompetes the young buntings by calling more loudly, more persistently, and getting most of the food brought by the parents, and it crowds them into the bottom of the nest and occasionally even pushes them out of the nest, and the young are attended and brooded less in the parasitized nests than in unparasitized nests (Dearborn 1996, 1999, Dearborn et al. 1998, RBP). In mixed broods in captivity, when young were reared for studies of song development, the nestling cowbirds trample the nestling bunting, climb on it, and peck it around the head (RBP).

Nest Predation

Broods with a cowbird nestling were more likely to be taken by a predator than were unparasitized broods. In Michigan, at Niles, for nests with cowbird chick, 53 (47.3%) of 112 parasitized broods and 143 (28.7%) of 626 unparasitized broods were taken by predators; at the George Reserve the numbers were 16 (36.4%) of 44 parasitized broods and 106 (22.7%) of 468 unparasitized broods. The higher risk of predation in parasitized broods may have been due to the cowbird chick begging and attracting predators, to the begging noise or to the sight of the parent making more trips to the nest. In addition, nests with a cowbird egg were more likely to be taken by a predator than were nests with only bunting eggs: at Niles 75 of 263 (31.1%) parasitized clutches and 145 (19.3%) of 793 unparasitized clutches, and at the George Reserve 39 (28.7%) of 136 parasitized clutches and 96 (17.3%) of 556 unparasitized clutches were taken by a predator. Higher risk of predation of the entire clutch of eggs in parasitized nests may be due to greater visibility of the nest both to a laying cowbird and then to a predator (Payne and Payne 1998).

Reduced survival of nestling buntings in broods with a nestling cowbird may also be affected by predation, when predators locate a nest by hearing; a three-day-old cowbird's begging calls at unattended nests are audible at a distance of 2 m, whereas bunting begging calls are not audible until 6 d after hatching (Dearborn 1999, RBP).

Postfledgling Survival

In addition to effects in the nests, young cowbirds affect the success of young buntings after fledging, perhaps in competition for parental care after leaving the nest. Postfledgling survival of young was not monitored during breeding season, but was monitored in return of local-born young the following breeding season. At Niles, only one young bunting returned from any brood in which a cowbird fledged, 1 of 64 buntings that fledged from a parasitized brood, in contrast to 125 of 1413 buntings that fledged from broods that did not fledge a cowbid; 9.3% of fledglings from unparasitized broods and 1.7% of fledglings with a cowbird. At the George Reserve, no buntings returned from broods that fledged a cowbird (of 32 nestlings fledged) and 27 of 1276 young buntings that fledged no cowbird (Payne and Payne 1998).

In Michigan, in 947 nests where bunting eggs were laid but no buntings survived to fledge and observer interference was unlikely, predation was the main source of nest failure (of eggs, 40.5%; of young, 32.9%). Nest predation often involved loss of entire clutch or brood. Cowbird parasitism less often caused total failure of the buntings, when the bunting deserted a parastitized clutch or the nestling cowbird outcompeted all the bunting nestlings for parental care (9.2% of cases of complete loss of a nest). Other sources of brood loss included weather (cold and rain), nest tipping due to its deterioration or attachment to inadequate supports, and starvation of the nestlings, sometimes with disappearance of known death of female on the road. A male does not feed its starving young even when its mate disappears (RBP).

Adult Reproductive Success And Survival

For adult buntings, females that were parasitized had a lower seasonal success in fledging buntings. At Niles, females not parasitized had the highest breeding success for the season, but not significantly higher than females that fledged a cowbird; they had higher successes than females whose cowbird failed to hatch or to fledge. At the George Reserve, unparasitized females fledged more buntings than did parasitized females, due to the shorter time available for a second brood. Rearing a cowbird chick to fledging had no effect on a later nest of the adults within a season, or on their survival and reproductive success into a later year, compared with adults that rear only bunting chicks. Females that were successful were more likely to survive and return next season as females that reared only their own buntings. The cost in terms of future reproductive value of rearing a cowbird appears to be no greater than the cost of the breeding success that is lost in their parasitized brood (Payne and Payne 1998).

Cost Effects Of Cowbird Parasitism On Bunting Success

In summary, the cost of cowbird parasitism is largely accounted for by female cowbird's removal of a bunting egg from the bunting clutch before the cowbird lays her own egg. The difference in number of bunting eggs in unparasitized nests and parasitized nests is greater than 1.0, and that is more than half the difference in the number of young buntings that fledge from the nests. Nevertheless, the higher risk of predation in parasitized nests, the lower the growth rate of bunting young in nests in competition with cowbird young for parental care, and the lower rate of survival in bunting young that fledge together with a young cowbird, all show additional costs of being parasitized beyond the initial loss of an egg taken when the female cowbird is laying.

Success Of Brood-Parasitic Cowbirds

In Michigan over 8 yr, cowbird young fledged from 76 of 411 (18.5%) parasitized nests where bunting eggs were laid and nesting success was known and was not affected by observer disturbance. In the successful cases of brood parasitism, a single cowbird fledged in most broods and two cowbirds fledged in five broods (Payne and Payne 1998). None of the 140 banded cowbirds that survived to fledge from a bunting nest were seen in a later year (RBP).

Young leave the nest as early as 8 d of age if disturbed, as late as 14 d during cool weather when not disturbed. Most fledge 9–12 days after hatching. Calls of the first young leaving nest attract its siblings; all leave within an hour. Young also leave when female gives rapid chips and flutters over nest in response to a person nearby, and presumably to an approaching predator. It is unknown whether the parent normally controls the time of departure. Fledged young sometimes return and roost at night in nest, especially if they have left before day 10. Young perch within a few meters of nest during their first fledgling hours and are fed there by parents. Buntings leave the nest before the flight feathers are fully grown but can fly upward into a tree within hours of leaving the nest.

Juvenile body feathers are nearly fully-grown when the young bird leaves the nest; the tail and wings are short. Growth after fledging has not been tracked in marked birds. Wing and tail feathers continue to grow for at least a week, and young add body mass, particularly breast muscle, to adult size after they leave the nest. Tail grows continually from fledging (10-20 mm) to independence at 30 d (48-50mm) (Yuri 2002). Young are independent by 3 wk after fledging. Nestlings reared by hand continue to beg for food to day 35 after hatching, three weeks after the time they would have fledged; they also pick up seeds in a cage by day 28 after hatching (RBP).

While in the care of a parent, brood usually stays together. Occasionally a male remains on territory and cares for one young from his brood while the female departs with another fledgling, but not known if these are permanent splits or short-term separations. Female may move to a new territory with one or more of her young, where she remains and breeds again with a new mate. Males on territories where the female takes her brood do not feed the young, although they give alarm calls (RBP).

Immature birds often flock together on their breeding grounds, either accompanied by adults or not, as one or both parents may attempt and complete a subsequent nesting cycle. When juveniles are accompanied by adults, the older males do not sing.