Black-billed cuckoo
Many cuckoos are obligate brood parasites, meaning they only lay eggs in other birds' nests and never take care of their own young. Birds that do this, such as the common cuckoo or the brown-headed cowbird, lay relatively small eggs because their expected hosts are usually smaller birds. Cuckoos in the genus "Coccyzus", lay relatively large eggs even though they still parasitize smaller birds. Yellow warblers are the smallest birds recorded caring for black-billed cuckoo eggs. In experiments, these wood warblers were found to accept the cuckoo eggs 63% of the time even though their own eggs are only a quarter of the size. Even with the size difference, the warbler parent is often still able to raise the cuckoo as long as it can provide the chick with sufficient nutrition and incubation.
The black-billed cuckoo was originally placed in genus "Cuculus". This genus comprises Old World cuckoos that are all obligate brood parasites. Upon further genetic testing, it was placed into genus "Coccyzus" that comprises nine species of New World cuckoos. The black-billed and yellow-billed cuckoos are the only two that migrate into North America while the other seven are permanent residents of either Central or South America. Even though these two cuckoos overlap in range, they are not sister taxa. This suggests that the two species invaded North America separately. Although family Cuculidae is monophyletic, the genus "Coccyzus" was found to be polyphyletic.
Reproduction and life cycle of the golden eagle
Mating and egg-laying timing for golden eagle is variable depending on the locality. Copulation normally lasts 10–20 seconds. Mating seems to occur around 40–46 days before the initial egg-laying. Unusual mating systems of three birds have been recorded in Scotland and Sweden, usually the third being an immature of either sex. These may simply reflect a relatively unstable period of population adjustment. Occasionally, pairs have been recorded copulating outside of the context of fertilization, possibly for pair maintenance and displacement activity. In Japan, a pair was seen mating 46 days before egg laying and in the United States 55 days after egg-laying. The date of egg laying has been directly correlated by the latitude where the pair lives. In the U.S., egg laying can be from anywhere between January and September, though is usually in March or later. In Scotland, egg-laying occurs in March to mid-April. The earliest median laying date in 25 international studies was December 3 in Oman; the latest median date of egg-laying was May 7 in sub-Arctic Alaska. Egg laying starts in the dry season as early as late November around Mali and Niger while the median egg laying date in southwestern Morocco is January 15. In Ethiopia, the estimated range of egg-laying dates ranged from October 24 to January 5. The median egg laying day in Arctic Russia was May 1. In the Kilbuck and Ahklun Mountains area, three pairs had eggs hatched from May 14 to 23, and young fledged from July 8 to August 10 with a median date of July 23 for 11 nests. Two nests on the Kisaralik River still contained young on July 24, and four nests contained young from 16 to 21 July. Clutches have been recorded range in size from 1 to 4 eggs, though two is the norm around the range. Records of single egg clutches are fairly common in Europe, with 3 being rare there but seemingly more common in North America, where up to 12% of clutches include 3 eggs. A 4 egg clutch is considered exceptional. The lowest mean clutch size known from surveys is 1.82 eggs in the Altai Mountains of Russia. A mean of 48 clutches from Algeria and Tunisia showed an average of 1.89 eggs per nests, with a range of 1 to 3. The female laid an average number of eggs of 1.99 in 332 clutches from 8 studies in 5 of the Western United States. The largest mean clutch size across the range was 2.1 in Montana. In the wild, eggs are typically laid at 3 to 5 day intervals, with records in captivity of intervals of up to 7 or 10 days. The eggs vary from all white to white with cinnamon or brown spots and blotches. Scottish eggs averaged in size and weigh about . In Armenia, the average egg measured and weighed . In North America, eggs average , with a range of lengths from and a range of width of . Eggs of golden eagles in California weighed from , averaging . The incubation period lasts from 41 to 45 days, averaging about 42.4 days, with previously reported claims of as low as 33 days from North America now known to be erroneous. Females do a majority of, but not all, of the incubating and largely attain their own food up to the stage of egg-laying, after which they are typically fed by the male. The female may continue to grab most of their own food in areas where carrion is easily accessible. If made to forage excessively during the incubation stage by a non-attentive mate, the female may abandon the nesting attempt. In Idaho, the female did 84% of the incubation during the day, with the male doing about 16%. At night, the female generally appears to do all of the incubating. In Japan, the female did 90% of the observed incubating.